[Mesa-stable] [Mesa-dev] [PATCH 3/4] i965/bufmgr: Add a set_tiling helper

[Kenneth Graunke]

On Friday, January 12, 2018 1:28:00 AM PST Chris Wilson wrote:
> Quoting Jason Ekstrand (2018-01-12 01:40:52)
> > This helper should be used carefully as setting tiling is a racy
> > operation since it potentially interacts with other processes.  Still,
> > it is a useful thing to be able to do.
> > 
> > Cc: mesa-stable at lists.freedesktop.org
> > ---
> >  src/mesa/drivers/dri/i965/brw_bufmgr.c | 27 +++++++++++++++++++++++++++
> >  src/mesa/drivers/dri/i965/brw_bufmgr.h | 10 ++++++++++
> >  2 files changed, 37 insertions(+)
> > 
> > diff --git a/src/mesa/drivers/dri/i965/brw_bufmgr.c b/src/mesa/drivers/dri/i965/brw_bufmgr.c
> > index 469895e..dbd13dd 100644
> > --- a/src/mesa/drivers/dri/i965/brw_bufmgr.c
> > +++ b/src/mesa/drivers/dri/i965/brw_bufmgr.c
> > @@ -1133,6 +1133,33 @@ brw_bo_get_tiling(struct brw_bo *bo, uint32_t *tiling_mode,
> >     return 0;
> >  }
> >  
> > +int
> > +brw_bo_set_tiling(struct brw_bo *bo, uint32_t tiling_mode,
> > +                  uint32_t stride)
> > +{
> > +   struct brw_bufmgr *bufmgr = bo->bufmgr;
> > +
> > +   mtx_lock(&bufmgr->lock);
> 
> This mutex protects the buffer cache, which should not be containing
> this bo. Changing the tiling of a shared active bo also should not
> happen because the other parties will have already derived state from
> the older tiling. So I don't see the purpose of this mutex here.
> 
> If we will need exclusive access to a bo, let's have a bo->lock.
> -Chris

I agree with Chris, I don't like this locking.  Looking more closely,
I don't think patches 3-4 are what you want at all.

intel_create_image_from_fds_common calls brw_bo_gem_create_from_prime.
It's the only caller of that function.

brw_bo_gem_create_from_prime allocates a new BO, does GET_TILING, and
inserts it into the cache.  This is pointless after your patch 4,
because you immediately whack it.

So...instead...just make brw_bo_gem_create_from_prime take a tiling,
and SET_TILING it...don't bother with GET_TILING at all.  Then, your
BO isn't in the cache yet (or you have the cache locked), and it's
safe without a race and redundant work.
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