drm/i915: Fixup the free space logic in ring_prepare
authorAkash Goel <akash.goel@intel.com>
Fri, 11 Mar 2016 09:26:42 +0000 (14:56 +0530)
committerChris Wilson <chris@chris-wilson.co.uk>
Sat, 9 Apr 2016 10:37:48 +0000 (11:37 +0100)
commit782f6bc0aba037436d6a04d19b23f8b61020a576
tree94696fc6f0a1ad095f26e5ccb837c4a2001ca4d8
parentcffa781e59070cf115525a57e8f6042bd4e2bf7c
drm/i915: Fixup the free space logic in ring_prepare

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@chris-wilson.co.uk>
Signed-off-by: Akash Goel <akash.goel@intel.com>
Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Link: http://patchwork.freedesktop.org/patch/msgid/1457688402-10411-1-git-send-email-akash.goel@intel.com
Reviewed-by: Chris Wilson <chris@chris-wilson.co.uk>
Cc: stable@vger.kernel.org
drivers/gpu/drm/i915/intel_lrc.c
drivers/gpu/drm/i915/intel_ringbuffer.c