On 6 August 2018 at 15:41, Marcin Wojtas <mw@xxxxxxxxxxxx> wrote: > Hi Mikulas, > > pon., 6 sie 2018 o 14:42 Robin Murphy <robin.murphy@xxxxxxx> 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?
