[1/3] drm/i915/perf: rework aging tail workaround

Submitted by Umesh Nerlige Ramappa on Sept. 13, 2019, 11:06 p.m.

Details

Message ID 20190913230620.15906-2-umesh.nerlige.ramappa@intel.com
State New
Headers show
Series "drm/i915/perf: Enable non-power-of-2 OA report sizes" ( rev: 1 ) in Intel GFX

Not browsing as part of any series.

Commit Message

Umesh Nerlige Ramappa Sept. 13, 2019, 11:06 p.m.
From: Lionel Landwerlin <lionel.g.landwerlin@intel.com>

Right now the workaround against the OA tail pointer race condition
requires at least twice the internal kernel polling timer to make any
data available.

This changes introduce checks on the OA data written into the circular
buffer to make as much data as possible available on the first
iteration of the polling timer.

v2: Use OA_TAKEN macro without the gtt_offset (Lionel)

Signed-off-by: Lionel Landwerlin <lionel.g.landwerlin@intel.com>
---
 drivers/gpu/drm/i915/i915_drv.h  |  30 ++---
 drivers/gpu/drm/i915/i915_perf.c | 200 ++++++++++++++-----------------
 2 files changed, 103 insertions(+), 127 deletions(-)

Patch hide | download patch | download mbox

diff --git a/drivers/gpu/drm/i915/i915_drv.h b/drivers/gpu/drm/i915/i915_drv.h
index bf600888b3f1..876aeaf3568e 100644
--- a/drivers/gpu/drm/i915/i915_drv.h
+++ b/drivers/gpu/drm/i915/i915_drv.h
@@ -1180,21 +1180,11 @@  struct i915_perf_stream {
 		spinlock_t ptr_lock;
 
 		/**
-		 * One 'aging' tail pointer and one 'aged' tail pointer ready to
-		 * used for reading.
-		 *
-		 * Initial values of 0xffffffff are invalid and imply that an
-		 * update is required (and should be ignored by an attempted
-		 * read)
-		 */
-		struct {
-			u32 offset;
-		} tails[2];
-
-		/**
-		 * Index for the aged tail ready to read() data up to.
+		 * The last HW tail reported by HW. The data
+		 * might not have made it to memory yet
+		 * though.
 		 */
-		unsigned int aged_tail_idx;
+		u32 aging_tail;
 
 		/**
 		 * A monotonic timestamp for when the current aging tail pointer
@@ -1210,6 +1200,12 @@  struct i915_perf_stream {
 		 * OA buffer data to userspace.
 		 */
 		u32 head;
+
+		/**
+		 * The last verified tail that can be read
+		 * by user space
+		 */
+		u32 tail;
 	} oa_buffer;
 };
 
@@ -1693,6 +1689,12 @@  struct drm_i915_private {
 		 */
 		struct ratelimit_state spurious_report_rs;
 
+		/**
+		 * For rate limiting any notifications of tail pointer
+		 * race.
+		 */
+		struct ratelimit_state tail_pointer_race;
+
 		struct i915_oa_config test_config;
 
 		u32 gen7_latched_oastatus1;
diff --git a/drivers/gpu/drm/i915/i915_perf.c b/drivers/gpu/drm/i915/i915_perf.c
index c1b764233761..50b6d154fd46 100644
--- a/drivers/gpu/drm/i915/i915_perf.c
+++ b/drivers/gpu/drm/i915/i915_perf.c
@@ -237,23 +237,14 @@ 
  * for this earlier, as part of the oa_buffer_check to avoid lots of redundant
  * read() attempts.
  *
- * In effect we define a tail pointer for reading that lags the real tail
- * pointer by at least %OA_TAIL_MARGIN_NSEC nanoseconds, which gives enough
- * time for the corresponding reports to become visible to the CPU.
- *
- * To manage this we actually track two tail pointers:
- *  1) An 'aging' tail with an associated timestamp that is tracked until we
- *     can trust the corresponding data is visible to the CPU; at which point
- *     it is considered 'aged'.
- *  2) An 'aged' tail that can be used for read()ing.
- *
- * The two separate pointers let us decouple read()s from tail pointer aging.
- *
- * The tail pointers are checked and updated at a limited rate within a hrtimer
- * callback (the same callback that is used for delivering EPOLLIN events)
- *
- * Initially the tails are marked invalid with %INVALID_TAIL_PTR which
- * indicates that an updated tail pointer is needed.
+ * We workaround this issue in oa_buffer_check() by reading the reports in the
+ * OA buffer, starting from the tail reported by the HW until we find 2
+ * consecutive reports with their first 2 dwords of not at 0. Those dwords are
+ * also set to 0 once read and the whole buffer is cleared upon OA buffer
+ * initialization. The first dword is the reason for this report while the
+ * second is the timestamp, making the chances of having those 2 fields at 0
+ * fairly unlikely. A more detailed explanation is available in
+ * oa_buffer_check().
  *
  * Most of the implementation details for this workaround are in
  * oa_buffer_check_unlocked() and _append_oa_reports()
@@ -266,7 +257,6 @@ 
  * enabled without any periodic sampling.
  */
 #define OA_TAIL_MARGIN_NSEC	100000ULL
-#define INVALID_TAIL_PTR	0xffffffff
 
 /* frequency for checking whether the OA unit has written new reports to the
  * circular OA buffer...
@@ -457,10 +447,10 @@  static u32 gen7_oa_hw_tail_read(struct i915_perf_stream *stream)
 static bool oa_buffer_check_unlocked(struct i915_perf_stream *stream)
 {
 	struct drm_i915_private *dev_priv = stream->dev_priv;
+	u32 gtt_offset = i915_ggtt_offset(stream->oa_buffer.vma);
 	int report_size = stream->oa_buffer.format_size;
 	unsigned long flags;
-	unsigned int aged_idx;
-	u32 head, hw_tail, aged_tail, aging_tail;
+	u32 hw_tail;
 	u64 now;
 
 	/* We have to consider the (unlikely) possibility that read() errors
@@ -469,16 +459,6 @@  static bool oa_buffer_check_unlocked(struct i915_perf_stream *stream)
 	 */
 	spin_lock_irqsave(&stream->oa_buffer.ptr_lock, flags);
 
-	/* NB: The head we observe here might effectively be a little out of
-	 * date (between head and tails[aged_idx].offset if there is currently
-	 * a read() in progress.
-	 */
-	head = stream->oa_buffer.head;
-
-	aged_idx = stream->oa_buffer.aged_tail_idx;
-	aged_tail = stream->oa_buffer.tails[aged_idx].offset;
-	aging_tail = stream->oa_buffer.tails[!aged_idx].offset;
-
 	hw_tail = dev_priv->perf.ops.oa_hw_tail_read(stream);
 
 	/* The tail pointer increases in 64 byte increments,
@@ -488,63 +468,75 @@  static bool oa_buffer_check_unlocked(struct i915_perf_stream *stream)
 
 	now = ktime_get_mono_fast_ns();
 
-	/* Update the aged tail
-	 *
-	 * Flip the tail pointer available for read()s once the aging tail is
-	 * old enough to trust that the corresponding data will be visible to
-	 * the CPU...
-	 *
-	 * Do this before updating the aging pointer in case we may be able to
-	 * immediately start aging a new pointer too (if new data has become
-	 * available) without needing to wait for a later hrtimer callback.
-	 */
-	if (aging_tail != INVALID_TAIL_PTR &&
-	    ((now - stream->oa_buffer.aging_timestamp) >
-	     OA_TAIL_MARGIN_NSEC)) {
-
-		aged_idx ^= 1;
-		stream->oa_buffer.aged_tail_idx = aged_idx;
+	if (hw_tail == stream->oa_buffer.aging_tail) {
+		/* If the HW tail hasn't move since the last check and the HW
+		 * tail has been aging for long enough, declare it the new
+		 * tail.
+		 */
+		if ((now - stream->oa_buffer.aging_timestamp) >
+		    OA_TAIL_MARGIN_NSEC) {
+			stream->oa_buffer.tail =
+				stream->oa_buffer.aging_tail;
+		}
+	} else {
+		u32 head, tail, landed_report_heads;
 
-		aged_tail = aging_tail;
+		/* NB: The head we observe here might effectively be a little out of
+		 * date (between head and tails[aged_idx].offset if there is currently
+		 * a read() in progress.
+		 */
+		head = stream->oa_buffer.head - gtt_offset;
 
-		/* Mark that we need a new pointer to start aging... */
-		stream->oa_buffer.tails[!aged_idx].offset = INVALID_TAIL_PTR;
-		aging_tail = INVALID_TAIL_PTR;
-	}
+		hw_tail -= gtt_offset;
+		tail = hw_tail;
 
-	/* Update the aging tail
-	 *
-	 * We throttle aging tail updates until we have a new tail that
-	 * represents >= one report more data than is already available for
-	 * reading. This ensures there will be enough data for a successful
-	 * read once this new pointer has aged and ensures we will give the new
-	 * pointer time to age.
-	 */
-	if (aging_tail == INVALID_TAIL_PTR &&
-	    (aged_tail == INVALID_TAIL_PTR ||
-	     OA_TAKEN(hw_tail, aged_tail) >= report_size)) {
-		struct i915_vma *vma = stream->oa_buffer.vma;
-		u32 gtt_offset = i915_ggtt_offset(vma);
-
-		/* Be paranoid and do a bounds check on the pointer read back
-		 * from hardware, just in case some spurious hardware condition
-		 * could put the tail out of bounds...
+		/* Walk the stream backward until we find at least 2 reports
+		 * with dword 0 & 1 not at 0. Since the circular buffer
+		 * pointers progress by increments of 64 bytes and that
+		 * reports can be up to 256 bytes long, we can't tell whether
+		 * a report has fully landed in memory before the first 2
+		 * dwords of the following report have effectively landed.
+		 *
+		 * This is assuming that the writes of the OA unit land in
+		 * memory in the order they were written to.
+		 * If not : (╯°□°)╯︵ ┻━┻
 		 */
-		if (hw_tail >= gtt_offset &&
-		    hw_tail < (gtt_offset + OA_BUFFER_SIZE)) {
-			stream->oa_buffer.tails[!aged_idx].offset =
-				aging_tail = hw_tail;
-			stream->oa_buffer.aging_timestamp = now;
-		} else {
-			DRM_ERROR("Ignoring spurious out of range OA buffer tail pointer = %u\n",
-				  hw_tail);
+		landed_report_heads = 0;
+		while (OA_TAKEN(tail, head) >= report_size) {
+			u32 previous_tail = (tail - report_size) & (OA_BUFFER_SIZE - 1);
+			u8 *report = stream->oa_buffer.vaddr + previous_tail;
+			u32 *report32 = (void *) report;
+
+			/* Head of the report indicated by the HW tail register has
+			 * indeed landed into memory.
+			 */
+			if (report32[0] != 0 || report[1] != 0) {
+				landed_report_heads++;
+
+				if (landed_report_heads >= 2)
+					break;
+			}
+
+			tail = previous_tail;
+		}
+
+		if (abs(tail - hw_tail) >= (2 * report_size)) {
+			if (__ratelimit(&dev_priv->perf.tail_pointer_race)) {
+				DRM_NOTE("unlanded report(s) head=0x%x "
+					 "tail=0x%x hw_tail=0x%x\n",
+					 head, tail, hw_tail);
+			}
 		}
+
+		stream->oa_buffer.tail = gtt_offset + tail;
+		stream->oa_buffer.aging_tail = gtt_offset + hw_tail;
+		stream->oa_buffer.aging_timestamp = now;
 	}
 
 	spin_unlock_irqrestore(&stream->oa_buffer.ptr_lock, flags);
 
-	return aged_tail == INVALID_TAIL_PTR ?
-		false : OA_TAKEN(aged_tail, head) >= report_size;
+	return OA_TAKEN(stream->oa_buffer.tail - gtt_offset,
+			stream->oa_buffer.head - gtt_offset) >= report_size;
 }
 
 /**
@@ -662,7 +654,6 @@  static int gen8_append_oa_reports(struct i915_perf_stream *stream,
 	u32 mask = (OA_BUFFER_SIZE - 1);
 	size_t start_offset = *offset;
 	unsigned long flags;
-	unsigned int aged_tail_idx;
 	u32 head, tail;
 	u32 taken;
 	int ret = 0;
@@ -673,18 +664,10 @@  static int gen8_append_oa_reports(struct i915_perf_stream *stream,
 	spin_lock_irqsave(&stream->oa_buffer.ptr_lock, flags);
 
 	head = stream->oa_buffer.head;
-	aged_tail_idx = stream->oa_buffer.aged_tail_idx;
-	tail = stream->oa_buffer.tails[aged_tail_idx].offset;
+	tail = stream->oa_buffer.tail;
 
 	spin_unlock_irqrestore(&stream->oa_buffer.ptr_lock, flags);
 
-	/*
-	 * An invalid tail pointer here means we're still waiting for the poll
-	 * hrtimer callback to give us a pointer
-	 */
-	if (tail == INVALID_TAIL_PTR)
-		return -EAGAIN;
-
 	/*
 	 * NB: oa_buffer.head/tail include the gtt_offset which we don't want
 	 * while indexing relative to oa_buf_base.
@@ -812,13 +795,10 @@  static int gen8_append_oa_reports(struct i915_perf_stream *stream,
 		}
 
 		/*
-		 * The above reason field sanity check is based on
-		 * the assumption that the OA buffer is initially
-		 * zeroed and we reset the field after copying so the
-		 * check is still meaningful once old reports start
-		 * being overwritten.
+		 * Clear out the first 2 dword as a mean to detect unlanded
+		 * reports.
 		 */
-		report32[0] = 0;
+		report32[0] = report32[1] = 0;
 	}
 
 	if (start_offset != *offset) {
@@ -950,7 +930,6 @@  static int gen7_append_oa_reports(struct i915_perf_stream *stream,
 	u32 mask = (OA_BUFFER_SIZE - 1);
 	size_t start_offset = *offset;
 	unsigned long flags;
-	unsigned int aged_tail_idx;
 	u32 head, tail;
 	u32 taken;
 	int ret = 0;
@@ -961,17 +940,10 @@  static int gen7_append_oa_reports(struct i915_perf_stream *stream,
 	spin_lock_irqsave(&stream->oa_buffer.ptr_lock, flags);
 
 	head = stream->oa_buffer.head;
-	aged_tail_idx = stream->oa_buffer.aged_tail_idx;
-	tail = stream->oa_buffer.tails[aged_tail_idx].offset;
+	tail = stream->oa_buffer.tail;
 
 	spin_unlock_irqrestore(&stream->oa_buffer.ptr_lock, flags);
 
-	/* An invalid tail pointer here means we're still waiting for the poll
-	 * hrtimer callback to give us a pointer
-	 */
-	if (tail == INVALID_TAIL_PTR)
-		return -EAGAIN;
-
 	/* NB: oa_buffer.head/tail include the gtt_offset which we don't want
 	 * while indexing relative to oa_buf_base.
 	 */
@@ -1026,13 +998,10 @@  static int gen7_append_oa_reports(struct i915_perf_stream *stream,
 		if (ret)
 			break;
 
-		/* The above report-id field sanity check is based on
-		 * the assumption that the OA buffer is initially
-		 * zeroed and we reset the field after copying so the
-		 * check is still meaningful once old reports start
-		 * being overwritten.
+		/* Clear out the first 2 dwords as a mean to detect unlanded
+		 * reports.
 		 */
-		report32[0] = 0;
+		report32[0] = report32[1] = 0;
 	}
 
 	if (start_offset != *offset) {
@@ -1411,8 +1380,8 @@  static void gen7_init_oa_buffer(struct i915_perf_stream *stream)
 	I915_WRITE(GEN7_OASTATUS1, gtt_offset | OABUFFER_SIZE_16M); /* tail */
 
 	/* Mark that we need updated tail pointers to read from... */
-	stream->oa_buffer.tails[0].offset = INVALID_TAIL_PTR;
-	stream->oa_buffer.tails[1].offset = INVALID_TAIL_PTR;
+	stream->oa_buffer.aging_tail =
+		stream->oa_buffer.tail = gtt_offset;
 
 	spin_unlock_irqrestore(&stream->oa_buffer.ptr_lock, flags);
 
@@ -1468,8 +1437,8 @@  static void gen8_init_oa_buffer(struct i915_perf_stream *stream)
 	I915_WRITE(GEN8_OATAILPTR, gtt_offset & GEN8_OATAILPTR_MASK);
 
 	/* Mark that we need updated tail pointers to read from... */
-	stream->oa_buffer.tails[0].offset = INVALID_TAIL_PTR;
-	stream->oa_buffer.tails[1].offset = INVALID_TAIL_PTR;
+	stream->oa_buffer.aging_tail =
+		stream->oa_buffer.tail = gtt_offset;
 
 	/*
 	 * Reset state used to recognise context switches, affecting which
@@ -3672,6 +3641,11 @@  void i915_perf_init(struct drm_i915_private *dev_priv)
 		ratelimit_set_flags(&dev_priv->perf.spurious_report_rs,
 				    RATELIMIT_MSG_ON_RELEASE);
 
+		ratelimit_state_init(&dev_priv->perf.tail_pointer_race,
+				     5 * HZ, 10);
+		ratelimit_set_flags(&dev_priv->perf.tail_pointer_race,
+				    RATELIMIT_MSG_ON_RELEASE);
+
 		dev_priv->perf.initialized = true;
 	}
 }

Comments

On 14/09/2019 02:06, Umesh Nerlige Ramappa wrote:
> From: Lionel Landwerlin <lionel.g.landwerlin@intel.com>
>
> Right now the workaround against the OA tail pointer race condition
> requires at least twice the internal kernel polling timer to make any
> data available.
>
> This changes introduce checks on the OA data written into the circular
> buffer to make as much data as possible available on the first
> iteration of the polling timer.
>
> v2: Use OA_TAKEN macro without the gtt_offset (Lionel)
>
> Signed-off-by: Lionel Landwerlin <lionel.g.landwerlin@intel.com>
> ---
>   drivers/gpu/drm/i915/i915_drv.h  |  30 ++---
>   drivers/gpu/drm/i915/i915_perf.c | 200 ++++++++++++++-----------------
>   2 files changed, 103 insertions(+), 127 deletions(-)
>
> diff --git a/drivers/gpu/drm/i915/i915_drv.h b/drivers/gpu/drm/i915/i915_drv.h
> index bf600888b3f1..876aeaf3568e 100644
> --- a/drivers/gpu/drm/i915/i915_drv.h
> +++ b/drivers/gpu/drm/i915/i915_drv.h
> @@ -1180,21 +1180,11 @@ struct i915_perf_stream {
>   		spinlock_t ptr_lock;
>   
>   		/**
> -		 * One 'aging' tail pointer and one 'aged' tail pointer ready to
> -		 * used for reading.
> -		 *
> -		 * Initial values of 0xffffffff are invalid and imply that an
> -		 * update is required (and should be ignored by an attempted
> -		 * read)
> -		 */
> -		struct {
> -			u32 offset;
> -		} tails[2];
> -
> -		/**
> -		 * Index for the aged tail ready to read() data up to.
> +		 * The last HW tail reported by HW. The data
> +		 * might not have made it to memory yet
> +		 * though.
>   		 */
> -		unsigned int aged_tail_idx;
> +		u32 aging_tail;
>   
>   		/**
>   		 * A monotonic timestamp for when the current aging tail pointer
> @@ -1210,6 +1200,12 @@ struct i915_perf_stream {
>   		 * OA buffer data to userspace.
>   		 */
>   		u32 head;
> +
> +		/**
> +		 * The last verified tail that can be read
> +		 * by user space
> +		 */
> +		u32 tail;
>   	} oa_buffer;
>   };
>   
> @@ -1693,6 +1689,12 @@ struct drm_i915_private {
>   		 */
>   		struct ratelimit_state spurious_report_rs;
>   
> +		/**
> +		 * For rate limiting any notifications of tail pointer
> +		 * race.
> +		 */
> +		struct ratelimit_state tail_pointer_race;
> +
>   		struct i915_oa_config test_config;
>   
>   		u32 gen7_latched_oastatus1;
> diff --git a/drivers/gpu/drm/i915/i915_perf.c b/drivers/gpu/drm/i915/i915_perf.c
> index c1b764233761..50b6d154fd46 100644
> --- a/drivers/gpu/drm/i915/i915_perf.c
> +++ b/drivers/gpu/drm/i915/i915_perf.c
> @@ -237,23 +237,14 @@
>    * for this earlier, as part of the oa_buffer_check to avoid lots of redundant
>    * read() attempts.
>    *
> - * In effect we define a tail pointer for reading that lags the real tail
> - * pointer by at least %OA_TAIL_MARGIN_NSEC nanoseconds, which gives enough
> - * time for the corresponding reports to become visible to the CPU.
> - *
> - * To manage this we actually track two tail pointers:
> - *  1) An 'aging' tail with an associated timestamp that is tracked until we
> - *     can trust the corresponding data is visible to the CPU; at which point
> - *     it is considered 'aged'.
> - *  2) An 'aged' tail that can be used for read()ing.
> - *
> - * The two separate pointers let us decouple read()s from tail pointer aging.
> - *
> - * The tail pointers are checked and updated at a limited rate within a hrtimer
> - * callback (the same callback that is used for delivering EPOLLIN events)
> - *
> - * Initially the tails are marked invalid with %INVALID_TAIL_PTR which
> - * indicates that an updated tail pointer is needed.
> + * We workaround this issue in oa_buffer_check() by reading the reports in the
> + * OA buffer, starting from the tail reported by the HW until we find 2
> + * consecutive reports with their first 2 dwords of not at 0. Those dwords are
> + * also set to 0 once read and the whole buffer is cleared upon OA buffer
> + * initialization. The first dword is the reason for this report while the
> + * second is the timestamp, making the chances of having those 2 fields at 0
> + * fairly unlikely. A more detailed explanation is available in
> + * oa_buffer_check().
>    *
>    * Most of the implementation details for this workaround are in
>    * oa_buffer_check_unlocked() and _append_oa_reports()
> @@ -266,7 +257,6 @@
>    * enabled without any periodic sampling.
>    */
>   #define OA_TAIL_MARGIN_NSEC	100000ULL
> -#define INVALID_TAIL_PTR	0xffffffff
>   
>   /* frequency for checking whether the OA unit has written new reports to the
>    * circular OA buffer...
> @@ -457,10 +447,10 @@ static u32 gen7_oa_hw_tail_read(struct i915_perf_stream *stream)
>   static bool oa_buffer_check_unlocked(struct i915_perf_stream *stream)
>   {
>   	struct drm_i915_private *dev_priv = stream->dev_priv;
> +	u32 gtt_offset = i915_ggtt_offset(stream->oa_buffer.vma);
>   	int report_size = stream->oa_buffer.format_size;
>   	unsigned long flags;
> -	unsigned int aged_idx;
> -	u32 head, hw_tail, aged_tail, aging_tail;
> +	u32 hw_tail;
>   	u64 now;
>   
>   	/* We have to consider the (unlikely) possibility that read() errors
> @@ -469,16 +459,6 @@ static bool oa_buffer_check_unlocked(struct i915_perf_stream *stream)
>   	 */
>   	spin_lock_irqsave(&stream->oa_buffer.ptr_lock, flags);
>   
> -	/* NB: The head we observe here might effectively be a little out of
> -	 * date (between head and tails[aged_idx].offset if there is currently
> -	 * a read() in progress.
> -	 */
> -	head = stream->oa_buffer.head;
> -
> -	aged_idx = stream->oa_buffer.aged_tail_idx;
> -	aged_tail = stream->oa_buffer.tails[aged_idx].offset;
> -	aging_tail = stream->oa_buffer.tails[!aged_idx].offset;
> -
>   	hw_tail = dev_priv->perf.ops.oa_hw_tail_read(stream);
>   
>   	/* The tail pointer increases in 64 byte increments,
> @@ -488,63 +468,75 @@ static bool oa_buffer_check_unlocked(struct i915_perf_stream *stream)
>   
>   	now = ktime_get_mono_fast_ns();
>   
> -	/* Update the aged tail
> -	 *
> -	 * Flip the tail pointer available for read()s once the aging tail is
> -	 * old enough to trust that the corresponding data will be visible to
> -	 * the CPU...
> -	 *
> -	 * Do this before updating the aging pointer in case we may be able to
> -	 * immediately start aging a new pointer too (if new data has become
> -	 * available) without needing to wait for a later hrtimer callback.
> -	 */
> -	if (aging_tail != INVALID_TAIL_PTR &&
> -	    ((now - stream->oa_buffer.aging_timestamp) >
> -	     OA_TAIL_MARGIN_NSEC)) {
> -
> -		aged_idx ^= 1;
> -		stream->oa_buffer.aged_tail_idx = aged_idx;
> +	if (hw_tail == stream->oa_buffer.aging_tail) {
> +		/* If the HW tail hasn't move since the last check and the HW
> +		 * tail has been aging for long enough, declare it the new
> +		 * tail.
> +		 */
> +		if ((now - stream->oa_buffer.aging_timestamp) >
> +		    OA_TAIL_MARGIN_NSEC) {
> +			stream->oa_buffer.tail =
> +				stream->oa_buffer.aging_tail;
> +		}
> +	} else {
> +		u32 head, tail, landed_report_heads;
>   
> -		aged_tail = aging_tail;
> +		/* NB: The head we observe here might effectively be a little out of
> +		 * date (between head and tails[aged_idx].offset if there is currently
> +		 * a read() in progress.
> +		 */
> +		head = stream->oa_buffer.head - gtt_offset;
>   
> -		/* Mark that we need a new pointer to start aging... */
> -		stream->oa_buffer.tails[!aged_idx].offset = INVALID_TAIL_PTR;
> -		aging_tail = INVALID_TAIL_PTR;
> -	}
> +		hw_tail -= gtt_offset;
> +		tail = hw_tail;
>   
> -	/* Update the aging tail
> -	 *
> -	 * We throttle aging tail updates until we have a new tail that
> -	 * represents >= one report more data than is already available for
> -	 * reading. This ensures there will be enough data for a successful
> -	 * read once this new pointer has aged and ensures we will give the new
> -	 * pointer time to age.
> -	 */
> -	if (aging_tail == INVALID_TAIL_PTR &&
> -	    (aged_tail == INVALID_TAIL_PTR ||
> -	     OA_TAKEN(hw_tail, aged_tail) >= report_size)) {
> -		struct i915_vma *vma = stream->oa_buffer.vma;
> -		u32 gtt_offset = i915_ggtt_offset(vma);
> -
> -		/* Be paranoid and do a bounds check on the pointer read back
> -		 * from hardware, just in case some spurious hardware condition
> -		 * could put the tail out of bounds...
> +		/* Walk the stream backward until we find at least 2 reports
> +		 * with dword 0 & 1 not at 0. Since the circular buffer
> +		 * pointers progress by increments of 64 bytes and that
> +		 * reports can be up to 256 bytes long, we can't tell whether
> +		 * a report has fully landed in memory before the first 2
> +		 * dwords of the following report have effectively landed.
> +		 *
> +		 * This is assuming that the writes of the OA unit land in
> +		 * memory in the order they were written to.
> +		 * If not : (╯°□°)╯︵ ┻━┻
>   		 */
> -		if (hw_tail >= gtt_offset &&
> -		    hw_tail < (gtt_offset + OA_BUFFER_SIZE)) {
> -			stream->oa_buffer.tails[!aged_idx].offset =
> -				aging_tail = hw_tail;
> -			stream->oa_buffer.aging_timestamp = now;
> -		} else {
> -			DRM_ERROR("Ignoring spurious out of range OA buffer tail pointer = %u\n",
> -				  hw_tail);
> +		landed_report_heads = 0;
> +		while (OA_TAKEN(tail, head) >= report_size) {
> +			u32 previous_tail = (tail - report_size) & (OA_BUFFER_SIZE - 1);
> +			u8 *report = stream->oa_buffer.vaddr + previous_tail;
> +			u32 *report32 = (void *) report;
> +
> +			/* Head of the report indicated by the HW tail register has
> +			 * indeed landed into memory.
> +			 */
> +			if (report32[0] != 0 || report[1] != 0) {


Just noticed this is wrong, it should be : if (report32[0] != || 
report32[1] != 0) {

report is not a pointer to a dword.


-Lionel


> +				landed_report_heads++;
> +
> +				if (landed_report_heads >= 2)
> +					break;
> +			}
> +
> +			tail = previous_tail;
> +		}
> +
> +		if (abs(tail - hw_tail) >= (2 * report_size)) {
> +			if (__ratelimit(&dev_priv->perf.tail_pointer_race)) {
> +				DRM_NOTE("unlanded report(s) head=0x%x "
> +					 "tail=0x%x hw_tail=0x%x\n",
> +					 head, tail, hw_tail);
> +			}
>   		}
> +
> +		stream->oa_buffer.tail = gtt_offset + tail;
> +		stream->oa_buffer.aging_tail = gtt_offset + hw_tail;
> +		stream->oa_buffer.aging_timestamp = now;
>   	}
>   
>   	spin_unlock_irqrestore(&stream->oa_buffer.ptr_lock, flags);
>   
> -	return aged_tail == INVALID_TAIL_PTR ?
> -		false : OA_TAKEN(aged_tail, head) >= report_size;
> +	return OA_TAKEN(stream->oa_buffer.tail - gtt_offset,
> +			stream->oa_buffer.head - gtt_offset) >= report_size;
>   }
>   
>   /**
> @@ -662,7 +654,6 @@ static int gen8_append_oa_reports(struct i915_perf_stream *stream,
>   	u32 mask = (OA_BUFFER_SIZE - 1);
>   	size_t start_offset = *offset;
>   	unsigned long flags;
> -	unsigned int aged_tail_idx;
>   	u32 head, tail;
>   	u32 taken;
>   	int ret = 0;
> @@ -673,18 +664,10 @@ static int gen8_append_oa_reports(struct i915_perf_stream *stream,
>   	spin_lock_irqsave(&stream->oa_buffer.ptr_lock, flags);
>   
>   	head = stream->oa_buffer.head;
> -	aged_tail_idx = stream->oa_buffer.aged_tail_idx;
> -	tail = stream->oa_buffer.tails[aged_tail_idx].offset;
> +	tail = stream->oa_buffer.tail;
>   
>   	spin_unlock_irqrestore(&stream->oa_buffer.ptr_lock, flags);
>   
> -	/*
> -	 * An invalid tail pointer here means we're still waiting for the poll
> -	 * hrtimer callback to give us a pointer
> -	 */
> -	if (tail == INVALID_TAIL_PTR)
> -		return -EAGAIN;
> -
>   	/*
>   	 * NB: oa_buffer.head/tail include the gtt_offset which we don't want
>   	 * while indexing relative to oa_buf_base.
> @@ -812,13 +795,10 @@ static int gen8_append_oa_reports(struct i915_perf_stream *stream,
>   		}
>   
>   		/*
> -		 * The above reason field sanity check is based on
> -		 * the assumption that the OA buffer is initially
> -		 * zeroed and we reset the field after copying so the
> -		 * check is still meaningful once old reports start
> -		 * being overwritten.
> +		 * Clear out the first 2 dword as a mean to detect unlanded
> +		 * reports.
>   		 */
> -		report32[0] = 0;
> +		report32[0] = report32[1] = 0;
>   	}
>   
>   	if (start_offset != *offset) {
> @@ -950,7 +930,6 @@ static int gen7_append_oa_reports(struct i915_perf_stream *stream,
>   	u32 mask = (OA_BUFFER_SIZE - 1);
>   	size_t start_offset = *offset;
>   	unsigned long flags;
> -	unsigned int aged_tail_idx;
>   	u32 head, tail;
>   	u32 taken;
>   	int ret = 0;
> @@ -961,17 +940,10 @@ static int gen7_append_oa_reports(struct i915_perf_stream *stream,
>   	spin_lock_irqsave(&stream->oa_buffer.ptr_lock, flags);
>   
>   	head = stream->oa_buffer.head;
> -	aged_tail_idx = stream->oa_buffer.aged_tail_idx;
> -	tail = stream->oa_buffer.tails[aged_tail_idx].offset;
> +	tail = stream->oa_buffer.tail;
>   
>   	spin_unlock_irqrestore(&stream->oa_buffer.ptr_lock, flags);
>   
> -	/* An invalid tail pointer here means we're still waiting for the poll
> -	 * hrtimer callback to give us a pointer
> -	 */
> -	if (tail == INVALID_TAIL_PTR)
> -		return -EAGAIN;
> -
>   	/* NB: oa_buffer.head/tail include the gtt_offset which we don't want
>   	 * while indexing relative to oa_buf_base.
>   	 */
> @@ -1026,13 +998,10 @@ static int gen7_append_oa_reports(struct i915_perf_stream *stream,
>   		if (ret)
>   			break;
>   
> -		/* The above report-id field sanity check is based on
> -		 * the assumption that the OA buffer is initially
> -		 * zeroed and we reset the field after copying so the
> -		 * check is still meaningful once old reports start
> -		 * being overwritten.
> +		/* Clear out the first 2 dwords as a mean to detect unlanded
> +		 * reports.
>   		 */
> -		report32[0] = 0;
> +		report32[0] = report32[1] = 0;
>   	}
>   
>   	if (start_offset != *offset) {
> @@ -1411,8 +1380,8 @@ static void gen7_init_oa_buffer(struct i915_perf_stream *stream)
>   	I915_WRITE(GEN7_OASTATUS1, gtt_offset | OABUFFER_SIZE_16M); /* tail */
>   
>   	/* Mark that we need updated tail pointers to read from... */
> -	stream->oa_buffer.tails[0].offset = INVALID_TAIL_PTR;
> -	stream->oa_buffer.tails[1].offset = INVALID_TAIL_PTR;
> +	stream->oa_buffer.aging_tail =
> +		stream->oa_buffer.tail = gtt_offset;
>   
>   	spin_unlock_irqrestore(&stream->oa_buffer.ptr_lock, flags);
>   
> @@ -1468,8 +1437,8 @@ static void gen8_init_oa_buffer(struct i915_perf_stream *stream)
>   	I915_WRITE(GEN8_OATAILPTR, gtt_offset & GEN8_OATAILPTR_MASK);
>   
>   	/* Mark that we need updated tail pointers to read from... */
> -	stream->oa_buffer.tails[0].offset = INVALID_TAIL_PTR;
> -	stream->oa_buffer.tails[1].offset = INVALID_TAIL_PTR;
> +	stream->oa_buffer.aging_tail =
> +		stream->oa_buffer.tail = gtt_offset;
>   
>   	/*
>   	 * Reset state used to recognise context switches, affecting which
> @@ -3672,6 +3641,11 @@ void i915_perf_init(struct drm_i915_private *dev_priv)
>   		ratelimit_set_flags(&dev_priv->perf.spurious_report_rs,
>   				    RATELIMIT_MSG_ON_RELEASE);
>   
> +		ratelimit_state_init(&dev_priv->perf.tail_pointer_race,
> +				     5 * HZ, 10);
> +		ratelimit_set_flags(&dev_priv->perf.tail_pointer_race,
> +				    RATELIMIT_MSG_ON_RELEASE);
> +
>   		dev_priv->perf.initialized = true;
>   	}
>   }