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SubjectRe: framebuffer corruption due to overlapping stp instructions on arm64


On Mon, 6 Aug 2018, Catalin Marinas wrote:

> On Mon, Aug 06, 2018 at 05:47:36PM +0200, Ard Biesheuvel wrote:
> > On 6 August 2018 at 14:42, Robin Murphy <robin.murphy@arm.com> wrote:
> > > On 06/08/18 11:25, Mikulas Patocka wrote:
> > > [...]
> > >>>
> > >>> None of this explains why some transactions fail to make it across
> > >>> entirely. The overlapping writes in question write the same data to
> > >>> the memory locations that are covered by both, and so the ordering in
> > >>> which the transactions are received should not affect the outcome.
> > >>
> > >> You're right that the corruption couldn't be explained just by reordering
> > >> writes. My hypothesis is that the PCIe controller tries to disambiguate
> > >> the overlapping writes, but the disambiguation logic was not tested and it
> > >> is buggy. If there's a barrier between the overlapping writes, the PCIe
> > >> controller won't see any overlapping writes, so it won't trigger the
> > >> faulty disambiguation logic and it works.
> > >>
> > >> Could the ARM engineers look if there's some chicken bit in Cortex-A72
> > >> that could insert barriers between non-cached writes automatically?
> > >
> > > I don't think there is, and even if there was I imagine it would have a
> > > pretty hideous effect on non-coherent DMA buffers and the various other
> > > places in which we have Normal-NC mappings of actual system RAM.
> >
> > Looking at the A72 manual, there is one chicken bit that looks like it
> > may be related:
> >
> > CPUACTLR_EL1 bit #50:
> >
> > 0 Enables store streaming on NC/GRE memory type. This is the reset value.
> > 1 Disables store streaming on NC/GRE memory type.
> >
> > so putting something like
> >
> > mrs x0, S3_1_C15_C2_0
> > orr x0, x0, #(1 << 50)
> > msr S3_1_C15_C2_0, x0
> >
> > in __cpu_setup() would be worth a try.
>
> Note that access to this register may be disabled at EL3 by firmware
> (ACTLR_EL3.CPUACTLR).
>
> FWIW, Mikulas' test seems to run fine on a ThunderX1 with AMD
> FirePro W2100 (on /dev/fb1)

I have the EDK EFI firmware sources (and I can load it from a SD card, so
there's no risk of bricking the board), so I can insert the write into it,
if you say where.

Mikulas

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