Re: framebuffer corruption due to overlapping stp instructions on arm64

Catalin Marinas
2018-08-06T17:14:04Z

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)

--
Catalin