Re: [PATCH] drm/amdgpu/dc: Stop dma_resv_lock inversion in commit_tail

[Alex Deucher <alexdeucher@gmail.com>]

Applied.  Thanks!

Alex

On Tue, Jul 28, 2020 at 2:56 AM Christian König
<christian.koenig@amd.com> wrote:
>
> Am 27.07.20 um 23:30 schrieb Daniel Vetter:
> > Trying to grab dma_resv_lock while in commit_tail before we've done
> > all the code that leads to the eventual signalling of the vblank event
> > (which can be a dma_fence) is deadlock-y. Don't do that.
> >
> > Here the solution is easy because just grabbing locks to read
> > something races anyway. We don't need to bother, READ_ONCE is
> > equivalent. And avoids the locking issue.
> >
> > v2: Also take into account tmz_surface boolean, plus just delete the
> > old code.
> >
> > Cc: linux-media@vger.kernel.org
> > Cc: linaro-mm-sig@lists.linaro.org
> > Cc: linux-rdma@vger.kernel.org
> > Cc: amd-gfx@lists.freedesktop.org
> > Cc: intel-gfx@lists.freedesktop.org
> > Cc: Chris Wilson <chris@chris-wilson.co.uk>
> > Cc: Maarten Lankhorst <maarten.lankhorst@linux.intel.com>
> > Cc: Christian König <christian.koenig@amd.com>
> > Signed-off-by: Daniel Vetter <daniel.vetter@intel.com>
> > ---
> > DC-folks, I think this split out patch from my series here
> >
> > https://nam11.safelinks.protection.outlook.com/?url=https%3A%2F%2Flore.kernel.org%2Fdri-devel%2F20200707201229.472834-1-daniel.vetter%40ffwll.ch%2F&amp;data=02%7C01%7Cchristian.koenig%40amd.com%7C8a4f5736682a4b5c943e08d832747ab1%7C3dd8961fe4884e608e11a82d994e183d%7C0%7C0%7C637314823145521840&amp;sdata=qd7Nrox62Lr%2FXWbJJFVskg9RYL4%2FoRVCFjR6rUDMA5E%3D&amp;reserved=0
> >
> > should be ready for review/merging. I fixed it up a bit so that it's not
> > just a gross hack :-)
> >
> > Cheers, Daniel
> >
> >
> > ---
> >   .../gpu/drm/amd/display/amdgpu_dm/amdgpu_dm.c | 19 ++++++-------------
> >   1 file changed, 6 insertions(+), 13 deletions(-)
> >
> > diff --git a/drivers/gpu/drm/amd/display/amdgpu_dm/amdgpu_dm.c b/drivers/gpu/drm/amd/display/amdgpu_dm/amdgpu_dm.c
> > index 21ec64fe5527..a20b62b1f2ef 100644
> > --- a/drivers/gpu/drm/amd/display/amdgpu_dm/amdgpu_dm.c
> > +++ b/drivers/gpu/drm/amd/display/amdgpu_dm/amdgpu_dm.c
> > @@ -6959,20 +6959,13 @@ static void amdgpu_dm_commit_planes(struct drm_atomic_state *state,
> >                       DRM_ERROR("Waiting for fences timed out!");
> >
> >               /*
> > -              * TODO This might fail and hence better not used, wait
> > -              * explicitly on fences instead
> > -              * and in general should be called for
> > -              * blocking commit to as per framework helpers
> > +              * We cannot reserve buffers here, which means the normal flag
> > +              * access functions don't work. Paper over this with READ_ONCE,
> > +              * but maybe the flags are invariant enough that not even that
> > +              * would be needed.
> >                */
> > -             r = amdgpu_bo_reserve(abo, true);
> > -             if (unlikely(r != 0))
> > -                     DRM_ERROR("failed to reserve buffer before flip\n");
> > -
> > -             amdgpu_bo_get_tiling_flags(abo, &tiling_flags);
> > -
> > -             tmz_surface = amdgpu_bo_encrypted(abo);
> > -
> > -             amdgpu_bo_unreserve(abo);
> > +             tiling_flags = READ_ONCE(abo->tiling_flags);
> > +             tmz_surface = READ_ONCE(abo->flags) & AMDGPU_GEM_CREATE_ENCRYPTED;
>
> Yeah, the abo->flags are mostly fixed after creation, especially the
> encrypted flag can't change or we corrupt page table tables. So that
> should work fine.
>
> Anybody who picks this up feel free to add an Reviewed-by: Christian
> König <christian.koenig@amd.com>.
>
> Regards,
> Christian.
>
> >
> >               fill_dc_plane_info_and_addr(
> >                       dm->adev, new_plane_state, tiling_flags,
>
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