[PATCH 1/3] drm/i915: Avoid the branch in computing intel_ring_space()

[PATCH 1/3] drm/i915: Avoid the branch in computing intel_ring_space()

[Chris Wilson]

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.

Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Cc: Mika Kuoppala <mika.kuoppala@intel.com>
---
 drivers/gpu/drm/i915/intel_ringbuffer.c | 20 ++++++++++----------
 drivers/gpu/drm/i915/intel_ringbuffer.h | 30 ++++++++++++++++--------------
 2 files changed, 26 insertions(+), 24 deletions(-)

diff --git a/drivers/gpu/drm/i915/intel_ringbuffer.c b/drivers/gpu/drm/i915/intel_ringbuffer.c
index 6836efb7e3d2..16739352f057 100644
--- a/drivers/gpu/drm/i915/intel_ringbuffer.c
+++ b/drivers/gpu/drm/i915/intel_ringbuffer.c
@@ -39,12 +39,15 @@
  */
 #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);
 }
 
 void intel_ring_update_space(struct intel_ring *ring)
@@ -1619,12 +1622,9 @@ static int wait_for_space(struct drm_i915_gem_request *req, int bytes)
 	GEM_BUG_ON(!req->reserved_space);
 
 	list_for_each_entry(target, &ring->request_list, ring_link) {
-		unsigned space;
-
 		/* Would completion of this request free enough space? */
-		space = __intel_ring_space(target->postfix, ring->emit,
-					   ring->size);
-		if (space >= bytes)
+		if (bytes <= __intel_ring_space(target->postfix,
+						ring->emit, ring->size))
 			break;
 	}
 
diff --git a/drivers/gpu/drm/i915/intel_ringbuffer.h b/drivers/gpu/drm/i915/intel_ringbuffer.h
index 96710b616efb..1b1099b94912 100644
--- a/drivers/gpu/drm/i915/intel_ringbuffer.h
+++ b/drivers/gpu/drm/i915/intel_ringbuffer.h
@@ -17,17 +17,6 @@
 #define CACHELINE_BYTES 64
 #define CACHELINE_DWORDS (CACHELINE_BYTES / sizeof(uint32_t))
 
-/*
- * Gen2 BSpec "1. Programming Environment" / 1.4.4.6 "Ring Buffer Use"
- * Gen3 BSpec "vol1c Memory Interface Functions" / 2.3.4.5 "Ring Buffer Use"
- * Gen4+ BSpec "vol1c Memory Interface and Command Stream" / 5.3.4.5 "Ring Buffer Use"
- *
- * "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."
- */
-#define I915_RING_FREE_SPACE 64
-
 struct intel_hw_status_page {
 	struct i915_vma *vma;
 	u32 *page_addr;
@@ -145,9 +134,9 @@ struct intel_ring {
 	u32 tail;
 	u32 emit;
 
-	int space;
-	int size;
-	int effective_size;
+	u32 space;
+	u32 size;
+	u32 effective_size;
 };
 
 struct i915_gem_context;
@@ -547,6 +536,19 @@ assert_ring_tail_valid(const struct intel_ring *ring, unsigned int tail)
 	 */
 	GEM_BUG_ON(!IS_ALIGNED(tail, 8));
 	GEM_BUG_ON(tail >= ring->size);
+
+	/* "Ring Buffer Use"
+	 *	Gen2 BSpec "1. Programming Environment" / 1.4.4.6
+	 *	Gen3 BSpec "1c Memory Interface Functions" / 2.3.4.5
+	 *	Gen4+ BSpec "1c Memory Interface and Command Stream" / 5.3.4.5
+	 * "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."
+	 */
+#define cacheline(a) round_down(a, CACHELINE_BYTES)
+	GEM_BUG_ON(cacheline(tail) == cacheline(ring->head) &&
+		   tail < ring->head);
+#undef cacheline
 }
 
 static inline unsigned int
-- 
2.11.0

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[PATCH 1/3] drm/i915: Avoid the branch in computing intel_ring_space()

[Chris Wilson]

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@chris-wilson.co.uk>
Cc: Mika Kuoppala <mika.kuoppala@intel.com>
---
 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 29b5afac7856..5947bba20c4a 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);
 }
 
 void intel_ring_update_space(struct intel_ring *ring)
@@ -1669,12 +1673,9 @@ static int wait_for_space(struct drm_i915_gem_request *req, int bytes)
 	GEM_BUG_ON(!req->reserved_space);
 
 	list_for_each_entry(target, &ring->request_list, ring_link) {
-		unsigned space;
-
 		/* Would completion of this request free enough space? */
-		space = __intel_ring_space(target->postfix, ring->emit,
-					   ring->size);
-		if (space >= bytes)
+		if (bytes <= __intel_ring_space(target->postfix,
+						ring->emit, ring->size))
 			break;
 	}
 
@@ -1743,11 +1744,11 @@ u32 *intel_ring_begin(struct drm_i915_gem_request *req, int num_dwords)
 	}
 
 	GEM_BUG_ON(ring->emit > ring->size - bytes);
+	GEM_BUG_ON(ring->space < bytes);
 	cs = ring->vaddr + ring->emit;
 	GEM_DEBUG_EXEC(memset(cs, POISON_INUSE, bytes));
 	ring->emit += bytes;
 	ring->space -= bytes;
-	GEM_BUG_ON(ring->space < 0);
 
 	return cs;
 }
diff --git a/drivers/gpu/drm/i915/intel_ringbuffer.h b/drivers/gpu/drm/i915/intel_ringbuffer.h
index 02d741ef99ad..64584d134681 100644
--- a/drivers/gpu/drm/i915/intel_ringbuffer.h
+++ b/drivers/gpu/drm/i915/intel_ringbuffer.h
@@ -17,17 +17,6 @@
 #define CACHELINE_BYTES 64
 #define CACHELINE_DWORDS (CACHELINE_BYTES / sizeof(uint32_t))
 
-/*
- * Gen2 BSpec "1. Programming Environment" / 1.4.4.6 "Ring Buffer Use"
- * Gen3 BSpec "vol1c Memory Interface Functions" / 2.3.4.5 "Ring Buffer Use"
- * Gen4+ BSpec "vol1c Memory Interface and Command Stream" / 5.3.4.5 "Ring Buffer Use"
- *
- * "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."
- */
-#define I915_RING_FREE_SPACE 64
-
 struct intel_hw_status_page {
 	struct i915_vma *vma;
 	u32 *page_addr;
@@ -145,9 +134,9 @@ struct intel_ring {
 	u32 tail;
 	u32 emit;
 
-	int space;
-	int size;
-	int effective_size;
+	u32 space;
+	u32 size;
+	u32 effective_size;
 };
 
 struct i915_gem_context;
@@ -548,6 +537,25 @@ assert_ring_tail_valid(const struct intel_ring *ring, unsigned int tail)
 	 */
 	GEM_BUG_ON(!IS_ALIGNED(tail, 8));
 	GEM_BUG_ON(tail >= ring->size);
+
+	/*
+	 * "Ring Buffer Use"
+	 *	Gen2 BSpec "1. Programming Environment" / 1.4.4.6
+	 *	Gen3 BSpec "1c Memory Interface Functions" / 2.3.4.5
+	 *	Gen4+ BSpec "1c Memory Interface and Command Stream" / 5.3.4.5
+	 * "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."
+	 *
+	 * We use ring->head as the last known location of the actual RING_HEAD,
+	 * it may have advanced but in the worst case it is equally the same
+	 * as ring->head and so we should never program RING_TAIL to advance
+	 * into the same cacheline as ring->head.
+	 */
+#define cacheline(a) round_down(a, CACHELINE_BYTES)
+	GEM_BUG_ON(cacheline(tail) == cacheline(ring->head) &&
+		   tail < ring->head);
+#undef cacheline
 }
 
 static inline unsigned int
-- 
2.11.0

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[PATCH 2/3] drm/i915: Report the ring->space from intel_ring_update_space()

[Chris Wilson]

Some callers immediately want to know the current ring->space after
calling intel_ring_update_space(), which we can freely provide via the
return parameter.

Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Reviewed-by: Joonas Lahtinen <joonas.lahtinen@linux.intel.com>
---
 drivers/gpu/drm/i915/intel_ringbuffer.c | 12 ++++++++----
 drivers/gpu/drm/i915/intel_ringbuffer.h |  2 +-
 2 files changed, 9 insertions(+), 5 deletions(-)

diff --git a/drivers/gpu/drm/i915/intel_ringbuffer.c b/drivers/gpu/drm/i915/intel_ringbuffer.c
index 5947bba20c4a..c46e5439d379 100644
--- a/drivers/gpu/drm/i915/intel_ringbuffer.c
+++ b/drivers/gpu/drm/i915/intel_ringbuffer.c
@@ -51,9 +51,14 @@ static unsigned int __intel_ring_space(unsigned int head,
 	return (head - tail - CACHELINE_BYTES) & (size - 1);
 }
 
-void intel_ring_update_space(struct intel_ring *ring)
+unsigned int intel_ring_update_space(struct intel_ring *ring)
 {
-	ring->space = __intel_ring_space(ring->head, ring->emit, ring->size);
+	unsigned int space;
+
+	space = __intel_ring_space(ring->head, ring->emit, ring->size);
+
+	ring->space = space;
+	return space;
 }
 
 static int
@@ -1657,8 +1662,7 @@ static int wait_for_space(struct drm_i915_gem_request *req, int bytes)
 
 	lockdep_assert_held(&req->i915->drm.struct_mutex);
 
-	intel_ring_update_space(ring);
-	if (ring->space >= bytes)
+	if (intel_ring_update_space(ring) >= bytes)
 		return 0;
 
 	/*
diff --git a/drivers/gpu/drm/i915/intel_ringbuffer.h b/drivers/gpu/drm/i915/intel_ringbuffer.h
index 64584d134681..4a599e3480a9 100644
--- a/drivers/gpu/drm/i915/intel_ringbuffer.h
+++ b/drivers/gpu/drm/i915/intel_ringbuffer.h
@@ -486,7 +486,7 @@ int intel_ring_pin(struct intel_ring *ring,
 		   struct drm_i915_private *i915,
 		   unsigned int offset_bias);
 void intel_ring_reset(struct intel_ring *ring, u32 tail);
-void intel_ring_update_space(struct intel_ring *ring);
+unsigned int intel_ring_update_space(struct intel_ring *ring);
 void intel_ring_unpin(struct intel_ring *ring);
 void intel_ring_free(struct intel_ring *ring);
 
-- 
2.11.0

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[PATCH 3/3] drm/i915: Micro-optimise hotpath through intel_ring_begin()

[Chris Wilson]

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@chris-wilson.co.uk>
---
 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;
+		} else  {
+			/*
+			 * 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 from the start of ringbuffer.
+			 */
+			total_bytes = req->reserved_space + remain_actual;
+		}
 	}
 
-	if (wait_bytes > ring->space) {
-		int ret = wait_for_space(req, wait_bytes);
+	if (unlikely(total_bytes > ring->space)) {
+		int ret = wait_for_space(req, total_bytes);
 		if (unlikely(ret))
 			return ERR_PTR(ret);
 	}
 
 	if (unlikely(need_wrap)) {
-		GEM_BUG_ON(remain_actual > ring->space);
-		GEM_BUG_ON(ring->emit + remain_actual > ring->size);
+		need_wrap &= ~1;
+		GEM_BUG_ON(need_wrap > ring->space);
+		GEM_BUG_ON(ring->emit + need_wrap > ring->size);
 
 		/* Fill the tail with MI_NOOP */
-		memset(ring->vaddr + ring->emit, 0, remain_actual);
+		memset(ring->vaddr + ring->emit, 0, need_wrap);
 		ring->emit = 0;
-		ring->space -= remain_actual;
+		ring->space -= need_wrap;
 	}
 
 	GEM_BUG_ON(ring->emit > ring->size - bytes);
-- 
2.11.0

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✓ Fi.CI.BAT: success for series starting with [1/3] drm/i915: Avoid the branch in computing intel_ring_space()

[Patchwork]

== Series Details ==

Series: series starting with [1/3] drm/i915: Avoid the branch in computing intel_ring_space()
URL   : https://patchwork.freedesktop.org/series/23878/
State : success

== Summary ==

Series 23878v1 Series without cover letter
https://patchwork.freedesktop.org/api/1.0/series/23878/revisions/1/mbox/

Test gem_exec_flush:
        Subgroup basic-batch-kernel-default-uc:
                pass       -> FAIL       (fi-snb-2600) fdo#100007
Test gem_exec_suspend:
        Subgroup basic-s4-devices:
                pass       -> DMESG-WARN (fi-kbl-7560u) fdo#100125

fdo#100007 https://bugs.freedesktop.org/show_bug.cgi?id=100007
fdo#100125 https://bugs.freedesktop.org/show_bug.cgi?id=100125

fi-bdw-5557u     total:278  pass:267  dwarn:0   dfail:0   fail:0   skip:11  time:429s
fi-bdw-gvtdvm    total:278  pass:256  dwarn:8   dfail:0   fail:0   skip:14  time:430s
fi-bsw-n3050     total:278  pass:242  dwarn:0   dfail:0   fail:0   skip:36  time:572s
fi-bxt-j4205     total:278  pass:259  dwarn:0   dfail:0   fail:0   skip:19  time:506s
fi-bxt-t5700     total:278  pass:258  dwarn:0   dfail:0   fail:0   skip:20  time:567s
fi-byt-j1900     total:278  pass:254  dwarn:0   dfail:0   fail:0   skip:24  time:492s
fi-byt-n2820     total:278  pass:250  dwarn:0   dfail:0   fail:0   skip:28  time:484s
fi-hsw-4770      total:278  pass:262  dwarn:0   dfail:0   fail:0   skip:16  time:412s
fi-hsw-4770r     total:278  pass:262  dwarn:0   dfail:0   fail:0   skip:16  time:406s
fi-ilk-650       total:278  pass:228  dwarn:0   dfail:0   fail:0   skip:50  time:414s
fi-ivb-3520m     total:278  pass:260  dwarn:0   dfail:0   fail:0   skip:18  time:492s
fi-ivb-3770      total:278  pass:260  dwarn:0   dfail:0   fail:0   skip:18  time:474s
fi-kbl-7500u     total:278  pass:260  dwarn:0   dfail:0   fail:0   skip:18  time:462s
fi-kbl-7560u     total:278  pass:267  dwarn:1   dfail:0   fail:0   skip:10  time:568s
fi-skl-6260u     total:278  pass:268  dwarn:0   dfail:0   fail:0   skip:10  time:458s
fi-skl-6700hq    total:278  pass:261  dwarn:0   dfail:0   fail:0   skip:17  time:572s
fi-skl-6700k     total:278  pass:256  dwarn:4   dfail:0   fail:0   skip:18  time:462s
fi-skl-6770hq    total:278  pass:268  dwarn:0   dfail:0   fail:0   skip:10  time:494s
fi-skl-gvtdvm    total:278  pass:265  dwarn:0   dfail:0   fail:0   skip:13  time:430s
fi-snb-2520m     total:278  pass:250  dwarn:0   dfail:0   fail:0   skip:28  time:522s
fi-snb-2600      total:278  pass:248  dwarn:0   dfail:0   fail:1   skip:29  time:410s

26ef9256ab34901a953bd5338a3c94388c745c99 drm-tip: 2017y-05m-03d-10h-11m-12s UTC integration manifest
fdbe0a2 drm/i915: Micro-optimise hotpath through intel_ring_begin()
90964bf drm/i915: Report the ring->space from intel_ring_update_space()
1c35c55 drm/i915: Avoid the branch in computing intel_ring_space()

== Logs ==

For more details see: https://intel-gfx-ci.01.org/CI/Patchwork_4609/
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Re: [PATCH 1/3] drm/i915: Avoid the branch in computing intel_ring_space()

[Mika Kuoppala]

Chris Wilson <chris@chris-wilson.co.uk> writes:

> 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@chris-wilson.co.uk>
> Cc: Mika Kuoppala <mika.kuoppala@intel.com>
> ---
>  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 29b5afac7856..5947bba20c4a 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)

Why not u32 here too?

>  {
> -	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);

I almost went and complained about the incorrect calculation here
but rereading the commit message I regained my senses.

>  }
>  
>  void intel_ring_update_space(struct intel_ring *ring)
> @@ -1669,12 +1673,9 @@ static int wait_for_space(struct drm_i915_gem_request *req, int bytes)

Bytes should be converted also to unsigned at some point.

>  	GEM_BUG_ON(!req->reserved_space);
>  
>  	list_for_each_entry(target, &ring->request_list, ring_link) {
> -		unsigned space;
> -
>  		/* Would completion of this request free enough space? */
> -		space = __intel_ring_space(target->postfix, ring->emit,
> -					   ring->size);
> -		if (space >= bytes)
> +		if (bytes <= __intel_ring_space(target->postfix,
> +						ring->emit, ring->size))
>  			break;
>  	}
>  
> @@ -1743,11 +1744,11 @@ u32 *intel_ring_begin(struct drm_i915_gem_request *req, int num_dwords)
>  	}
>  
>  	GEM_BUG_ON(ring->emit > ring->size - bytes);
> +	GEM_BUG_ON(ring->space < bytes);
>  	cs = ring->vaddr + ring->emit;
>  	GEM_DEBUG_EXEC(memset(cs, POISON_INUSE, bytes));
>  	ring->emit += bytes;
>  	ring->space -= bytes;
> -	GEM_BUG_ON(ring->space < 0);
>  
>  	return cs;
>  }
> diff --git a/drivers/gpu/drm/i915/intel_ringbuffer.h b/drivers/gpu/drm/i915/intel_ringbuffer.h
> index 02d741ef99ad..64584d134681 100644
> --- a/drivers/gpu/drm/i915/intel_ringbuffer.h
> +++ b/drivers/gpu/drm/i915/intel_ringbuffer.h
> @@ -17,17 +17,6 @@
>  #define CACHELINE_BYTES 64
>  #define CACHELINE_DWORDS (CACHELINE_BYTES / sizeof(uint32_t))
>  
> -/*
> - * Gen2 BSpec "1. Programming Environment" / 1.4.4.6 "Ring Buffer Use"
> - * Gen3 BSpec "vol1c Memory Interface Functions" / 2.3.4.5 "Ring Buffer Use"
> - * Gen4+ BSpec "vol1c Memory Interface and Command Stream" / 5.3.4.5 "Ring Buffer Use"
> - *
> - * "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."
> - */
> -#define I915_RING_FREE_SPACE 64
> -
>  struct intel_hw_status_page {
>  	struct i915_vma *vma;
>  	u32 *page_addr;
> @@ -145,9 +134,9 @@ struct intel_ring {
>  	u32 tail;
>  	u32 emit;
>  
> -	int space;
> -	int size;
> -	int effective_size;
> +	u32 space;
> +	u32 size;
> +	u32 effective_size;
>  };
>  
>  struct i915_gem_context;
> @@ -548,6 +537,25 @@ assert_ring_tail_valid(const struct intel_ring *ring, unsigned int tail)
>  	 */
>  	GEM_BUG_ON(!IS_ALIGNED(tail, 8));
>  	GEM_BUG_ON(tail >= ring->size);
> +
> +	/*
> +	 * "Ring Buffer Use"
> +	 *	Gen2 BSpec "1. Programming Environment" / 1.4.4.6
> +	 *	Gen3 BSpec "1c Memory Interface Functions" / 2.3.4.5
> +	 *	Gen4+ BSpec "1c Memory Interface and Command Stream" / 5.3.4.5
> +	 * "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."
> +	 *
> +	 * We use ring->head as the last known location of the actual RING_HEAD,
> +	 * it may have advanced but in the worst case it is equally the same
> +	 * as ring->head and so we should never program RING_TAIL to advance
> +	 * into the same cacheline as ring->head.
> +	 */
> +#define cacheline(a) round_down(a, CACHELINE_BYTES)
> +	GEM_BUG_ON(cacheline(tail) == cacheline(ring->head) &&
> +		   tail < ring->head);

Nitpick, ring->head > tail should have fit the quote above better.

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

> +#undef cacheline
>  }
>  
>  static inline unsigned int
> -- 
> 2.11.0
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Re: [PATCH 1/3] drm/i915: Avoid the branch in computing intel_ring_space()

[Chris Wilson]

On Wed, May 03, 2017 at 02:53:43PM +0300, Mika Kuoppala wrote:
> Chris Wilson <chris@chris-wilson.co.uk> writes:
> 
> > 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@chris-wilson.co.uk>
> > Cc: Mika Kuoppala <mika.kuoppala@intel.com>
> > ---
> >  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 29b5afac7856..5947bba20c4a 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)
> 
> Why not u32 here too?

Because there's no need to constrain the compiler's freedom in choosing
a natural uint.
 
> >  {
> > -	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);
> 
> I almost went and complained about the incorrect calculation here
> but rereading the commit message I regained my senses.
> 
> >  }
> >  
> >  void intel_ring_update_space(struct intel_ring *ring)
> > @@ -1669,12 +1673,9 @@ static int wait_for_space(struct drm_i915_gem_request *req, int bytes)
> 
> Bytes should be converted also to unsigned at some point.

Might as well do it now, indeed.

> 
> >  	GEM_BUG_ON(!req->reserved_space);
> >  
> >  	list_for_each_entry(target, &ring->request_list, ring_link) {
> > -		unsigned space;
> > -
> >  		/* Would completion of this request free enough space? */
> > -		space = __intel_ring_space(target->postfix, ring->emit,
> > -					   ring->size);
> > -		if (space >= bytes)
> > +		if (bytes <= __intel_ring_space(target->postfix,
> > +						ring->emit, ring->size))
> >  			break;
> >  	}
> >  
> > @@ -1743,11 +1744,11 @@ u32 *intel_ring_begin(struct drm_i915_gem_request *req, int num_dwords)
> >  	}
> >  
> >  	GEM_BUG_ON(ring->emit > ring->size - bytes);
> > +	GEM_BUG_ON(ring->space < bytes);
> >  	cs = ring->vaddr + ring->emit;
> >  	GEM_DEBUG_EXEC(memset(cs, POISON_INUSE, bytes));
> >  	ring->emit += bytes;
> >  	ring->space -= bytes;
> > -	GEM_BUG_ON(ring->space < 0);
> >  
> >  	return cs;
> >  }
> > diff --git a/drivers/gpu/drm/i915/intel_ringbuffer.h b/drivers/gpu/drm/i915/intel_ringbuffer.h
> > index 02d741ef99ad..64584d134681 100644
> > --- a/drivers/gpu/drm/i915/intel_ringbuffer.h
> > +++ b/drivers/gpu/drm/i915/intel_ringbuffer.h
> > @@ -17,17 +17,6 @@
> >  #define CACHELINE_BYTES 64
> >  #define CACHELINE_DWORDS (CACHELINE_BYTES / sizeof(uint32_t))
> >  
> > -/*
> > - * Gen2 BSpec "1. Programming Environment" / 1.4.4.6 "Ring Buffer Use"
> > - * Gen3 BSpec "vol1c Memory Interface Functions" / 2.3.4.5 "Ring Buffer Use"
> > - * Gen4+ BSpec "vol1c Memory Interface and Command Stream" / 5.3.4.5 "Ring Buffer Use"
> > - *
> > - * "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."
> > - */
> > -#define I915_RING_FREE_SPACE 64
> > -
> >  struct intel_hw_status_page {
> >  	struct i915_vma *vma;
> >  	u32 *page_addr;
> > @@ -145,9 +134,9 @@ struct intel_ring {
> >  	u32 tail;
> >  	u32 emit;
> >  
> > -	int space;
> > -	int size;
> > -	int effective_size;
> > +	u32 space;
> > +	u32 size;
> > +	u32 effective_size;
> >  };
> >  
> >  struct i915_gem_context;
> > @@ -548,6 +537,25 @@ assert_ring_tail_valid(const struct intel_ring *ring, unsigned int tail)
> >  	 */
> >  	GEM_BUG_ON(!IS_ALIGNED(tail, 8));
> >  	GEM_BUG_ON(tail >= ring->size);
> > +
> > +	/*
> > +	 * "Ring Buffer Use"
> > +	 *	Gen2 BSpec "1. Programming Environment" / 1.4.4.6
> > +	 *	Gen3 BSpec "1c Memory Interface Functions" / 2.3.4.5
> > +	 *	Gen4+ BSpec "1c Memory Interface and Command Stream" / 5.3.4.5
> > +	 * "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."
> > +	 *
> > +	 * We use ring->head as the last known location of the actual RING_HEAD,
> > +	 * it may have advanced but in the worst case it is equally the same
> > +	 * as ring->head and so we should never program RING_TAIL to advance
> > +	 * into the same cacheline as ring->head.
> > +	 */
> > +#define cacheline(a) round_down(a, CACHELINE_BYTES)
> > +	GEM_BUG_ON(cacheline(tail) == cacheline(ring->head) &&
> > +		   tail < ring->head);
> 
> Nitpick, ring->head > tail should have fit the quote above better.

It's hard though. Here there is an equivalence between ring->tail and
RING_TAIL, but the relationship between ring->head and RING_HEAD is much
more relaxed, so I felt worth commenting upon following our last
discussion. The function is assert_ring_tail_valid() so I assume the
ring->tail == RING_TAIL is clear enough and I don't need to make the
comment even more convoluted.
-Chris

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Re: [PATCH 1/3] drm/i915: Avoid the branch in computing intel_ring_space()

[Chris Wilson]

On Wed, May 03, 2017 at 01:02:40PM +0100, Chris Wilson wrote:
> On Wed, May 03, 2017 at 02:53:43PM +0300, Mika Kuoppala wrote:
> > >  void intel_ring_update_space(struct intel_ring *ring)
> > > @@ -1669,12 +1673,9 @@ static int wait_for_space(struct drm_i915_gem_request *req, int bytes)
> > 
> > Bytes should be converted also to unsigned at some point.
> 
> Might as well do it now, indeed.

Actually I'll do it in patch 3 as there are nearby similar changes.
-Chris

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Re: [PATCH 3/3] drm/i915: Micro-optimise hotpath through intel_ring_begin()

[Mika Kuoppala]

Chris Wilson <chris@chris-wilson.co.uk> 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@chris-wilson.co.uk>
> ---
>  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.

-Mika

> +		} else  {
> +			/*
> +			 * 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 from the start of ringbuffer.
> +			 */
> +			total_bytes = req->reserved_space + remain_actual;
> +		}
>  	}
>  
> -	if (wait_bytes > ring->space) {
> -		int ret = wait_for_space(req, wait_bytes);
> +	if (unlikely(total_bytes > ring->space)) {
> +		int ret = wait_for_space(req, total_bytes);
>  		if (unlikely(ret))
>  			return ERR_PTR(ret);
>  	}
>  
>  	if (unlikely(need_wrap)) {
> -		GEM_BUG_ON(remain_actual > ring->space);
> -		GEM_BUG_ON(ring->emit + remain_actual > ring->size);
> +		need_wrap &= ~1;
> +		GEM_BUG_ON(need_wrap > ring->space);
> +		GEM_BUG_ON(ring->emit + need_wrap > ring->size);
>  
>  		/* Fill the tail with MI_NOOP */
> -		memset(ring->vaddr + ring->emit, 0, remain_actual);
> +		memset(ring->vaddr + ring->emit, 0, need_wrap);
>  		ring->emit = 0;
> -		ring->space -= remain_actual;
> +		ring->space -= need_wrap;
>  	}
>  
>  	GEM_BUG_ON(ring->emit > ring->size - bytes);
> -- 
> 2.11.0
>
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Re: [PATCH 3/3] drm/i915: Micro-optimise hotpath through intel_ring_begin()

[Chris Wilson]

On Thu, May 04, 2017 at 03:11:45PM +0300, Mika Kuoppala wrote:
> Chris Wilson <chris@chris-wilson.co.uk> 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@chris-wilson.co.uk>
> > ---
> >  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

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Re: [PATCH 3/3] drm/i915: Micro-optimise hotpath through intel_ring_begin()

[Mika Kuoppala]

Chris Wilson <chris@chris-wilson.co.uk> writes:

> On Thu, May 04, 2017 at 03:11:45PM +0300, Mika Kuoppala wrote:
>> Chris Wilson <chris@chris-wilson.co.uk> 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@chris-wilson.co.uk>
>> > ---
>> >  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@intel.com>
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Re: [PATCH 3/3] drm/i915: Micro-optimise hotpath through intel_ring_begin()

[Chris Wilson]

On Thu, May 04, 2017 at 03:59:05PM +0300, Mika Kuoppala wrote:
> Chris Wilson <chris@chris-wilson.co.uk> writes:
> 
> > On Thu, May 04, 2017 at 03:11:45PM +0300, Mika Kuoppala wrote:
> >> Chris Wilson <chris@chris-wilson.co.uk> 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@chris-wilson.co.uk>
> >> > ---
> >> >  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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