Re: [PATCH v8 03/10] arm_ffa: introduce Arm FF-A low-level driver

Abdellatif El Khlifi
2023-01-19T16:32:10Z

On Wed, Jan 18, 2023 at 08:59:32AM -0500, Tom Rini wrote:
> On Wed, Jan 18, 2023 at 01:46:54PM +0000, Sudeep Holla wrote:
> > On Wed, Jan 18, 2023 at 12:49 PM Tom Rini <trini@konsulko.com> wrote:
> >
> > >
> > > I guess the problem comes down to, can we have one discovery method that
> > > everyone shares, or do we have to let everyone invent a new discovery
> > > method every time?
> >
> >
> > No one needs to invent any discovery method every time if the firmware
> > specification
> > provides one and as Rob mentioned many times in the thread, all new firmware
> > specification must provide one and we are trying to make sure that is
> > the case with all new
> > specs from Arm.
> >
> >
> > > FF-A, Op-tee, U-Boot, coreboot, barebox (and
> > > everyone else I'm unintentionally forgetting) could just discover these
> > > things via device tree.
> >
> >
> > I leave that to the individual projects to decide and agree but
> > fundamentally if
> > the specification provides a way to discover, not sure why we are even
> > discussing
> > an alternative method here.
> >
> >
> > > Or, we could all write our own code to perform
> > > the discovery.
> >
> >
> > For what reason ? I can understand if there is no discovery mechanism but
> > that's not the
> > case in $subject.
> >
> >
> > > And when RISC-V comes along with similar functionality,
> > > we could probe their device tree and see they've implemented the same
> > > concept, but a little differently, but still have the discovery portion
> > > be in the device tree. To which it sounds like your answer is "not in
> > > the device tree".
> > >
> >
> > I see U-boot seem to have made a decision to create DT node for each and
> > everything
> > that needs to be added to DM which seems bit unfortunate but I don't
> > understand the
> > history/motive/background for it but I respect the decision if it is
> > already made.
> >
> > These firmware interfaces are standard on all Arm platforms and can be
> > discovered
> > based on PSCI/SMCCC. Not using the same and use DT node needs unnecessary
> > addition of DT nodes for all the f/w i/f on all the platforms that need the
> > support when
> > one can be just discovered.
> >
> > Sorry for the sudden appearance on this thread, I was avoiding getting into
> > this but thought
> > I will at least express my opinion and also the way the firmware
> > specifications from Arm is
> > expected to be evolved from now on. With that I will leave it to you and
> > other U-boot
> > maintainers and the community in general to decide the right course in this
> > case.
>
> To be clear, if the position is that "this is what everyone else will
> use, really" then yes, we'll follow this in U-Boot.

Hi Simon, Tom,

The FF-A transport is a SW bus and is not associated to any HW peripheral or
undiscoverable base address.

There is only 1 way of discovering the FF-A bus and it's through the FF-A SW
interfaces. The FF-A spec [1] describes this in details.

Discovering means gathering information about the FF-A framework such as:
the FF-A version, supported features, secure partitions number and attributes.

Please refer to the following paragraphs for more details: [2], [3], [4], [5]

The core driver provided by this patchset implements the Setup and discovery interfaces
in addition to direct messaging.

The driver provides ffa_bus_discover() API that allows to discover the FF-A bus
as described by the spec and in the FF-A driver readme [6].

We expect and highly recommend FF-A users to always discover the FF-A bus using ffa_bus_discover() API.

A use case is provided which is the EFI MM communication [7].

ffa_bus_discover() does the following:

- creates, binds and probes the arm_ffa device
- at probe level, discovery FF-A interfaces are called to try to discover the FF-A framework
- when all discovery interfaces succeed, probing is successful and FF-A bus is ready to use
- if one of the discovery interfaces fails, the arm_ffa device is removed from the DM and
FF-A bus can not be used

Cheers
Abdellatif

[1]: FF-A spec version 1.0, https://developer.arm.com/documentation/den0077/latest/

[2] 2.8 Partition identification and discovery

All FF-A components can discover the identities and properties of other partitions through the FFA_PARTITION_INFO_GET
interface. Once discovered, the IDs must be used in the messaging interfaces to identify the target of a message.

[3] 4.2.2.2 Buffer discovery and setup

This version of the Framework enables discovery and setup of RX/TX buffer pairs between FF-A components as
follows.

...

2. An endpoint could allocate the buffer pair and use the FFA_RXTX_MAP interface to map it with the
Hypervisor or SPM as applicable.

[4] 4.2.2.3 Buffer attributes

An endpoint must discover the minimum size and alignment boundary for the RX/TX buffers by passing the
function ID of the FFA_RXTX_MAP ABI as input in the FFA_FEATURES interface (see 8.2 FFA_FEATURES).

[5] 6.1.1 Common compliance requirements

- It must be possible for the FF-A components at an FF-A instance to discover the presence and version number
of their Framework implementations through the FFA_VERSION interface (see 8.1 FFA_VERSION).

- All FF-A components must implement support for Setup and discovery interfaces described in Chapter 8
Setup and discovery interfaces. These interfaces are as follows.

FFA_VERSION.
FFA_FEATURES.
FFA_RX_RELEASE.
FFA_RXTX_MAP.
FFA_RXTX_UNMAP.
FFA_PARTITION_INFO_GET.
FFA_ID_GET.

- It must be possible for an FF-A component, at the lower EL at an FF-A instance to use the FFA_FEATURES
interface (see 8.2 FFA_FEATURES) to discover if an FF-A interface is implemented by the FF-A component
at the higher EL.

[6] FF-A support readme: https://lore.kernel.org/all/20221122131751.22747-4-abdellatif.elkhlifi@arm.com/#Z31doc:arch:arm64.ffa.rst
[7] FF-A MM comms: https://lore.kernel.org/all/20221122131751.22747-10-abdellatif.elkhlifi@arm.com/
>
> --
> Tom