On Thu, May 04, 2017 at 03:59:05PM +0300, Mika Kuoppala wrote:
> Chris Wilson <chris@xxxxxxxxxxxxxxxxxx> writes:
>
> > On Thu, May 04, 2017 at 03:11:45PM +0300, Mika Kuoppala wrote:
> >> Chris Wilson <chris@xxxxxxxxxxxxxxxxxx> writes:
> >>
> >> > Typically, there is space available within the ring and if not we have
> >> > to wait (by definition a slow path). Rearrange the code to reduce the
> >> > number of branches and stack size for the hotpath, accomodating a slight
> >> > growth for the wait.
> >> >
> >> > v2: Fix the new assert that packets are not larger than the actual ring.
> >> >
> >> > Signed-off-by: Chris Wilson <chris@xxxxxxxxxxxxxxxxxx>
> >> > ---
> >> > drivers/gpu/drm/i915/intel_ringbuffer.c | 63 +++++++++++++++++----------------
> >> > 1 file changed, 33 insertions(+), 30 deletions(-)
> >> >
> >> > diff --git a/drivers/gpu/drm/i915/intel_ringbuffer.c b/drivers/gpu/drm/i915/intel_ringbuffer.c
> >> > index c46e5439d379..53123c1cfcc5 100644
> >> > --- a/drivers/gpu/drm/i915/intel_ringbuffer.c
> >> > +++ b/drivers/gpu/drm/i915/intel_ringbuffer.c
> >> > @@ -1654,7 +1654,7 @@ static int ring_request_alloc(struct drm_i915_gem_request *request)
> >> > return 0;
> >> > }
> >> >
> >> > -static int wait_for_space(struct drm_i915_gem_request *req, int bytes)
> >> > +static noinline int wait_for_space(struct drm_i915_gem_request *req, int bytes)
> >> > {
> >> > struct intel_ring *ring = req->ring;
> >> > struct drm_i915_gem_request *target;
> >> > @@ -1702,49 +1702,52 @@ static int wait_for_space(struct drm_i915_gem_request *req, int bytes)
> >> > u32 *intel_ring_begin(struct drm_i915_gem_request *req, int num_dwords)
> >> > {
> >> > struct intel_ring *ring = req->ring;
> >> > - int remain_actual = ring->size - ring->emit;
> >> > - int remain_usable = ring->effective_size - ring->emit;
> >> > - int bytes = num_dwords * sizeof(u32);
> >> > - int total_bytes, wait_bytes;
> >> > - bool need_wrap = false;
> >> > + const unsigned int remain_usable = ring->effective_size - ring->emit;
> >> > + const unsigned int bytes = num_dwords * sizeof(u32);
> >> > + unsigned int need_wrap = 0;
> >> > + unsigned int total_bytes;
> >> > u32 *cs;
> >> >
> >> > total_bytes = bytes + req->reserved_space;
> >> > + GEM_BUG_ON(total_bytes > ring->effective_size);
> >> >
> >> > - if (unlikely(bytes > remain_usable)) {
> >> > - /*
> >> > - * Not enough space for the basic request. So need to flush
> >> > - * out the remainder and then wait for base + reserved.
> >> > - */
> >> > - wait_bytes = remain_actual + total_bytes;
> >> > - need_wrap = true;
> >> > - } else if (unlikely(total_bytes > remain_usable)) {
> >> > - /*
> >> > - * The base request will fit but the reserved space
> >> > - * falls off the end. So we don't need an immediate wrap
> >> > - * and only need to effectively wait for the reserved
> >> > - * size space from the start of ringbuffer.
> >> > - */
> >> > - wait_bytes = remain_actual + req->reserved_space;
> >> > - } else {
> >> > - /* No wrapping required, just waiting. */
> >> > - wait_bytes = total_bytes;
> >> > + if (unlikely(total_bytes > remain_usable)) {
> >> > + const int remain_actual = ring->size - ring->emit;
> >> > +
> >> > + if (bytes > remain_usable) {
> >> > + /*
> >> > + * Not enough space for the basic request. So need to
> >> > + * flush out the remainder and then wait for
> >> > + * base + reserved.
> >> > + */
> >> > + total_bytes += remain_actual;
> >> > + need_wrap = remain_actual | 1;
> >>
> >> Your remain_actual should never reach zero. So in here
> >> forcing the lowest bit on, and later off, seems superfluous.
> >
> > Why can't we fill up to the last byte with commands? remain_actual is
> > just (size - tail) and we don't force a wrap until emit crosses the
> > boundary (and not before). We hit remain_actual == 0 in practice.
> > -Chris
>
> My mistake, was thinking postwrap.
>
> num_dwords and second parameter to wait_for_space should be unsigned.
You predictive algorithm is working fine though. Applied after your
suggestion from patch 1.
Thanks,
-Chris
--
Chris Wilson, Intel Open Source Technology Centre
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