On Thu, May 04, 2017 at 04:17:13PM +0200, Michal Wajdeczko wrote: > On Thu, May 04, 2017 at 02:08:44PM +0100, Chris Wilson wrote: > > Exploit the power-of-two ring size to compute the space across the > > wraparound using a mask rather than a if. Convert to unsigned integers > > so the operation is well defined. > > > > References: https://bugs.freedesktop.org/show_bug.cgi?id=99671 > > Signed-off-by: Chris Wilson <chris@xxxxxxxxxxxxxxxxxx> > > Cc: Mika Kuoppala <mika.kuoppala@xxxxxxxxx> > > Reviewed-by: Mika Kuoppala <mika.kuoppala@xxxxxxxxx> > > --- > > drivers/gpu/drm/i915/intel_ringbuffer.c | 23 +++++++++++---------- > > drivers/gpu/drm/i915/intel_ringbuffer.h | 36 ++++++++++++++++++++------------- > > 2 files changed, 34 insertions(+), 25 deletions(-) > > > > diff --git a/drivers/gpu/drm/i915/intel_ringbuffer.c b/drivers/gpu/drm/i915/intel_ringbuffer.c > > index 3ce1c87dec46..e7ef04cc071b 100644 > > --- a/drivers/gpu/drm/i915/intel_ringbuffer.c > > +++ b/drivers/gpu/drm/i915/intel_ringbuffer.c > > @@ -39,12 +39,16 @@ > > */ > > #define LEGACY_REQUEST_SIZE 200 > > > > -static int __intel_ring_space(int head, int tail, int size) > > +static unsigned int __intel_ring_space(unsigned int head, > > + unsigned int tail, > > + unsigned int size) > > { > > - int space = head - tail; > > - if (space <= 0) > > - space += size; > > - return space - I915_RING_FREE_SPACE; > > + /* > > + * "If the Ring Buffer Head Pointer and the Tail Pointer are on the > > + * same cacheline, the Head Pointer must not be greater than the Tail > > + * Pointer." > > + */ > > + return (head - tail - CACHELINE_BYTES) & (size - 1); > > Btw, as you exploit power-of-two ring size here, maybe it is worth to repeat > > GEM_BUG_ON(!is_power_of_2(size)); > > to emphase this assumption in the code (not only in the commit message)? I did check we had an is_power_of_2() check in intel_engine_create_ring. Might be worth asserting here as well as there's a little disconnect between the function and ring->size. -Chris -- Chris Wilson, Intel Open Source Technology Centre _______________________________________________ Intel-gfx mailing list Intel-gfx@xxxxxxxxxxxxxxxxxxxxx https://lists.freedesktop.org/mailman/listinfo/intel-gfx
