Re: framebuffer corruption due to overlapping stp instructions on arm64

[Arnd Bergmann <arnd@arndb.de>]

On Wed, Aug 8, 2018 at 6:22 PM Mikulas Patocka <mpatocka@redhat.com> wrote:
>
> On Wed, 8 Aug 2018, Catalin Marinas wrote:
>
> > On Wed, Aug 08, 2018 at 02:26:11PM +0000, David Laight wrote:
> > > From: Mikulas Patocka
> > > > Sent: 08 August 2018 14:47
> > > ...
> > > > The problem on ARM is that I see data corruption when the overlapping
> > > > unaligned writes are done just by a single core.
> > >
> > > Is this a sequence of unaligned writes (that shouldn't modify the
> > > same physical locations) or an aligned write followed by an
> > > unaligned one that updates part of the earlier write.
> > > (Or the opposite order?)
> >
> > In the memcpy() case, there can be a sequence of unaligned writes but
> > they would not modify the same byte (so no overlapping address at the
> > byte level).
>
> They do modify the same byte, but with the same value. Suppose that you
> want to copy a piece of data that is between 8 and 16 bytes long. You can
> do this:
>
> add src_end, src, len
> add dst_end, dst, len
> ldr x0, [src]
> ldr x1, [src_end - 8]
> str x0, [dst]
> str x1, [dst_end - 8]
>
> The ARM64 memcpy uses this trick heavily in order to reduce branching, and
> this is what makes the PCIe controller choke.

So when a single unaligned 'stp' gets translated into a PCIe with TLP
with length=5 (20 bytes) and LastBE = ~1stBE, write combining the
overlapping stores gives us a TLP with a longer length (5..8 for two
stores), and byte-enable bits that are not exactly a complement.

If the explanation is just that of the byte-enable settings of the merged
TLP are wrong, maybe the problem is that one of them is always
the complement of the other, which would work for power-of-two
length but not the odd length of the TLP post write-combining?

      Arnd