Re: framebuffer corruption due to overlapping stp instructions on arm64

[Ard Biesheuvel <ard.biesheuvel@linaro.org>]

On 6 August 2018 at 15:41, Marcin Wojtas <mw@semihalf.com> wrote:
> Hi Mikulas,
>
> pon., 6 sie 2018 o 14:42 Robin Murphy <robin.murphy@arm.com> napisał(a):
>>
>> 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.
>>
>> > I observe these kinds of corruptions:
>> > - failing to write a few bytes
>>
>> That could potentially be explained by the reordering/atomicity issues
>> Matt mentioned, i.e. the load is observing part of the store, before the
>> store has fully completed.
>>
>> > - writing a few bytes that were written 16 bytes before
>> > - writing a few bytes that were written 16 bytes after
>>
>> Those sound more like the interconnect or root complex ignoring the byte
>> strobes on an unaligned burst, of which I think the simplistic view
>> would be "it's broken".
>>
>> FWIW I stuck my old Nvidia 7600GT card in my Arm Juno r2 board (2x
>> Cortex-A72), built your test program natively with GCC 8.1.1 at -O2, and
>> it's still happily flickering pixels in the corner of the console after
>> nearly an hour (in parallel with some iperf3 just to ensure plenty of
>> PCIe traffic). I would strongly suspect this issue is particular to
>> Armada 8k, so its' probably one for the Marvell folks to take a closer
>> look at - I believe some previous interconnect issues on those SoCs were
>> actually fixable in firmware.
>>
>>
>
> On my Macchiato I use GT630 card (nuveau driver) + debian + xfce
> desktop and in dual monitor mode, I could run a couple of 1080p
> streams. All smooth and I've never noticed any image corruption
> whatsoever (I spent a lot of time in front of such setup). Just to be
> on a safe side, can you send me a bootlog and your board revision? I'd
> like to see your firware version and type.
>

Hi Marcin,

Could you please try running his reproducer?