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 [off-list ref] wrote:

quoted

On Fri, Nov 09, 2018 at 01:01:55PM -0500, Rob Clark wrote:

quoted

On Mon, Oct 29, 2018 at 3:09 PM Will Deacon [off-list ref] wrote:

quoted

On Thu, Sep 27, 2018 at 06:46:07PM -0400, Rob Clark wrote:

quoted

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 <redacted>
---
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..

quoted

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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?

Will