Re: [PATCH 18/18] drm/i915: Simplify calling engine->sync_to

Joonas Lahtinen
2016-07-22T08:59:30Z

On ke, 2016-07-20 at 14:12 +0100, Chris Wilson wrote:
> Since requests can no longer be generated as a side-effect of
> intel_ring_begin(), we know that the seqno will be unchanged during
> ring-emission. This predicatablity then means we do not have to check
> for the seqno wrapping around whilst emitting the semaphore for
> engine->sync_to().
>
> Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
> ---
>  drivers/gpu/drm/i915/i915_drv.h         |  2 +-
>  drivers/gpu/drm/i915/i915_gem.c         | 13 ++-----
>  drivers/gpu/drm/i915/i915_gem_request.c |  9 +----
>  drivers/gpu/drm/i915/intel_ringbuffer.c | 64 ++++++++++++---------------------
>  drivers/gpu/drm/i915/intel_ringbuffer.h |  5 ++-
>  5 files changed, 30 insertions(+), 63 deletions(-)
>
> diff --git a/drivers/gpu/drm/i915/i915_drv.h b/drivers/gpu/drm/i915/i915_drv.h
> index f188c9a9b746..c374b8687d87 100644
> --- a/drivers/gpu/drm/i915/i915_drv.h
> +++ b/drivers/gpu/drm/i915/i915_drv.h
> @@ -1757,7 +1757,7 @@ struct drm_i915_private {
>   struct i915_gem_context *kernel_context;
>   struct intel_engine_cs engine[I915_NUM_ENGINES];
>   struct drm_i915_gem_object *semaphore_obj;
> - uint32_t last_seqno, next_seqno;
> + u32 next_seqno;
>
>   struct drm_dma_handle *status_page_dmah;
>   struct resource mch_res;
> diff --git a/drivers/gpu/drm/i915/i915_gem.c b/drivers/gpu/drm/i915/i915_gem.c
> index 9fdecef34fa8..0b7a0e6f9dd1 100644
> --- a/drivers/gpu/drm/i915/i915_gem.c
> +++ b/drivers/gpu/drm/i915/i915_gem.c
> @@ -2864,22 +2864,15 @@ __i915_gem_object_sync(struct drm_i915_gem_object *obj,
>   i915_gem_object_retire_request(obj, from);
>   } else {
>   int idx = intel_engine_sync_index(from->engine, to->engine);
> - u32 seqno = i915_gem_request_get_seqno(from);
> -
> - if (seqno <= from->engine->semaphore.sync_seqno[idx])
> + if (from->fence.seqno <= from->engine->semaphore.sync_seqno[idx])
>   return 0;
>
>   trace_i915_gem_ring_sync_to(to, from);
> - ret = to->engine->semaphore.sync_to(to, from->engine, seqno);
> + ret = to->engine->semaphore.sync_to(to, from);
>   if (ret)
>   return ret;
>
> - /* We use last_read_req because sync_to()
> -  * might have just caused seqno wrap under
> -  * the radar.
> -  */
> - from->engine->semaphore.sync_seqno[idx] =
> - i915_gem_request_get_seqno(obj->last_read_req[from->engine->id]);
> + from->engine->semaphore.sync_seqno[idx] = from->fence.seqno;
>   }
>
>   return 0;
> diff --git a/drivers/gpu/drm/i915/i915_gem_request.c b/drivers/gpu/drm/i915/i915_gem_request.c
> index 3e633b47213c..dfdb86c8a433 100644
> --- a/drivers/gpu/drm/i915/i915_gem_request.c
> +++ b/drivers/gpu/drm/i915/i915_gem_request.c
> @@ -264,14 +264,7 @@ int i915_gem_set_seqno(struct drm_device *dev, u32 seqno)
>   if (ret)
>   return ret;
>
> - /* Carefully set the last_seqno value so that wrap
> -  * detection still works
> -  */
>   dev_priv->next_seqno = seqno;
> - dev_priv->last_seqno = seqno - 1;
> - if (dev_priv->last_seqno == 0)
> - dev_priv->last_seqno--;
> -
>   return 0;
>  }
>
> @@ -288,7 +281,7 @@ static int i915_gem_get_seqno(struct drm_i915_private *dev_priv, u32 *seqno)
>   dev_priv->next_seqno = 1;
>   }
>
> - *seqno = dev_priv->last_seqno = dev_priv->next_seqno++;
> + *seqno = dev_priv->next_seqno++;
>   return 0;
>  }
>
> diff --git a/drivers/gpu/drm/i915/intel_ringbuffer.c b/drivers/gpu/drm/i915/intel_ringbuffer.c
> index 8ae25bcc876e..bfeb16025327 100644
> --- a/drivers/gpu/drm/i915/intel_ringbuffer.c
> +++ b/drivers/gpu/drm/i915/intel_ringbuffer.c
> @@ -1494,12 +1494,6 @@ static int gen8_render_emit_request(struct drm_i915_gem_request *req)
>   return 0;
>  }
>
> -static inline bool i915_gem_has_seqno_wrapped(struct drm_i915_private *dev_priv,
> -       u32 seqno)
> -{
> - return dev_priv->last_seqno < seqno;
> -}
> -
>  /**
>   * intel_ring_sync - sync the waiter to the signaller on seqno
>   *
> @@ -1509,24 +1503,23 @@ static inline bool i915_gem_has_seqno_wrapped(struct drm_i915_private *dev_priv,
>   */
>
>  static int
> -gen8_ring_sync(struct drm_i915_gem_request *waiter_req,
> -        struct intel_engine_cs *signaller,
> -        u32 seqno)
> +gen8_ring_sync(struct drm_i915_gem_request *wait,

Why not to, from here too or in the header then, when they're revamped
in the series? To bring some clarity. Maybe wait and signal in header
too rather.

> +        struct drm_i915_gem_request *signal)
>  {
> - struct intel_ring *waiter = waiter_req->ring;
> - struct drm_i915_private *dev_priv = waiter_req->i915;
> - u64 offset = GEN8_WAIT_OFFSET(waiter_req->engine, signaller->id);
> + struct intel_ring *waiter = wait->ring;

Just call this "ring" to reduce confusion of renaming the other
variable, then the ring_begin(wait) ring_emit() convention makes more
sense.

> + struct drm_i915_private *dev_priv = wait->i915;
> + u64 offset = GEN8_WAIT_OFFSET(wait->engine, signal->engine->id);
>   struct i915_hw_ppgtt *ppgtt;
>   int ret;
>
> - ret = intel_ring_begin(waiter_req, 4);
> + ret = intel_ring_begin(wait, 4);
>   if (ret)
>   return ret;
>
>   intel_ring_emit(waiter, MI_SEMAPHORE_WAIT |
>   MI_SEMAPHORE_GLOBAL_GTT |
>   MI_SEMAPHORE_SAD_GTE_SDD);
> - intel_ring_emit(waiter, seqno);
> + intel_ring_emit(waiter, signal->fence.seqno);
>   intel_ring_emit(waiter, lower_32_bits(offset));
>   intel_ring_emit(waiter, upper_32_bits(offset));
>   intel_ring_advance(waiter);
> @@ -1536,48 +1529,37 @@ gen8_ring_sync(struct drm_i915_gem_request *waiter_req,
>    * We do this on the i915_switch_context() following the wait and
>    * before the dispatch.
>    */
> - ppgtt = waiter_req->ctx->ppgtt;
> - if (ppgtt && waiter_req->engine->id != RCS)
> - ppgtt->pd_dirty_rings |= intel_engine_flag(waiter_req->engine);
> + ppgtt = wait->ctx->ppgtt;

This could be moved to initialization line, like elsewhere.

> + if (ppgtt && wait->engine->id != RCS)
> + ppgtt->pd_dirty_rings |= intel_engine_flag(wait->engine);
>   return 0;
>  }
>
>  static int
> -gen6_ring_sync(struct drm_i915_gem_request *waiter_req,
> -        struct intel_engine_cs *signaller,
> -        u32 seqno)
> +gen6_ring_sync(struct drm_i915_gem_request *wait,
> +        struct drm_i915_gem_request *signal)
>  {
> - struct intel_ring *waiter = waiter_req->ring;
> + struct intel_ring *waiter = wait->ring;
>   u32 dw1 = MI_SEMAPHORE_MBOX |
>     MI_SEMAPHORE_COMPARE |
>     MI_SEMAPHORE_REGISTER;
> - u32 wait_mbox = signaller->semaphore.mbox.wait[waiter_req->engine->id];
> + u32 wait_mbox = signal->engine->semaphore.mbox.wait[wait->engine->id];
>   int ret;
>
> - /* Throughout all of the GEM code, seqno passed implies our current
> -  * seqno is >= the last seqno executed. However for hardware the
> -  * comparison is strictly greater than.
> -  */
> - seqno -= 1;
> -

Finally we get rid of this \o/

Reviewed-by: Joonas Lahtinen <joonas.lahtinen@linux.intel.com>

>   WARN_ON(wait_mbox == MI_SEMAPHORE_SYNC_INVALID);
>
> - ret = intel_ring_begin(waiter_req, 4);
> + ret = intel_ring_begin(wait, 4);
>   if (ret)
>   return ret;
>
> - /* If seqno wrap happened, omit the wait with no-ops */
> - if (likely(!i915_gem_has_seqno_wrapped(waiter_req->i915, seqno))) {
> - intel_ring_emit(waiter, dw1 | wait_mbox);
> - intel_ring_emit(waiter, seqno);
> - intel_ring_emit(waiter, 0);
> - intel_ring_emit(waiter, MI_NOOP);
> - } else {
> - intel_ring_emit(waiter, MI_NOOP);
> - intel_ring_emit(waiter, MI_NOOP);
> - intel_ring_emit(waiter, MI_NOOP);
> - intel_ring_emit(waiter, MI_NOOP);
> - }
> + intel_ring_emit(waiter, dw1 | wait_mbox);
> + /* Throughout all of the GEM code, seqno passed implies our current
> +  * seqno is >= the last seqno executed. However for hardware the
> +  * comparison is strictly greater than.
> +  */
> + intel_ring_emit(waiter, signal->fence.seqno - 1);
> + intel_ring_emit(waiter, 0);
> + intel_ring_emit(waiter, MI_NOOP);
>   intel_ring_advance(waiter);
>
>   return 0;
> diff --git a/drivers/gpu/drm/i915/intel_ringbuffer.h b/drivers/gpu/drm/i915/intel_ringbuffer.h
> index 08e86204a3d5..65cb6adf26ca 100644
> --- a/drivers/gpu/drm/i915/intel_ringbuffer.h
> +++ b/drivers/gpu/drm/i915/intel_ringbuffer.h
> @@ -275,9 +275,8 @@ struct intel_engine_cs {
>   };
>
>   /* AKA wait() */
> - int (*sync_to)(struct drm_i915_gem_request *to_req,
> -    struct intel_engine_cs *from,
> -    u32 seqno);
> + int (*sync_to)(struct drm_i915_gem_request *to,
> +    struct drm_i915_gem_request *from);
>   int (*signal)(struct drm_i915_gem_request *signaller_req);
>   } semaphore;
>
--
Joonas Lahtinen
Open Source Technology Center
Intel Corporation