[Intel-gfx] [PATCH 29/38] drm/i915: Remove locking for get_tiling

[Daniel Vetter]

On Wed, Jun 08, 2016 at 11:11:07AM +0100, Chris Wilson wrote:
> On Wed, Jun 08, 2016 at 12:02:01PM +0200, Daniel Vetter wrote:
> > On Fri, Jun 03, 2016 at 05:55:44PM +0100, Chris Wilson wrote:
> > > Since we are not concerned with userspace racing itself with set-tiling
> > > (the order is indeterminant even if we take a lock), then we can safely
> > > read back the single obj->tiling_mode and do the static lookup of
> > > swizzle mode without having to take a lock.
> > > 
> > > get-tiling is reasonably frequent due to the back-channel passing around
> > > of tiling parameters in DRI2/DRI3.
> > > 
> > > Signed-off-by: Chris Wilson <chris at chris-wilson.co.uk>
> > > ---
> > >  drivers/gpu/drm/i915/i915_gem_tiling.c | 8 ++------
> > >  1 file changed, 2 insertions(+), 6 deletions(-)
> > > 
> > > diff --git a/drivers/gpu/drm/i915/i915_gem_tiling.c b/drivers/gpu/drm/i915/i915_gem_tiling.c
> > > index 326de7eae101..d6acd0a27c06 100644
> > > --- a/drivers/gpu/drm/i915/i915_gem_tiling.c
> > > +++ b/drivers/gpu/drm/i915/i915_gem_tiling.c
> > > @@ -302,10 +302,8 @@ i915_gem_get_tiling(struct drm_device *dev, void *data,
> > >  	if (!obj)
> > >  		return -ENOENT;
> > >  
> > > -	mutex_lock(&dev->struct_mutex);
> > > -
> > >  	args->tiling_mode = obj->tiling_mode;
> > 
> > READ_ONCE here. With that Reviewed-by: Daniel Vetter <daniel.vetter at ffwll.ch>
> 
> obj->tiling_mode is still a bitfield. Not yet convinced of extracting
> it, but avoiding the lock for get_tiling is useful.

With all the recent discussions the past years about gcc becoming more and
more creative with exploiting the undefined parts of C99 I'm just a bit
paranoid. Would be awesome if we could unbitfield this one beforehand ...
-Daniel
-- 
Daniel Vetter
Software Engineer, Intel Corporation
http://blog.ffwll.ch