[Intel-gfx] [PATCH] drm/i915: Fixup the free space logic in ring_prepare

[Chris Wilson]

On Fri, Mar 11, 2016 at 02:56:42PM +0530, akash.goel at intel.com wrote:
> From: Akash Goel <akash.goel at intel.com>
> 
> Currently for the case where there is enough space at the end of Ring
> buffer for accommodating only the base request, the wrapround is done
> immediately and as a result the base request gets added at the start
> of Ring buffer. But there may not be enough free space at the beginning
> to accommodate the base request, as before the wraparound, the wait was
> effectively done for the reserved_size free space from the start of
> Ring buffer. In such a case there is a potential of Ring buffer overflow,
> the instructions at the head of Ring (ACTHD) can get overwritten.
> 
> Since the base request can fit in the remaining space, there is no need
> to wraparound immediately. The wraparound will anyway happen later when
> the reserved part starts getting used.
> 
> Cc: Chris Wilson <chris at chris-wilson.co.uk>
> Signed-off-by: Akash Goel <akash.goel at intel.com>

>From an earlier review:

> The benefit (other than that bug fix which could affect ilk in extreme
> circumstances, though I guess the timing is a little too short for
> igt/gem_ringfill to hit it - we can fill the ring with ease, but we would
> have to overwrite after wrap whilst the ACTHD was still in the zone in
> order for it to actualy fallover. Hmm, if we added a BUG for tail overflow
> that might have a chance of detecting the issue) should be that we
> squeeze in perhaps even another whole request before the ring is
> exhausted and so reserved-space is less wasteful on gen that do not need
> the full reservation.
>
> Reviewed-by: Chris Wilson <chris at chris-wilson.co.uk>
>
> In theory this could be impacting upon Ironlakes in the real-world, so a
> candidate for stable@
-Chris

-- 
Chris Wilson, Intel Open Source Technology Centre