[Intel-gfx] [PATCH] drm/i915/perf: Do not clear pollin for small user read buffers

[Chris Wilson]

Quoting Dixit, Ashutosh (2020-04-03 18:45:09)
> On Fri, 03 Apr 2020 09:17:14 -0700, Chris Wilson wrote:
> >
> > Quoting Ashutosh Dixit (2020-04-03 02:01:20)
> > > It is wrong to block the user thread in the next poll when OA data is
> > > already available which could not fit in the user buffer provided in
> > > the previous read. In several cases the exact user buffer size is not
> > > known. Blocking user space in poll can lead to data loss when the
> > > buffer size used is smaller than the available data.
> > >
> > > This change fixes this issue and allows user space to read all OA data
> > > even when using a buffer size smaller than the available data using
> > > multiple non-blocking reads rather than staying blocked in poll till
> > > the next timer interrupt.
> > >
> > > v2: Fix ret value for blocking reads (Umesh)
> > > v3: Mistake during patch send (Ashutosh)
> > > v4: Remove -EAGAIN from comment (Umesh)
> > > v5: Improve condition for clearing pollin and return (Lionel)
> > > v6: Improve blocking read loop and other cleanups (Lionel)
> > > v7: Added Cc stable
> > >
> > > Cc: Umesh Nerlige Ramappa <umesh.nerlige.ramappa at intel.com>
> > > Cc: <stable at vger.kernel.org>
> > > Reviewed-by: Lionel Landwerlin <lionel.g.landwerlin at intel.com>
> > > Signed-off-by: Ashutosh Dixit <ashutosh.dixit at intel.com>
> >
> > Did you manage to devise a test case? It is nice (some might say
> > important) to pair a patch for stable with its regression test.
> 
> Yes there is a test case here:
> 
> https://patchwork.freedesktop.org/series/75100/#rev3
> 
> Lionel verified that it is fails on stable kernels here:
> 
> https://patchwork.freedesktop.org/patch/358873/?series=75100&rev=1

Ta. Pushed both,
-Chris