Re: [RFC PATCH v4 6/8] xen/arm: scmi: introduce SCI SCMI SMC multi-agent driver

[Julien Grall <julien@xxxxxxx>]

Hi Stefano,

On 22/06/2025 23:15, Stefano Stabellini wrote:
> On Thu, 19 Jun 2025, Oleksii Moisieiev wrote:
> > On 18/06/2025 02:22, Stefano Stabellini wrote:
> > > On Thu, 12 Jun 2025, Oleksii Moisieiev wrote:
> > > > [1]:https://git.iliana.fyi/linux/patch/?id=d5141f37c42e0b833863f157ac4cee203b2ba3d2
> > > Keep in mind that [0] refers specifically to access to MMIO regions. I
> > > assume that the SCMI shared buffers are on normal memory? Regarding [1],
> > > it makes sense if Linux is trying to support shared memory over MMIO.
> > >
> > > Looking at one of your replies below, I am guessing the memory buffers
> > > are actually in normal memory but the issue is that TF-A is mapping them
> > > as uncacheable. Is that correct?
> > >
> > > In that case, I still don't understand why a simple memcpy would not be
> > > sufficient. Can you check?
> > >
> > > If yes, then for now I would just simplify it down to memcpy. When
> > > someone adds support for an SCMI server elsewhere we could look into
> > > adding a more sophisticated memcpy and we can look at the details at
> > > that point in time. Specifically, I am not convinced that memcpy_toio
> > > and memcpy_fromio would work if the SCMI server is on a separate
> > > non-coherent microcontroller.
> > According to the TF-A implementation  SCMI memory
> >
> > is mapped with the  flags: MT_DEVICE (like for  stm32mp1) or
> > MT_NON_CACHEABLE (for rpi3)
> >
> > So probably you're right. I will check with simple memcpy.
> There is a difference between MT_DEVICE and MT_NON_CACHEABLE: as far as
> I know MT_DEVICE requires aligned accesses while MT_NON_CACHEABLE does
> not.
>
> However, as I wrote in the other email, if I am not mistaken the current
> implementation of memcpy might work well for us anyway. (To be
> confirmed.)
I am not entirely sure what exactly you want to confirm. I have already 
mentioned several time that our memcpy() on arm64 is using unaligned 
access. So it can't be used for copying data to/from device memory area.
Are you looking for a different things?

Cheers,

--
Julien Grall