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

Thu Jan 19 17:40:45 CET 2023

On Thu, Jan 19, 2023 at 04:31:57PM +0000, Abdellatif El Khlifi wrote:
> 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 at 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/

Since I'm apparently being unclear, let me try again. Yes, fine, for
U-Boot, we'll go ahead and accept patches that implement this spec, and
not require device tree modifications.

But as also I believe been agreed on, this doesn't prevent some other
architecture / group from coming along and claiming it has a new unique
approach to this problem and so has to re-invent the discoverable
software bus wheel. Nor does it prevent some group from inventing a
different software defined bus and discovery method and not re-using any
of this work, because it too is special and unique and somehow different
enough.

-- 
Tom
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