[PATCH 01/16] docs: Add documentation for MHI bus

[PATCH 01/16] docs: Add documentation for MHI bus

[Manivannan Sadhasivam]

MHI (Modem Host Interface) is a communication protocol used by the
host processors to control and communicate with modems over a high
speed peripheral bus or shared memory. The MHI protocol has been
designed and developed by Qualcomm Innovation Center, Inc., for use
in their modems. This commit adds the documentation for the bus and
the implementation in Linux kernel.

This is based on the patch submitted by Sujeev Dias:
https://lkml.org/lkml/2018/7/9/987

Cc: Jonathan Corbet <corbet@lwn.net>
Cc: linux-doc@vger.kernel.org
Signed-off-by: Sujeev Dias <sdias@codeaurora.org>
Signed-off-by: Siddartha Mohanadoss <smohanad@codeaurora.org>
[mani: converted to .rst and splitted the patch]
Signed-off-by: Manivannan Sadhasivam <manivannan.sadhasivam@linaro.org>
---
 Documentation/index.rst        |   1 +
 Documentation/mhi/index.rst    |  18 +++
 Documentation/mhi/mhi.rst      | 218 +++++++++++++++++++++++++++++++++
 Documentation/mhi/topology.rst |  60 +++++++++
 4 files changed, 297 insertions(+)
 create mode 100644 Documentation/mhi/index.rst
 create mode 100644 Documentation/mhi/mhi.rst
 create mode 100644 Documentation/mhi/topology.rst

diff --git a/Documentation/index.rst b/Documentation/index.rst
index e99d0bd2589d..edc9b211bbff 100644
--- a/Documentation/index.rst
+++ b/Documentation/index.rst
@@ -133,6 +133,7 @@ needed).
    misc-devices/index
    mic/index
    scheduler/index
+   mhi/index
 
 Architecture-agnostic documentation
 -----------------------------------
diff --git a/Documentation/mhi/index.rst b/Documentation/mhi/index.rst
new file mode 100644
index 000000000000..1d8dec302780
--- /dev/null
+++ b/Documentation/mhi/index.rst
@@ -0,0 +1,18 @@
+.. SPDX-License-Identifier: GPL-2.0
+
+===
+MHI
+===
+
+.. toctree::
+   :maxdepth: 1
+
+   mhi
+   topology
+
+.. only::  subproject and html
+
+   Indices
+   =======
+
+   * :ref:`genindex`
diff --git a/Documentation/mhi/mhi.rst b/Documentation/mhi/mhi.rst
new file mode 100644
index 000000000000..718dbbdc7a04
--- /dev/null
+++ b/Documentation/mhi/mhi.rst
@@ -0,0 +1,218 @@
+.. SPDX-License-Identifier: GPL-2.0
+
+==========================
+MHI (Modem Host Interface)
+==========================
+
+This document provides information about the MHI protocol.
+
+Overview
+========
+
+MHI is a protocol developed by Qualcomm Innovation Center, Inc., It is used
+by the host processors to control and communicate with modem devices over high
+speed peripheral buses or shared memory. Even though MHI can be easily adapted
+to any peripheral buses, it is primarily used with PCIe based devices. MHI
+provides logical channels over the physical buses and allows transporting the
+modem protocols, such as IP data packets, modem control messages, and
+diagnostics over at least one of those logical channels. Also, the MHI
+protocol provides data acknowledgment feature and manages the power state of the
+modems via one or more logical channels.
+
+MHI Internals
+=============
+
+MMIO
+----
+
+MMIO (Memory mapped IO) consists of a set of registers in the device hardware,
+which are mapped to the host memory space by the peripheral buses like PCIe.
+Following are the major components of MMIO register space:
+
+MHI control registers: Access to MHI configurations registers
+
+MHI BHI registers: BHI (Boot Host Interface) registers are used by the host
+for downloading the firmware to the device before MHI initialization.
+
+Channel Doorbell array: Channel Doorbell (DB) registers used by the host to
+notify the device when there is new work to do.
+
+Event Doorbell array: Associated with event context array, the Event Doorbell
+(DB) registers are used by the host to notify the device when new events are
+available.
+
+Debug registers: A set of registers and counters used by the device to expose
+debugging information like performance, functional, and stability to the host.
+
+Data structures
+---------------
+
+All data structures used by MHI are in the host system memory. Using the
+physical interface, the device accesses those data structures. MHI data
+structures and data buffers in the host system memory regions are mapped for
+the device.
+
+Channel context array: All channel configurations are organized in channel
+context data array.
+
+Transfer rings: Used by the host to schedule work items for a channel. The
+transfer rings are organized as a circular queue of Transfer Descriptors (TD).
+
+Event context array: All event configurations are organized in the event context
+data array.
+
+Event rings: Used by the device to send completion and state transition messages
+to the host
+
+Command context array: All command configurations are organized in command
+context data array.
+
+Command rings: Used by the host to send MHI commands to the device. The command
+rings are organized as a circular queue of Command Descriptors (CD).
+
+Channels
+--------
+
+MHI channels are logical, unidirectional data pipes between a host and a device.
+The concept of channels in MHI is similar to endpoints in USB. MHI supports up
+to 256 channels. However, specific device implementations may support less than
+the maximum number of channels allowed.
+
+Two unidirectional channels with their associated transfer rings form a
+bidirectional data pipe, which can be used by the upper-layer protocols to
+transport application data packets (such as IP packets, modem control messages,
+diagnostics messages, and so on). Each channel is associated with a single
+transfer ring.
+
+Transfer rings
+--------------
+
+Transfers between the host and device are organized by channels and defined by
+Transfer Descriptors (TD). TDs are managed through transfer rings, which are
+defined for each channel between the device and host and reside in the host
+memory. TDs consist of one or more ring elements (or transfer blocks)::
+
+        [Read Pointer (RP)] ----------->[Ring Element] } TD
+        [Write Pointer (WP)]-           [Ring Element]
+                             -          [Ring Element]
+                              --------->[Ring Element]
+                                        [Ring Element]
+
+Below is the basic usage of transfer rings:
+
+* Host allocates memory for transfer ring.
+* Host sets the base pointer, read pointer, and write pointer in corresponding
+  channel context.
+* Ring is considered empty when RP == WP.
+* Ring is considered full when WP + 1 == RP.
+* RP indicates the next element to be serviced by the device.
+* When the host has a new buffer to send, it updates the ring element with
+  buffer information, increments the WP to the next element and rings the
+  associated channel DB.
+
+Event rings
+-----------
+
+Events from the device to host are organized in event rings and defined by Event
+Descriptors (ED). Event rings are used by the device to report events such as
+data transfer completion status, command completion status, and state changes
+to the host. Event rings are the array of EDs that resides in the host
+memory. EDs consist of one or more ring elements (or transfer blocks)::
+
+        [Read Pointer (RP)] ----------->[Ring Element] } ED
+        [Write Pointer (WP)]-           [Ring Element]
+                             -          [Ring Element]
+                              --------->[Ring Element]
+                                        [Ring Element]
+
+Below is the basic usage of event rings:
+
+* Host allocates memory for event ring.
+* Host sets the base pointer, read pointer, and write pointer in corresponding
+  channel context.
+* Both host and device has a local copy of RP, WP.
+* Ring is considered empty (no events to service) when WP + 1 == RP.
+* Ring is considered full of events when RP == WP.
+* When there is a new event the device needs to send, the device updates ED
+  pointed by RP, increments the RP to the next element and triggers the
+  interrupt.
+
+Ring Element
+------------
+
+A Ring Element is a data structure used to transfer a single block
+of data between the host and the device. Transfer ring element types contain a
+single buffer pointer, the size of the buffer, and additional control
+information. Other ring element types may only contain control and status
+information. For single buffer operations, a ring descriptor is composed of a
+single element. For large multi-buffer operations (such as scatter and gather),
+elements can be chained to form a longer descriptor.
+
+MHI Operations
+==============
+
+MHI States
+----------
+
+MHI_STATE_RESET
+~~~~~~~~~~~~~~~
+MHI is in reset state after power-up or hardware reset. The host is not allowed
+to access device MMIO register space.
+
+MHI_STATE_READY
+~~~~~~~~~~~~~~~
+MHI is ready for initialization. The host can start MHI initialization by
+programming MMIO registers.
+
+MHI_STATE_M0
+~~~~~~~~~~~~
+MHI is running and operational in the device. The host can start channels by
+issuing channel start command.
+
+MHI_STATE_M1
+~~~~~~~~~~~~
+MHI operation is suspended by the device. This state is entered when the
+device detects inactivity at the physical interface within a preset time.
+
+MHI_STATE_M2
+~~~~~~~~~~~~
+MHI is in low power state. MHI operation is suspended and the device may
+enter lower power mode.
+
+MHI_STATE_M3
+~~~~~~~~~~~~
+MHI operation stopped by the host. This state is entered when the host suspends
+MHI operation.
+
+MHI Initialization
+------------------
+
+After system boots, the device is enumerated over the physical interface.
+In the case of PCIe, the device is enumerated and assigned BAR-0 for
+the device's MMIO register space. To initialize the MHI in a device,
+the host performs the following operations:
+
+* Allocates the MHI context for event, channel and command arrays.
+* Initializes the context array, and prepares interrupts.
+* Waits until the device enters READY state.
+* Programs MHI MMIO registers and sets device into MHI_M0 state.
+* Waits for the device to enter M0 state.
+
+MHI Data Transfer
+-----------------
+
+MHI data transfer is initiated by the host to transfer data to the device.
+Following are the sequence of operations performed by the host to transfer
+data to device:
+
+* Host prepares TD with buffer information.
+* Host increments the WP of the corresponding channel transfer ring.
+* Host rings the channel DB register.
+* Device wakes up to process the TD.
+* Device generates a completion event for the processed TD by updating ED.
+* Device increments the RP of the corresponding event ring.
+* Device triggers IRQ to wake up the host.
+* Host wakes up and checks the event ring for completion event.
+* Host updates the WP of the corresponding event ring to indicate that the
+  data transfer has been completed successfully.
+
diff --git a/Documentation/mhi/topology.rst b/Documentation/mhi/topology.rst
new file mode 100644
index 000000000000..90d80a7f116d
--- /dev/null
+++ b/Documentation/mhi/topology.rst
@@ -0,0 +1,60 @@
+.. SPDX-License-Identifier: GPL-2.0
+
+============
+MHI Topology
+============
+
+This document provides information about the MHI topology modeling and
+representation in the kernel.
+
+MHI Controller
+--------------
+
+MHI controller driver manages the interaction with the MHI client devices
+such as the external modems and WiFi chipsets. It is also the MHI bus master
+which is in charge of managing the physical link between the host and device.
+It is however not involved in the actual data transfer as the data transfer
+is taken care by the physical bus such as PCIe. Each controller driver exposes
+channels and events based on the client device type.
+
+Below are the roles of the MHI controller driver:
+
+* Turns on the physical bus and establishes the link to the device
+* Configures IRQs, SMMU, and IOMEM
+* Allocates struct mhi_controller and registers with the MHI bus framework
+  with channel and event configurations using mhi_register_controller.
+* Initiates power on and shutdown sequence
+* Initiates suspend and resume power management operations of the device.
+
+MHI Device
+----------
+
+MHI device is the logical device which binds to a maximum of two MHI channels
+for bi-directional communication. Once MHI is in powered on state, the MHI
+core will create MHI devices based on the channel configuration exposed
+by the controller. There can be a single MHI device for each channel or for a
+couple of channels.
+
+Each supported device is enumerated in::
+
+        /sys/bus/mhi/devices/
+
+MHI Driver
+----------
+
+MHI driver is the client driver which binds to one or more MHI devices. The MHI
+driver sends and receives the upper-layer protocol packets like IP packets,
+modem control messages, and diagnostics messages over MHI. The MHI core will
+bind the MHI devices to the MHI driver.
+
+Each supported driver is enumerated in::
+
+        /sys/bus/mhi/drivers/
+
+Below are the roles of the MHI driver:
+
+* Registers the driver with the MHI bus framework using mhi_driver_register.
+* Prepares the device for transfer by calling mhi_prepare_for_transfer.
+* Initiates data transfer by calling mhi_queue_transfer.
+* Once the data transfer is finished, calls mhi_unprepare_from_transfer to
+  end data transfer.
-- 
2.17.1

Re: [PATCH 01/16] docs: Add documentation for MHI bus

[Arnd Bergmann]

On Thu, Jan 23, 2020 at 12:18 PM Manivannan Sadhasivam
<manivannan.sadhasivam@linaro.org> wrote:
> +============
> +MHI Topology
> +============
> +
> +This document provides information about the MHI topology modeling and
> +representation in the kernel.
> +
> +MHI Controller
> +--------------
> +
> +MHI controller driver manages the interaction with the MHI client devices
> +such as the external modems and WiFi chipsets. It is also the MHI bus master
> +which is in charge of managing the physical link between the host and device.
> +It is however not involved in the actual data transfer as the data transfer
> +is taken care by the physical bus such as PCIe. Each controller driver exposes
> +channels and events based on the client device type.
> +
> +Below are the roles of the MHI controller driver:
> +
> +* Turns on the physical bus and establishes the link to the device
> +* Configures IRQs, SMMU, and IOMEM
> +* Allocates struct mhi_controller and registers with the MHI bus framework
> +  with channel and event configurations using mhi_register_controller.
> +* Initiates power on and shutdown sequence
> +* Initiates suspend and resume power management operations of the device.

I don't see any callers of mhi_register_controller(). Did I just miss it or did
you not post one? I'm particularly interested in where the configuration comes
from, is this hardcoded in the driver, or parsed from firmware or from registers
in the hardware itself?

        Arnd

Re: [PATCH 01/16] docs: Add documentation for MHI bus

[Manivannan Sadhasivam]

On Thu, Jan 23, 2020 at 01:58:22PM +0100, Arnd Bergmann wrote:
> On Thu, Jan 23, 2020 at 12:18 PM Manivannan Sadhasivam
> <manivannan.sadhasivam@linaro.org> wrote:
> > +============
> > +MHI Topology
> > +============
> > +
> > +This document provides information about the MHI topology modeling and
> > +representation in the kernel.
> > +
> > +MHI Controller
> > +--------------
> > +
> > +MHI controller driver manages the interaction with the MHI client devices
> > +such as the external modems and WiFi chipsets. It is also the MHI bus master
> > +which is in charge of managing the physical link between the host and device.
> > +It is however not involved in the actual data transfer as the data transfer
> > +is taken care by the physical bus such as PCIe. Each controller driver exposes
> > +channels and events based on the client device type.
> > +
> > +Below are the roles of the MHI controller driver:
> > +
> > +* Turns on the physical bus and establishes the link to the device
> > +* Configures IRQs, SMMU, and IOMEM
> > +* Allocates struct mhi_controller and registers with the MHI bus framework
> > +  with channel and event configurations using mhi_register_controller.
> > +* Initiates power on and shutdown sequence
> > +* Initiates suspend and resume power management operations of the device.
> 
> I don't see any callers of mhi_register_controller(). Did I just miss it or did
> you not post one? I'm particularly interested in where the configuration comes
> from, is this hardcoded in the driver, or parsed from firmware or from registers
> in the hardware itself?
> 

I have not included the controller driver in this patchset. But you can take a
look at the ath11k controller driver here:
https://git.linaro.org/people/manivannan.sadhasivam/linux.git/tree/drivers/net/wireless/ath/ath11k/mhi.c?h=ath11k-qca6390-mhi#n13

So the configuration comes from the static structures defined in the controller
driver. Earlier revision derived the configuration from devicetree but there are
many cases where this MHI bus is being used in non DT environments like x86.
So inorder to be platform agnostic, we chose static declaration method.

In future we can add DT/ACPI support for the applicable parameters.

I will include the link to this controller driver in the cover letter of future
iterations.

Thanks,
Mani

>         Arnd

Re: [PATCH 01/16] docs: Add documentation for MHI bus

[Arnd Bergmann]

On Thu, Jan 23, 2020 at 2:10 PM Manivannan Sadhasivam
<manivannan.sadhasivam@linaro.org> wrote:
>
> On Thu, Jan 23, 2020 at 01:58:22PM +0100, Arnd Bergmann wrote:
> > On Thu, Jan 23, 2020 at 12:18 PM Manivannan Sadhasivam <manivannan.sadhasivam@linaro.org> wrote:
> >
> > I don't see any callers of mhi_register_controller(). Did I just miss it or did
> > you not post one? I'm particularly interested in where the configuration comes
> > from, is this hardcoded in the driver, or parsed from firmware or from registers
> > in the hardware itself?
> >
>
> I have not included the controller driver in this patchset. But you can take a
> look at the ath11k controller driver here:
> https://git.linaro.org/people/manivannan.sadhasivam/linux.git/tree/drivers/net/wireless/ath/ath11k/mhi.c?h=ath11k-qca6390-mhi#n13
>
> So the configuration comes from the static structures defined in the controller
> driver. Earlier revision derived the configuration from devicetree but there are
> many cases where this MHI bus is being used in non DT environments like x86.
> So inorder to be platform agnostic, we chose static declaration method.
>
> In future we can add DT/ACPI support for the applicable parameters.

What determines the configuration? Is this always something that is fixed
in hardware, or can some of the properties be changed based on what
firmware runs the device?

If this is determined by the firmware, maybe the configuration would also
need to be loaded from the file that contains the firmware, which in turn
could be a blob in DT.

     Arnd

Re: [PATCH 01/16] docs: Add documentation for MHI bus

[Manivannan Sadhasivam]

On Thu, Jan 23, 2020 at 02:19:51PM +0100, Arnd Bergmann wrote:
> On Thu, Jan 23, 2020 at 2:10 PM Manivannan Sadhasivam
> <manivannan.sadhasivam@linaro.org> wrote:
> >
> > On Thu, Jan 23, 2020 at 01:58:22PM +0100, Arnd Bergmann wrote:
> > > On Thu, Jan 23, 2020 at 12:18 PM Manivannan Sadhasivam <manivannan.sadhasivam@linaro.org> wrote:
> > >
> > > I don't see any callers of mhi_register_controller(). Did I just miss it or did
> > > you not post one? I'm particularly interested in where the configuration comes
> > > from, is this hardcoded in the driver, or parsed from firmware or from registers
> > > in the hardware itself?
> > >
> >
> > I have not included the controller driver in this patchset. But you can take a
> > look at the ath11k controller driver here:
> > https://git.linaro.org/people/manivannan.sadhasivam/linux.git/tree/drivers/net/wireless/ath/ath11k/mhi.c?h=ath11k-qca6390-mhi#n13
> >
> > So the configuration comes from the static structures defined in the controller
> > driver. Earlier revision derived the configuration from devicetree but there are
> > many cases where this MHI bus is being used in non DT environments like x86.
> > So inorder to be platform agnostic, we chose static declaration method.
> >
> > In future we can add DT/ACPI support for the applicable parameters.
> 
> What determines the configuration? Is this always something that is fixed
> in hardware, or can some of the properties be changed based on what
> firmware runs the device?
> 

AFAIK, these configurations are fixed in hardware (this could come from
the firmware I'm not sure but they don't change with firmware revisions
for sure)

The reason for defining in the driver itself implies that these don't
change. But I'll confirm this with Qcom folks.

Thanks,
Mani

> If this is determined by the firmware, maybe the configuration would also
> need to be loaded from the file that contains the firmware, which in turn
> could be a blob in DT.
> 
>      Arnd

Re: [PATCH 01/16] docs: Add documentation for MHI bus

[Jeffrey Hugo]

On 1/23/2020 6:30 AM, Manivannan Sadhasivam wrote:
> On Thu, Jan 23, 2020 at 02:19:51PM +0100, Arnd Bergmann wrote:
>> On Thu, Jan 23, 2020 at 2:10 PM Manivannan Sadhasivam
>> <manivannan.sadhasivam@linaro.org> wrote:
>>>
>>> On Thu, Jan 23, 2020 at 01:58:22PM +0100, Arnd Bergmann wrote:
>>>> On Thu, Jan 23, 2020 at 12:18 PM Manivannan Sadhasivam <manivannan.sadhasivam@linaro.org> wrote:
>>>>
>>>> I don't see any callers of mhi_register_controller(). Did I just miss it or did
>>>> you not post one? I'm particularly interested in where the configuration comes
>>>> from, is this hardcoded in the driver, or parsed from firmware or from registers
>>>> in the hardware itself?
>>>>
>>>
>>> I have not included the controller driver in this patchset. But you can take a
>>> look at the ath11k controller driver here:
>>> https://git.linaro.org/people/manivannan.sadhasivam/linux.git/tree/drivers/net/wireless/ath/ath11k/mhi.c?h=ath11k-qca6390-mhi#n13
>>>
>>> So the configuration comes from the static structures defined in the controller
>>> driver. Earlier revision derived the configuration from devicetree but there are
>>> many cases where this MHI bus is being used in non DT environments like x86.
>>> So inorder to be platform agnostic, we chose static declaration method.
>>>
>>> In future we can add DT/ACPI support for the applicable parameters.
>>
>> What determines the configuration? Is this always something that is fixed
>> in hardware, or can some of the properties be changed based on what
>> firmware runs the device?
>>
> 
> AFAIK, these configurations are fixed in hardware (this could come from
> the firmware I'm not sure but they don't change with firmware revisions
> for sure)
> 
> The reason for defining in the driver itself implies that these don't
> change. But I'll confirm this with Qcom folks.
> 
> Thanks,
> Mani
> 
>> If this is determined by the firmware, maybe the configuration would also
>> need to be loaded from the file that contains the firmware, which in turn
>> could be a blob in DT.
>>
>>       Arnd

We can't derive the configuration from hardware, and its something that 
is currently a priori known since the host (linux) needs to initialize 
the hardware with the configuration before it can communicate with the 
device (ie the on device FW).

99% of the time the configuration is fixed, however there have been 
instances where features have been added on the device, which result in 
new channels, which then impact the configuration.  In the cases I'm 
aware of this, both sides were updated in lockstep.  I don't know how 
upstream would handle it.  I'm thinking we can ignore that case until it 
comes up.

DT/ACPI is tricky, since the cases where we want this currently are 
essentially standalone PCI(e) cards.  Those are likely to be on systems 
which don't support DT (ie x86), and there really isn't a place in ACPI 
to put PCI(e) device configuration information, since its supposed to be 
a discoverable bus.

There are hardware limitations to the configuration, and that varies 
from device to device.  Since the host (linux) programs the 
configuration into the hardware, its possible for an invalid 
configuration to be programed, but I would expect that in the majority 
of cases (ie programming a channel that the device FW doesn't know 
about), there is no adverse impact.

-- 
Jeffrey Hugo
Qualcomm Technologies, Inc. is a member of the
Code Aurora Forum, a Linux Foundation Collaborative Project.

Re: [PATCH 01/16] docs: Add documentation for MHI bus

[Jeffrey Hugo]

On 1/23/2020 4:18 AM, Manivannan Sadhasivam wrote:
> MHI (Modem Host Interface) is a communication protocol used by the
> host processors to control and communicate with modems over a high
> speed peripheral bus or shared memory. The MHI protocol has been
> designed and developed by Qualcomm Innovation Center, Inc., for use
> in their modems. This commit adds the documentation for the bus and
> the implementation in Linux kernel.
> 
> This is based on the patch submitted by Sujeev Dias:
> https://lkml.org/lkml/2018/7/9/987
> 
> Cc: Jonathan Corbet <corbet@lwn.net>
> Cc: linux-doc@vger.kernel.org
> Signed-off-by: Sujeev Dias <sdias@codeaurora.org>
> Signed-off-by: Siddartha Mohanadoss <smohanad@codeaurora.org>
> [mani: converted to .rst and splitted the patch]
> Signed-off-by: Manivannan Sadhasivam <manivannan.sadhasivam@linaro.org>
> ---
>   Documentation/index.rst        |   1 +
>   Documentation/mhi/index.rst    |  18 +++
>   Documentation/mhi/mhi.rst      | 218 +++++++++++++++++++++++++++++++++
>   Documentation/mhi/topology.rst |  60 +++++++++
>   4 files changed, 297 insertions(+)
>   create mode 100644 Documentation/mhi/index.rst
>   create mode 100644 Documentation/mhi/mhi.rst
>   create mode 100644 Documentation/mhi/topology.rst
> 
> diff --git a/Documentation/index.rst b/Documentation/index.rst
> index e99d0bd2589d..edc9b211bbff 100644
> --- a/Documentation/index.rst
> +++ b/Documentation/index.rst
> @@ -133,6 +133,7 @@ needed).
>      misc-devices/index
>      mic/index
>      scheduler/index
> +   mhi/index
>   
>   Architecture-agnostic documentation
>   -----------------------------------
> diff --git a/Documentation/mhi/index.rst b/Documentation/mhi/index.rst
> new file mode 100644
> index 000000000000..1d8dec302780
> --- /dev/null
> +++ b/Documentation/mhi/index.rst
> @@ -0,0 +1,18 @@
> +.. SPDX-License-Identifier: GPL-2.0
> +
> +===
> +MHI
> +===
> +
> +.. toctree::
> +   :maxdepth: 1
> +
> +   mhi
> +   topology
> +
> +.. only::  subproject and html
> +
> +   Indices
> +   =======
> +
> +   * :ref:`genindex`
> diff --git a/Documentation/mhi/mhi.rst b/Documentation/mhi/mhi.rst
> new file mode 100644
> index 000000000000..718dbbdc7a04
> --- /dev/null
> +++ b/Documentation/mhi/mhi.rst
> @@ -0,0 +1,218 @@
> +.. SPDX-License-Identifier: GPL-2.0
> +
> +==========================
> +MHI (Modem Host Interface)
> +==========================
> +
> +This document provides information about the MHI protocol.
> +
> +Overview
> +========
> +
> +MHI is a protocol developed by Qualcomm Innovation Center, Inc., It is used

The "," suggests the sentence is going to continue, yet "it" has a 
capitol "I" like its the start of a new sentence.  This seems wrong to 
me.  Perhaps drop that final comma?

> +by the host processors to control and communicate with modem devices over high
> +speed peripheral buses or shared memory. Even though MHI can be easily adapted
> +to any peripheral buses, it is primarily used with PCIe based devices. MHI
> +provides logical channels over the physical buses and allows transporting the
> +modem protocols, such as IP data packets, modem control messages, and
> +diagnostics over at least one of those logical channels. Also, the MHI
> +protocol provides data acknowledgment feature and manages the power state of the
> +modems via one or more logical channels.
> +
> +MHI Internals
> +=============
> +
> +MMIO
> +----
> +
> +MMIO (Memory mapped IO) consists of a set of registers in the device hardware,
> +which are mapped to the host memory space by the peripheral buses like PCIe.
> +Following are the major components of MMIO register space:
> +
> +MHI control registers: Access to MHI configurations registers
> +
> +MHI BHI registers: BHI (Boot Host Interface) registers are used by the host
> +for downloading the firmware to the device before MHI initialization.
> +
> +Channel Doorbell array: Channel Doorbell (DB) registers used by the host to
> +notify the device when there is new work to do.
> +
> +Event Doorbell array: Associated with event context array, the Event Doorbell
> +(DB) registers are used by the host to notify the device when new events are
> +available.
> +
> +Debug registers: A set of registers and counters used by the device to expose
> +debugging information like performance, functional, and stability to the host.
> +
> +Data structures
> +---------------
> +
> +All data structures used by MHI are in the host system memory. Using the
> +physical interface, the device accesses those data structures. MHI data
> +structures and data buffers in the host system memory regions are mapped for
> +the device.
> +
> +Channel context array: All channel configurations are organized in channel
> +context data array.
> +
> +Transfer rings: Used by the host to schedule work items for a channel. The
> +transfer rings are organized as a circular queue of Transfer Descriptors (TD).
> +
> +Event context array: All event configurations are organized in the event context
> +data array.
> +
> +Event rings: Used by the device to send completion and state transition messages
> +to the host
> +
> +Command context array: All command configurations are organized in command
> +context data array.
> +
> +Command rings: Used by the host to send MHI commands to the device. The command
> +rings are organized as a circular queue of Command Descriptors (CD).
> +
> +Channels
> +--------
> +
> +MHI channels are logical, unidirectional data pipes between a host and a device.
> +The concept of channels in MHI is similar to endpoints in USB. MHI supports up
> +to 256 channels. However, specific device implementations may support less than
> +the maximum number of channels allowed.
> +
> +Two unidirectional channels with their associated transfer rings form a
> +bidirectional data pipe, which can be used by the upper-layer protocols to
> +transport application data packets (such as IP packets, modem control messages,
> +diagnostics messages, and so on). Each channel is associated with a single
> +transfer ring.
> +
> +Transfer rings
> +--------------
> +
> +Transfers between the host and device are organized by channels and defined by
> +Transfer Descriptors (TD). TDs are managed through transfer rings, which are
> +defined for each channel between the device and host and reside in the host
> +memory. TDs consist of one or more ring elements (or transfer blocks)::
> +
> +        [Read Pointer (RP)] ----------->[Ring Element] } TD
> +        [Write Pointer (WP)]-           [Ring Element]
> +                             -          [Ring Element]
> +                              --------->[Ring Element]
> +                                        [Ring Element]
> +
> +Below is the basic usage of transfer rings:
> +
> +* Host allocates memory for transfer ring.
> +* Host sets the base pointer, read pointer, and write pointer in corresponding
> +  channel context.
> +* Ring is considered empty when RP == WP.
> +* Ring is considered full when WP + 1 == RP.
> +* RP indicates the next element to be serviced by the device.
> +* When the host has a new buffer to send, it updates the ring element with
> +  buffer information, increments the WP to the next element and rings the
> +  associated channel DB.
> +
> +Event rings
> +-----------
> +
> +Events from the device to host are organized in event rings and defined by Event
> +Descriptors (ED). Event rings are used by the device to report events such as
> +data transfer completion status, command completion status, and state changes
> +to the host. Event rings are the array of EDs that resides in the host
> +memory. EDs consist of one or more ring elements (or transfer blocks)::
> +
> +        [Read Pointer (RP)] ----------->[Ring Element] } ED
> +        [Write Pointer (WP)]-           [Ring Element]
> +                             -          [Ring Element]
> +                              --------->[Ring Element]
> +                                        [Ring Element]
> +
> +Below is the basic usage of event rings:
> +
> +* Host allocates memory for event ring.
> +* Host sets the base pointer, read pointer, and write pointer in corresponding
> +  channel context.
> +* Both host and device has a local copy of RP, WP.
> +* Ring is considered empty (no events to service) when WP + 1 == RP.
> +* Ring is considered full of events when RP == WP.
> +* When there is a new event the device needs to send, the device updates ED
> +  pointed by RP, increments the RP to the next element and triggers the
> +  interrupt.
> +
> +Ring Element
> +------------
> +
> +A Ring Element is a data structure used to transfer a single block
> +of data between the host and the device. Transfer ring element types contain a
> +single buffer pointer, the size of the buffer, and additional control
> +information. Other ring element types may only contain control and status
> +information. For single buffer operations, a ring descriptor is composed of a
> +single element. For large multi-buffer operations (such as scatter and gather),
> +elements can be chained to form a longer descriptor.
> +
> +MHI Operations
> +==============
> +
> +MHI States
> +----------
> +
> +MHI_STATE_RESET
> +~~~~~~~~~~~~~~~
> +MHI is in reset state after power-up or hardware reset. The host is not allowed
> +to access device MMIO register space.
> +
> +MHI_STATE_READY
> +~~~~~~~~~~~~~~~
> +MHI is ready for initialization. The host can start MHI initialization by
> +programming MMIO registers.
> +
> +MHI_STATE_M0
> +~~~~~~~~~~~~
> +MHI is running and operational in the device. The host can start channels by
> +issuing channel start command.
> +
> +MHI_STATE_M1
> +~~~~~~~~~~~~
> +MHI operation is suspended by the device. This state is entered when the
> +device detects inactivity at the physical interface within a preset time.
> +
> +MHI_STATE_M2
> +~~~~~~~~~~~~
> +MHI is in low power state. MHI operation is suspended and the device may
> +enter lower power mode.
> +
> +MHI_STATE_M3
> +~~~~~~~~~~~~
> +MHI operation stopped by the host. This state is entered when the host suspends
> +MHI operation.
> +
> +MHI Initialization
> +------------------
> +
> +After system boots, the device is enumerated over the physical interface.
> +In the case of PCIe, the device is enumerated and assigned BAR-0 for
> +the device's MMIO register space. To initialize the MHI in a device,
> +the host performs the following operations:
> +
> +* Allocates the MHI context for event, channel and command arrays.
> +* Initializes the context array, and prepares interrupts.
> +* Waits until the device enters READY state.
> +* Programs MHI MMIO registers and sets device into MHI_M0 state.
> +* Waits for the device to enter M0 state.
> +
> +MHI Data Transfer
> +-----------------
> +
> +MHI data transfer is initiated by the host to transfer data to the device.
> +Following are the sequence of operations performed by the host to transfer
> +data to device:
> +
> +* Host prepares TD with buffer information.
> +* Host increments the WP of the corresponding channel transfer ring.
> +* Host rings the channel DB register.
> +* Device wakes up to process the TD.
> +* Device generates a completion event for the processed TD by updating ED.
> +* Device increments the RP of the corresponding event ring.
> +* Device triggers IRQ to wake up the host.
> +* Host wakes up and checks the event ring for completion event.
> +* Host updates the WP of the corresponding event ring to indicate that the
> +  data transfer has been completed successfully.
> +
> diff --git a/Documentation/mhi/topology.rst b/Documentation/mhi/topology.rst
> new file mode 100644
> index 000000000000..90d80a7f116d
> --- /dev/null
> +++ b/Documentation/mhi/topology.rst
> @@ -0,0 +1,60 @@
> +.. SPDX-License-Identifier: GPL-2.0
> +
> +============
> +MHI Topology
> +============
> +
> +This document provides information about the MHI topology modeling and
> +representation in the kernel.
> +
> +MHI Controller
> +--------------
> +
> +MHI controller driver manages the interaction with the MHI client devices
> +such as the external modems and WiFi chipsets. It is also the MHI bus master
> +which is in charge of managing the physical link between the host and device.
> +It is however not involved in the actual data transfer as the data transfer
> +is taken care by the physical bus such as PCIe. Each controller driver exposes
> +channels and events based on the client device type.
> +
> +Below are the roles of the MHI controller driver:
> +
> +* Turns on the physical bus and establishes the link to the device
> +* Configures IRQs, SMMU, and IOMEM

I'd recommend changing SMMU to IOMMU.  SMMU tends to be an ARM specific 
term, while IOMMU is the generic term, in my experience.

> +* Allocates struct mhi_controller and registers with the MHI bus framework
> +  with channel and event configurations using mhi_register_controller.
> +* Initiates power on and shutdown sequence
> +* Initiates suspend and resume power management operations of the device.
> +
> +MHI Device
> +----------
> +
> +MHI device is the logical device which binds to a maximum of two MHI channels
> +for bi-directional communication. Once MHI is in powered on state, the MHI
> +core will create MHI devices based on the channel configuration exposed
> +by the controller. There can be a single MHI device for each channel or for a
> +couple of channels.
> +
> +Each supported device is enumerated in::
> +
> +        /sys/bus/mhi/devices/
> +
> +MHI Driver
> +----------
> +
> +MHI driver is the client driver which binds to one or more MHI devices. The MHI
> +driver sends and receives the upper-layer protocol packets like IP packets,
> +modem control messages, and diagnostics messages over MHI. The MHI core will
> +bind the MHI devices to the MHI driver.
> +
> +Each supported driver is enumerated in::
> +
> +        /sys/bus/mhi/drivers/
> +
> +Below are the roles of the MHI driver:
> +
> +* Registers the driver with the MHI bus framework using mhi_driver_register.
> +* Prepares the device for transfer by calling mhi_prepare_for_transfer.
> +* Initiates data transfer by calling mhi_queue_transfer.
> +* Once the data transfer is finished, calls mhi_unprepare_from_transfer to
> +  end data transfer.
> 


-- 
Jeffrey Hugo
Qualcomm Technologies, Inc. is a member of the
Code Aurora Forum, a Linux Foundation Collaborative Project.

Re: [PATCH 01/16] docs: Add documentation for MHI bus

[Manivannan Sadhasivam]

On Thu, Jan 23, 2020 at 09:41:42AM -0700, Jeffrey Hugo wrote:
> On 1/23/2020 4:18 AM, Manivannan Sadhasivam wrote:
> > MHI (Modem Host Interface) is a communication protocol used by the
> > host processors to control and communicate with modems over a high
> > speed peripheral bus or shared memory. The MHI protocol has been
> > designed and developed by Qualcomm Innovation Center, Inc., for use
> > in their modems. This commit adds the documentation for the bus and
> > the implementation in Linux kernel.
> > 
> > This is based on the patch submitted by Sujeev Dias:
> > https://lkml.org/lkml/2018/7/9/987
> > 
> > Cc: Jonathan Corbet <corbet@lwn.net>
> > Cc: linux-doc@vger.kernel.org
> > Signed-off-by: Sujeev Dias <sdias@codeaurora.org>
> > Signed-off-by: Siddartha Mohanadoss <smohanad@codeaurora.org>
> > [mani: converted to .rst and splitted the patch]
> > Signed-off-by: Manivannan Sadhasivam <manivannan.sadhasivam@linaro.org>
> > ---
> >   Documentation/index.rst        |   1 +
> >   Documentation/mhi/index.rst    |  18 +++
> >   Documentation/mhi/mhi.rst      | 218 +++++++++++++++++++++++++++++++++
> >   Documentation/mhi/topology.rst |  60 +++++++++
> >   4 files changed, 297 insertions(+)
> >   create mode 100644 Documentation/mhi/index.rst
> >   create mode 100644 Documentation/mhi/mhi.rst
> >   create mode 100644 Documentation/mhi/topology.rst
> > 
> > diff --git a/Documentation/index.rst b/Documentation/index.rst
> > index e99d0bd2589d..edc9b211bbff 100644
> > --- a/Documentation/index.rst
> > +++ b/Documentation/index.rst
> > @@ -133,6 +133,7 @@ needed).
> >      misc-devices/index
> >      mic/index
> >      scheduler/index
> > +   mhi/index
> >   Architecture-agnostic documentation
> >   -----------------------------------
> > diff --git a/Documentation/mhi/index.rst b/Documentation/mhi/index.rst
> > new file mode 100644
> > index 000000000000..1d8dec302780
> > --- /dev/null
> > +++ b/Documentation/mhi/index.rst
> > @@ -0,0 +1,18 @@
> > +.. SPDX-License-Identifier: GPL-2.0
> > +
> > +===
> > +MHI
> > +===
> > +
> > +.. toctree::
> > +   :maxdepth: 1
> > +
> > +   mhi
> > +   topology
> > +
> > +.. only::  subproject and html
> > +
> > +   Indices
> > +   =======
> > +
> > +   * :ref:`genindex`
> > diff --git a/Documentation/mhi/mhi.rst b/Documentation/mhi/mhi.rst
> > new file mode 100644
> > index 000000000000..718dbbdc7a04
> > --- /dev/null
> > +++ b/Documentation/mhi/mhi.rst
> > @@ -0,0 +1,218 @@
> > +.. SPDX-License-Identifier: GPL-2.0
> > +
> > +==========================
> > +MHI (Modem Host Interface)
> > +==========================
> > +
> > +This document provides information about the MHI protocol.
> > +
> > +Overview
> > +========
> > +
> > +MHI is a protocol developed by Qualcomm Innovation Center, Inc., It is used
> 
> The "," suggests the sentence is going to continue, yet "it" has a capitol
> "I" like its the start of a new sentence.  This seems wrong to me.  Perhaps
> drop that final comma?
> 

Interesting find... Yes, the comma needs to be dropped :)

> > +by the host processors to control and communicate with modem devices over high
> > +speed peripheral buses or shared memory. Even though MHI can be easily adapted
> > +to any peripheral buses, it is primarily used with PCIe based devices. MHI
> > +provides logical channels over the physical buses and allows transporting the
> > +modem protocols, such as IP data packets, modem control messages, and
> > +diagnostics over at least one of those logical channels. Also, the MHI
> > +protocol provides data acknowledgment feature and manages the power state of the
> > +modems via one or more logical channels.
> > +
> > +MHI Internals
> > +=============
> > +
> > +MMIO
> > +----
> > +
> > +MMIO (Memory mapped IO) consists of a set of registers in the device hardware,
> > +which are mapped to the host memory space by the peripheral buses like PCIe.
> > +Following are the major components of MMIO register space:
> > +
> > +MHI control registers: Access to MHI configurations registers
> > +
> > +MHI BHI registers: BHI (Boot Host Interface) registers are used by the host
> > +for downloading the firmware to the device before MHI initialization.
> > +
> > +Channel Doorbell array: Channel Doorbell (DB) registers used by the host to
> > +notify the device when there is new work to do.
> > +
> > +Event Doorbell array: Associated with event context array, the Event Doorbell
> > +(DB) registers are used by the host to notify the device when new events are
> > +available.
> > +
> > +Debug registers: A set of registers and counters used by the device to expose
> > +debugging information like performance, functional, and stability to the host.
> > +
> > +Data structures
> > +---------------
> > +
> > +All data structures used by MHI are in the host system memory. Using the
> > +physical interface, the device accesses those data structures. MHI data
> > +structures and data buffers in the host system memory regions are mapped for
> > +the device.
> > +
> > +Channel context array: All channel configurations are organized in channel
> > +context data array.
> > +
> > +Transfer rings: Used by the host to schedule work items for a channel. The
> > +transfer rings are organized as a circular queue of Transfer Descriptors (TD).
> > +
> > +Event context array: All event configurations are organized in the event context
> > +data array.
> > +
> > +Event rings: Used by the device to send completion and state transition messages
> > +to the host
> > +
> > +Command context array: All command configurations are organized in command
> > +context data array.
> > +
> > +Command rings: Used by the host to send MHI commands to the device. The command
> > +rings are organized as a circular queue of Command Descriptors (CD).
> > +
> > +Channels
> > +--------
> > +
> > +MHI channels are logical, unidirectional data pipes between a host and a device.
> > +The concept of channels in MHI is similar to endpoints in USB. MHI supports up
> > +to 256 channels. However, specific device implementations may support less than
> > +the maximum number of channels allowed.
> > +
> > +Two unidirectional channels with their associated transfer rings form a
> > +bidirectional data pipe, which can be used by the upper-layer protocols to
> > +transport application data packets (such as IP packets, modem control messages,
> > +diagnostics messages, and so on). Each channel is associated with a single
> > +transfer ring.
> > +
> > +Transfer rings
> > +--------------
> > +
> > +Transfers between the host and device are organized by channels and defined by
> > +Transfer Descriptors (TD). TDs are managed through transfer rings, which are
> > +defined for each channel between the device and host and reside in the host
> > +memory. TDs consist of one or more ring elements (or transfer blocks)::
> > +
> > +        [Read Pointer (RP)] ----------->[Ring Element] } TD
> > +        [Write Pointer (WP)]-           [Ring Element]
> > +                             -          [Ring Element]
> > +                              --------->[Ring Element]
> > +                                        [Ring Element]
> > +
> > +Below is the basic usage of transfer rings:
> > +
> > +* Host allocates memory for transfer ring.
> > +* Host sets the base pointer, read pointer, and write pointer in corresponding
> > +  channel context.
> > +* Ring is considered empty when RP == WP.
> > +* Ring is considered full when WP + 1 == RP.
> > +* RP indicates the next element to be serviced by the device.
> > +* When the host has a new buffer to send, it updates the ring element with
> > +  buffer information, increments the WP to the next element and rings the
> > +  associated channel DB.
> > +
> > +Event rings
> > +-----------
> > +
> > +Events from the device to host are organized in event rings and defined by Event
> > +Descriptors (ED). Event rings are used by the device to report events such as
> > +data transfer completion status, command completion status, and state changes
> > +to the host. Event rings are the array of EDs that resides in the host
> > +memory. EDs consist of one or more ring elements (or transfer blocks)::
> > +
> > +        [Read Pointer (RP)] ----------->[Ring Element] } ED
> > +        [Write Pointer (WP)]-           [Ring Element]
> > +                             -          [Ring Element]
> > +                              --------->[Ring Element]
> > +                                        [Ring Element]
> > +
> > +Below is the basic usage of event rings:
> > +
> > +* Host allocates memory for event ring.
> > +* Host sets the base pointer, read pointer, and write pointer in corresponding
> > +  channel context.
> > +* Both host and device has a local copy of RP, WP.
> > +* Ring is considered empty (no events to service) when WP + 1 == RP.
> > +* Ring is considered full of events when RP == WP.
> > +* When there is a new event the device needs to send, the device updates ED
> > +  pointed by RP, increments the RP to the next element and triggers the
> > +  interrupt.
> > +
> > +Ring Element
> > +------------
> > +
> > +A Ring Element is a data structure used to transfer a single block
> > +of data between the host and the device. Transfer ring element types contain a
> > +single buffer pointer, the size of the buffer, and additional control
> > +information. Other ring element types may only contain control and status
> > +information. For single buffer operations, a ring descriptor is composed of a
> > +single element. For large multi-buffer operations (such as scatter and gather),
> > +elements can be chained to form a longer descriptor.
> > +
> > +MHI Operations
> > +==============
> > +
> > +MHI States
> > +----------
> > +
> > +MHI_STATE_RESET
> > +~~~~~~~~~~~~~~~
> > +MHI is in reset state after power-up or hardware reset. The host is not allowed
> > +to access device MMIO register space.
> > +
> > +MHI_STATE_READY
> > +~~~~~~~~~~~~~~~
> > +MHI is ready for initialization. The host can start MHI initialization by
> > +programming MMIO registers.
> > +
> > +MHI_STATE_M0
> > +~~~~~~~~~~~~
> > +MHI is running and operational in the device. The host can start channels by
> > +issuing channel start command.
> > +
> > +MHI_STATE_M1
> > +~~~~~~~~~~~~
> > +MHI operation is suspended by the device. This state is entered when the
> > +device detects inactivity at the physical interface within a preset time.
> > +
> > +MHI_STATE_M2
> > +~~~~~~~~~~~~
> > +MHI is in low power state. MHI operation is suspended and the device may
> > +enter lower power mode.
> > +
> > +MHI_STATE_M3
> > +~~~~~~~~~~~~
> > +MHI operation stopped by the host. This state is entered when the host suspends
> > +MHI operation.
> > +
> > +MHI Initialization
> > +------------------
> > +
> > +After system boots, the device is enumerated over the physical interface.
> > +In the case of PCIe, the device is enumerated and assigned BAR-0 for
> > +the device's MMIO register space. To initialize the MHI in a device,
> > +the host performs the following operations:
> > +
> > +* Allocates the MHI context for event, channel and command arrays.
> > +* Initializes the context array, and prepares interrupts.
> > +* Waits until the device enters READY state.
> > +* Programs MHI MMIO registers and sets device into MHI_M0 state.
> > +* Waits for the device to enter M0 state.
> > +
> > +MHI Data Transfer
> > +-----------------
> > +
> > +MHI data transfer is initiated by the host to transfer data to the device.
> > +Following are the sequence of operations performed by the host to transfer
> > +data to device:
> > +
> > +* Host prepares TD with buffer information.
> > +* Host increments the WP of the corresponding channel transfer ring.
> > +* Host rings the channel DB register.
> > +* Device wakes up to process the TD.
> > +* Device generates a completion event for the processed TD by updating ED.
> > +* Device increments the RP of the corresponding event ring.
> > +* Device triggers IRQ to wake up the host.
> > +* Host wakes up and checks the event ring for completion event.
> > +* Host updates the WP of the corresponding event ring to indicate that the
> > +  data transfer has been completed successfully.
> > +
> > diff --git a/Documentation/mhi/topology.rst b/Documentation/mhi/topology.rst
> > new file mode 100644
> > index 000000000000..90d80a7f116d
> > --- /dev/null
> > +++ b/Documentation/mhi/topology.rst
> > @@ -0,0 +1,60 @@
> > +.. SPDX-License-Identifier: GPL-2.0
> > +
> > +============
> > +MHI Topology
> > +============
> > +
> > +This document provides information about the MHI topology modeling and
> > +representation in the kernel.
> > +
> > +MHI Controller
> > +--------------
> > +
> > +MHI controller driver manages the interaction with the MHI client devices
> > +such as the external modems and WiFi chipsets. It is also the MHI bus master
> > +which is in charge of managing the physical link between the host and device.
> > +It is however not involved in the actual data transfer as the data transfer
> > +is taken care by the physical bus such as PCIe. Each controller driver exposes
> > +channels and events based on the client device type.
> > +
> > +Below are the roles of the MHI controller driver:
> > +
> > +* Turns on the physical bus and establishes the link to the device
> > +* Configures IRQs, SMMU, and IOMEM
> 
> I'd recommend changing SMMU to IOMMU.  SMMU tends to be an ARM specific
> term, while IOMMU is the generic term, in my experience.
> 

Ack.

Thanks a lot for the review!

Regards,
Mani

> > +* Allocates struct mhi_controller and registers with the MHI bus framework
> > +  with channel and event configurations using mhi_register_controller.
> > +* Initiates power on and shutdown sequence
> > +* Initiates suspend and resume power management operations of the device.
> > +
> > +MHI Device
> > +----------
> > +
> > +MHI device is the logical device which binds to a maximum of two MHI channels
> > +for bi-directional communication. Once MHI is in powered on state, the MHI
> > +core will create MHI devices based on the channel configuration exposed
> > +by the controller. There can be a single MHI device for each channel or for a
> > +couple of channels.
> > +
> > +Each supported device is enumerated in::
> > +
> > +        /sys/bus/mhi/devices/
> > +
> > +MHI Driver
> > +----------
> > +
> > +MHI driver is the client driver which binds to one or more MHI devices. The MHI
> > +driver sends and receives the upper-layer protocol packets like IP packets,
> > +modem control messages, and diagnostics messages over MHI. The MHI core will
> > +bind the MHI devices to the MHI driver.
> > +
> > +Each supported driver is enumerated in::
> > +
> > +        /sys/bus/mhi/drivers/
> > +
> > +Below are the roles of the MHI driver:
> > +
> > +* Registers the driver with the MHI bus framework using mhi_driver_register.
> > +* Prepares the device for transfer by calling mhi_prepare_for_transfer.
> > +* Initiates data transfer by calling mhi_queue_transfer.
> > +* Once the data transfer is finished, calls mhi_unprepare_from_transfer to
> > +  end data transfer.
> > 
> 
> 
> -- 
> Jeffrey Hugo
> Qualcomm Technologies, Inc. is a member of the
> Code Aurora Forum, a Linux Foundation Collaborative Project.