On Mon, Nov 26, 2018 at 07:31:48PM +0000, Will Deacon wrote: > Hi Rob, > > On Tue, Nov 13, 2018 at 08:12:35AM -0500, Rob Clark wrote: > > On Tue, Nov 13, 2018 at 1:32 AM Will Deacon <will.deacon@xxxxxxx> wrote: > > > 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. > > > > it is useful to debug the fault ;-) > > > > Although eventually we'll want to be able to do more than that, like > > have the fault trigger bringing in pages of a mmap'd file and that > > sort of thing. > > Right, and feels very strange to me if we have this bit set because it > means that your fault is no longer synchronous and therefore diverges > from the fault model that you get from the CPU, where you certainly > wouldn't expect stores appearing in the program after a faulting load to > be visible in memory. However, thinking harder about it, I suppose we're > already in a situation where the translations are handled out of order > in the absence of barriers, so maybe it's not the end of the world. > > Could you dump the FSR value that you see in the fault handler, please? > From my reading of the architecture spec, as long as clear all of the > fault bits in the irq handler, then your machine shouldn't die like it > does with HUPCFG=CFCFG=0.. > > > > > 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. > > > > Sure, it is a bug.. but it can be an application bug that is not > > something the userspace GL driver or kernel could do anything about. > > We shouldn't let this kill the GPU. If the application didn't have > > this much control, we wouldn't need an IOMMU in the first place[1]. > > With opencl compute, the userspace controlled shader has full blown > > pointers to GPU memory. > > > > And even in cases where it is a userspace GL driver bug, having some > > robustness to not completely kill the GPU makes debugging much easier. > > Something I do a lot when bringing up support for a new generation of > > GPU. > > > > I'm having a hard time understanding your objection to this. > > Returning zero's for non-faulting transactions is a *really bad idea*. > > Wait -- who said anything about returning zeroes? Where does that behaviour > appear in the architecture? I _think_ it is the bus implementation that returns zero on a terminated transaction but the effect on the GPU is the same. Jordan -- The Qualcomm Innovation Center, Inc. is a member of Code Aurora Forum, a Linux Foundation Collaborative Project
