Re: [PATCH] dmaengine: qcom: bam_dma: Add LOCK and UNLOCK flag bit support

[Vinod Koul <vkoul@kernel.org>]

On 27-01-21, 23:56, mdalam@codeaurora.org wrote:
> On 2021-01-19 22:15, Vinod Koul wrote:
> > On 18-01-21, 09:21, mdalam@codeaurora.org wrote:
> > > On 2021-01-15 11:28, Vinod Koul wrote:
> > > > On 14-01-21, 01:20, mdalam@codeaurora.org wrote:
> > > > > On 2021-01-12 15:40, Vinod Koul wrote:
> > > > > > On 12-01-21, 15:01, mdalam@codeaurora.org wrote:
> > > > > > > On 2020-12-21 23:03, mdalam@codeaurora.org wrote:
> > > > > > > > On 2020-12-21 14:53, Vinod Koul wrote:
> > > > > > > > > Hello,
> > > > > > > > >
> > > > > > > > > On 17-12-20, 20:07, Md Sadre Alam wrote:
> > > > > > > > > > This change will add support for LOCK & UNLOCK flag bit support
> > > > > > > > > > on CMD descriptor.
> > > > > > > > > >
> > > > > > > > > > If DMA_PREP_LOCK flag passed in prep_slave_sg then requester of this
> > > > > > > > > > transaction wanted to lock the DMA controller for this transaction so
> > > > > > > > > > BAM driver should set LOCK bit for the HW descriptor.
> > > > > > > > > >
> > > > > > > > > > If DMA_PREP_UNLOCK flag passed in prep_slave_sg then requester
> > > > > > > > > > of this
> > > > > > > > > > transaction wanted to unlock the DMA controller.so BAM driver
> > > > > > > > > > should set
> > > > > > > > > > UNLOCK bit for the HW descriptor.
> > > > > > > > >
> > > > > > > > > Can you explain why would we need to first lock and then unlock..? How
> > > > > > > > > would this be used in real world.
> > > > > > > > >
> > > > > > > > > I have read a bit of documentation but is unclear to me. Also should
> > > > > > > > > this be exposed as an API to users, sounds like internal to driver..?
> > > > > > > > >
> > > > > > > >
> > > > > > > > IPQ5018 SoC having only one Crypto Hardware Engine. This Crypto Hardware
> > > > > > > > Engine
> > > > > > > > will be shared between A53 core & ubi32 core. There is two separate
> > > > > > > > driver dedicated
> > > > > > > > to A53 core and ubi32 core. So to use Crypto Hardware Engine
> > > > > > > > parallelly for encryption/description
> > > > > > > > we need bam locking mechanism. if one driver will submit the request
> > > > > > > > for encryption/description
> > > > > > > > to Crypto then first it has to set LOCK flag bit on command descriptor
> > > > > > > > so that other pipes will
> > > > > > > > get locked.
> > > > > > > >
> > > > > > > > The Pipe Locking/Unlocking will be only on command-descriptor. Upon
> > > > > > > > encountering a command descriptor
> > > > > >
> > > > > > Can you explain what is a cmd descriptor?
> > > > >
> > > > >   In BAM pipe descriptor structure there is a field called CMD
> > > > > (Command
> > > > > descriptor).
> > > > >   CMD allows the SW to create descriptors of type Command which does
> > > > > not
> > > > > generate any data transmissions
> > > > >   but configures registers in the Peripheral (write operations, and
> > > > > read
> > > > > registers operations ).
> > > > >   Using command descriptor enables the SW to queue new configurations
> > > > > between data transfers in advance.
> > > >
> > > > What and when is the CMD descriptor used for..?
> > > 
> > >   CMD descriptor is mainly used for configuring controller register.
> > >   We can read/write controller register via BAM using CMD descriptor
> > > only.
> > >   CMD descriptor use command pipe for the transaction.
> > 
> > In which use cases would you need to issue cmd descriptors..?
> 
>   In IPQ5018 there is only one Crypto engine and it will get shared
>   between UBI32 core & A53 core. So here we need to use command
>   descriptor in-order to perform LOCKING/UNLOCKING mechanism. Since
>   LOCK/ULOCK flag we can set only on CMD descriptor.

So when will lock/unlock be performed? Can you please explain that..

> > 
> > > >
> > > > > >
> > > > > > > > with LOCK bit set, The BAM will lock all other pipes not related to
> > > > > > > > the current pipe group, and keep
> > > > > > > > handling the current pipe only until it sees the UNLOCK set then it
> > > > > > > > will release all locked pipes.
> > > > > > > > locked pipe will not fetch new descriptors even if it got event/events
> > > > > > > > adding more descriptors for
> > > > > > > > this pipe (locked pipe).
> > > > > > > >
> > > > > > > > No need to expose as an API to user because its internal to driver, so
> > > > > > > > while preparing command descriptor
> > > > > > > > just we have to update the LOCK/UNLOCK flag.
> > > > > >
> > > > > > So IIUC, no api right? it would be internal to driver..?
> > > > >
> > > > >   Yes its totally internal to deriver.
> > > >
> > > > So no need for this patch then, right?
> > > 
> > >   This patch is needed , because if some hardware will shared between
> > >   multiple core like A53 and ubi32 for example. In IPQ5018 there is
> > >   only one crypto engine and this will be shared between A53 core and
> > >   ubi32 core and in A53 core & ubi32 core there are different drivers
> > >   is getting used. So if encryption/decryption request come at same
> > >   time from both the driver then things will get messed up. So here we
> > >   need LOCKING mechanism.  If first request is from A53 core driver
> > >   then this driver should lock all the pipes other than pipe dedicated
> > >   to A53 core. So while preparing CMD descriptor driver should used
> > >   this flag "DMA_PREP_LOCK", Since LOCK and UNLOCK flag bit we can set
> > >   only on CMD descriptor. Once request processed then driver will set
> > >   UNLOCK flag on CMD descriptor. Driver should use this flag
> > >   "DMA_PREP_UNLOCK" while preparing CMD descriptor. Same logic will be
> > >   apply for ubi32 core driver as well.
> > 
> > Why cant this be applied at driver level, based on txn being issued it
> > can lock issue the txn and then unlock when done. I am not convinced yet
> > that this needs to be exported to users and can be managed by dmaengine
> > driver.
> 
>   The actual LOCK/UNLOCK flag should be set on hardware command descriptor.
>   so this flag setting should be done in DMA engine driver. The user of the
> DMA
>   driver like (in case of IPQ5018) Crypto can use flag "DMA_PREP_LOCK" &
> "DMA_PREP_UNLOCK"
>   while preparing CMD descriptor before submitting to the DMA engine. In DMA
> engine driver
>   we are checking these flasgs on CMD descriptor and setting actual
> LOCK/UNLOCK flag on hardware
>   descriptor.


I am not sure I comprehend this yet.. when is that we would need to do
this... is this for each txn submitted to dmaengine.. or something
else..

> 
>    if (flags & DMA_PREP_CMD) { <== check for descriptor type
> 		if (flags & DMA_PREP_LOCK)
> 			desc->flags |= cpu_to_le16(DESC_FLAG_LOCK); <== Actual LOCK flag setting
> on HW descriptor.
> 		if (flags & DMA_PREP_UNLOCK)
> 			desc->flags |= cpu_to_le16(DESC_FLAG_UNLOCK); <== Actual UNLOCK flag
> setting on HW descriptor.
> 	}
> > 
> > Thanks

-- 
~Vinod