[Intel-gfx] [PATCH] drm/i915: Fix RPS pointer passed from wait_ioctl to i915_wait_request

[Chris Wilson]

On Wed, Dec 02, 2015 at 09:13:46AM +0000, Chris Wilson wrote:
> In commit 2e1b873072dfe3bbcc158a9c21acde1ab0d36c55 [v4.2]
> Author: Chris Wilson <chris at chris-wilson.co.uk>
> Date:   Mon Apr 27 13:41:22 2015 +0100
> 
>     drm/i915: Convert RPS tracking to a intel_rps_client struct
> 
> we converted the __i915_wait_request() to take a new intel_rps_client
> struct (rather than having to pass fake drm_i915_file_private structs).
> However, due to use of passing a void pointer, I didn't spot one
> callsite in wait-ioctl was passing the wrong pointer.
> 
> Signed-off-by: Chris Wilson <chris at chris-wilson.co.uk>
> Cc: Daniel Vetter <daniel.vetter at ffwll.ch>
> Cc: stable at vger.kernel.org

Fwiw, the impact of this bug is zero. Along the rps path, we always
first call list_empty(rps) which when we pass in the wrong pointer
always evaluates to false and we return early and never chase the
invalid pointers.

The user visible impact is then wait-ioctl doesn't get the same
waitboosting as the other interfaces (set-domain, throttle), which is a
performance concern for the *very* few users of the wait interface.
There is also a libdrm_intel patch to use the wait-ioctl for
drm_intel_bo_wait_rendering() if anyone feels inclined to review
libdrm_intel patches.
-Chris

-- 
Chris Wilson, Intel Open Source Technology Centre