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.
Reviewed-by: Mika Kuoppala <mika.kuoppala@xxxxxxxxx>
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