Re: framebuffer corruption due to overlapping stp instructions on arm64

Marcin Wojtas
2018-08-06T13:41:50Z

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.

Thanks,
Marcin