On Fri, Nov 09, 2018 at 01:01:55PM -0500, Rob Clark wrote: > On Mon, Oct 29, 2018 at 3:09 PM Will Deacon <will.deacon@xxxxxxx> wrote: > > On Thu, Sep 27, 2018 at 06:46:07PM -0400, Rob Clark wrote: > > > We seem to need to set either this or CFCFG (stall), otherwise gpu > > > faults trigger problems with other in-flight transactions from the > > > GPU causing CP errors, etc. > > > > > > In the ARM SMMU spec, the 'Hit under previous context fault' bit is > > > described as: > > > > > > '0' - Stall or terminate subsequent transactions in the presence > > > of an outstanding context fault > > > '1' - Process all subsequent transactions independently of any > > > outstanding context fault. > > > > > > Since we don't enable CFCFG (stall) the behavior of terminating > > > other transactions makes sense. And is probably not what we want > > > (and definately not what we want for GPU). > > > > > > Signed-off-by: Rob Clark <robdclark@xxxxxxxxx> > > > --- > > > So I hit this issue a long time back on 820 (msm8996) and at the > > > time I solved it with a patch that enabled CFCFG. And it resurfaced > > > more recently on sdm845. But at the time CFCFG was rejected, iirc > > > because of concern that it would cause problems on other non-qcom > > > arm smmu implementations. And I think I forgot to send this version > > > of the solution. > > > > > > If enabling HUPCF is anticipated to cause problems on other ARM > > > SMMU implementations, I think I can come up with a variant of this > > > patch which conditionally enables it for snapdragon. > > > > > > Either way, I'd really like to get some variant of this fix merged > > > (and probably it would be a good idea for stable kernel branches > > > too), since current behaviour with the GPU means faults turn into > > > a fantastic cascade of fail. > > > > Can you describe how this fantastic cascade of fail improves with this > > patch, please? If you're getting context faults then something has already > > gone horribly wrong, so I'm trying to work out how this improves things. > > > > There are plenty of cases where getting iommu faults with a GPU is > "normal", or at least not something the kernel or even GL driver can > control. Such as? All the mainline driver does is print a diagnostic and clear the fault, which doesn't seem generally useful. > With this patch, you still get the iommu fault, but it doesn't cause > the gpu to crash. But without it, other memory accesses in flight > while the fault occurs, like the GPU command-processor reading further > ahead in the cmdstream to setup next draw, would return zero's, > causing the GPU to crash or get into a bad state. I get that part, but I don't understand why we're seeing faults in the first place and I worry that this patch is just the tip of the iceberg. It's also not clear that processing subsequent transactions is always the right thing to do in a world where we actually want to report (and handle) synchronous faults from devices. Will
