[Intel-gfx] [PATCH] drm/i915: Retry gtt fault when out of fence register

[Andi Shyti]

Hi Ville,

> If we can't find a free fence register to handle a fault in the GMADR
> range just return VM_FAULT_NOPAGE without populating the PTE so that
> userspace will retry the access and trigger another fault. Eventually
> we should find a free fence and the fault will get properly handled.
> 
> A further improvement idea might be to reserve a fence (or one per CPU?)
> for the express purpose of handling faults without having to retry. But
> that would require some additional work.
> 
> Looks like this may have gotten broken originally by
> commit 39965b376601 ("drm/i915: don't trash the gtt when running out of fences")
> as that changed the errno to -EDEADLK which wasn't handle by the gtt
> fault code either. But later in commit 2feeb52859fc ("drm/i915/gt: Fix
> -EDEADLK handling regression") I changed it again to -ENOBUFS as -EDEADLK
> was now getting used for the ww mutex dance. So this fix only makes
> sense after that last commit.
> 
> Cc: stable at vger.kernel.org
> Closes: https://gitlab.freedesktop.org/drm/intel/-/issues/9479
> Fixes: 2feeb52859fc ("drm/i915/gt: Fix -EDEADLK handling regression")
> Signed-off-by: Ville Syrjälä <ville.syrjala at linux.intel.com>

Reviewed-by: Andi Shyti <andi.shyti at linux.intel.com> 

Andi