From 99264a61dfcda41d86d0960cf2d4c0fc2758a773 Mon Sep 17 00:00:00 2001 From: Daniel Vetter Date: Wed, 15 Apr 2015 19:34:43 +0200 Subject: [PATCH] drm/vblank: Fixup and document timestamp update/read barriers MIME-Version: 1.0 Content-Type: text/plain; charset=utf8 Content-Transfer-Encoding: 8bit This was a bit too much cargo-culted, so lets make it solid: - vblank->count doesn't need to be an atomic, writes are always done under the protection of dev->vblank_time_lock. Switch to an unsigned long instead and update comments. Note that atomic_read is just a normal read of a volatile variable, so no need to audit all the read-side access specifically. - The barriers for the vblank counter seqlock weren't complete: The read-side was missing the first barrier between the counter read and the timestamp read, it only had a barrier between the ts and the counter read. We need both. - Barriers weren't properly documented. Since barriers only work if you have them on boths sides of the transaction it's prudent to reference where the other side is. To avoid duplicating the write-side comment 3 times extract a little store_vblank() helper. In that helper also assert that we do indeed hold dev->vblank_time_lock, since in some cases the lock is acquired a few functions up in the callchain. Spotted while reviewing a patch from Chris Wilson to add a fastpath to the vblank_wait ioctl. v2: Add comment to better explain how store_vblank works, suggested by Chris. v3: Peter noticed that as-is the 2nd smp_wmb is redundant with the implicit barrier in the spin_unlock. But that can only be proven by auditing all callers and my point in extracting this little helper was to localize all the locking into just one place. Hence I think that additional optimization is too risky. Cc: Chris Wilson Cc: Mario Kleiner Cc: Ville Syrjälä Cc: Michel Dänzer Cc: Peter Hurley Reviewed-by: Chris Wilson Reviewed-and-tested-by: Mario Kleiner Signed-off-by: Daniel Vetter --- Reading git-format-patch failed