[Intel-gfx] [PATCH 4/4] drm/i915: optional fewer warning patch

[Chris Wilson]

On Sat,  9 Apr 2011 13:31:22 -0700, Ben Widawsky <ben at bwidawsk.net> wrote:
> +	 *
> +	 * Intelligent users of the interface may do a force_wake_get() followed
> +	 * by many register reads and writes, knowing that the reference count
> +	 * is already incremented. So we do not want to warn on those.
>  	 */

Hmm, any place where we touch registers without holding a lock, unless
under exceptional conditions such as !SYSTEM_RUNNING, is a bug waiting to
happen. And probably already happened. Nasty, hard to reproduce race
conditions.

The complication is that some registers will be protected by
dev->config.mode_lock rather than dev->struct_mutex (and a very rare set
are protected by their own irq spinlocks). This sounds like a job for
lockdep... And a lot of documentation. Fortunately, we only have those two
conditions (KMS (detection and mode setting) vs GEM (execution and memory
managment)) and their intersection to worry about.

One of the first steps would be to review all the comments Jesse added to
document the KMS locking and back those up with assertions and updates.
Plenty of work to do before we can feel sure that the locking is sane.
-Chris

-- 
Chris Wilson, Intel Open Source Technology Centre