Re: [PATCH] arm64: cache: Lower ARCH_DMA_MINALIGN to 64 (L1_CACHE_BYTES)

[Arnd Bergmann <arnd@arndb.de>]

On Tue, Jul 6, 2021 at 6:20 PM Will Deacon <will@kernel.org> wrote:
>
> I think the million dollar question is whether the 128-byte cache-lines
> live in a cache above the PoC or not. If it's just a system level cache
> through which all DMA is "coherent", then it doesn't matter.

On Wed, Jul 7, 2021 at 10:24 AM Yassine Oudjana
<y.oudjana@protonmail.com> wrote:
>
> On Wednesday, July 7th, 2021 at 12:33 AM, Arnd Bergmann <arnd@arndb.de> wrote:
> > On Tue, Jul 6, 2021 at 7:15 PM Yassine Oudjana y.oudjana@protonmail.com wrote:
> > >
> > > $ numactl -C 0 line -M 1M
> > > 128
> > > $ numactl -C 3 line -M 1M
> > > 128
> >
> >     Can you rerun the the 'line' test with '-M 128K' to see if that confirms the 64
> >     byte L1 line size that the 'cache' test reported?
>
> $ numactl -C 0 line -M 128K
> 64
> $ numactl -C 3 line -M 128K
> 64

Ok, so this seems to confirm that the L1 uses 64 byte lines, while the L2 (or
possibly L3) uses 128 byte lines.

On Wed, Jul 7, 2021 at 12:27 AM Bjorn Andersson
<bjorn.andersson@linaro.org> wrote:
>
> I can confirm that MSM8996, and a few derivatives, has 128 byte cache lines.

Ok, thanks. Assuming this is an outer cache and the L1 indeed has a smaller line
size, can you also confirm that this 128 byte lines are north of the point of
coherency?

In other words, does the CTR_EL0.DminLine field also show 128 bytes
(in which case
it seems we already lost)? And if not, does a CPU store to the second half of a
128 byte L2 line cause DMA data in the first half to be clobbered?

       Arnd