[Intel-gfx] [PATCH 1/2] drm/i915: Add MOCS state dump to debugfs

[Chris Wilson]

Quoting Stuart Summers (2019-08-07 22:48:55)
> On Wed, 2019-08-07 at 22:29 +0100, Chris Wilson wrote:
> > Quoting Stuart Summers (2019-08-07 21:55:55)
> > > User applications might need to verify hardware configuration
> > > of the MOCS entries. To facilitate this debug, add a new debugfs
> > > entry to allow a dump of the MOCS state to verify expected values
> > > are set by i915.
> > 
> > User applications + debugfs? It's not an avenue for ABI.
> > 
> > If you really want to provide the settings back to userspace, look at
> > something like an i915_query or sysfs.
> > 
> > Or if you just mean igt, then add a Testcase:
> > 
> > If you just need to validate that we are setting and restoring them,
> > selftests.
> > 
> > If you need them for debugging errors, add them to the error state.
> 
> This was probably poorly worded, you're right. I'll update the commit
> message to be more specific.
> 
> I do want this for debugging, but not sure error state is the right
> place. This is for debugging performance issues, so no specific
> failures. If you feel sysfs or i915_query are more correct here, I can
> look at adding this there instead. Is there a reason we don't want this
> in debugfs specifically?

No, it was just the wording implied to me you had a use case for
clients, not just debugging the kernel.

Adding it to the error state (see i915_gpu_info) is not too bad an idea
if you need a sledgehammer to inspect the GPU state while a batch is
executing, but really it just sounds like you want to automate checking
the mocs registers against "ideal" state. They should be static, so once
they are set, so long as we are confident and check that they do not
change nor can be scribbled over by userspace, you only need to scan the
source :)

I will add that I wish we took a more complete snapshot of interesting
registers for the error state.
-Chris