drm/i915/perf: rework aging tail workaround

We're about to introduce an options to open the perf stream, giving
the user ability to configure how often it wants the kernel to poll
the OA registers for available data.

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)
v3: (Umesh)
- Rebase
- Change report to report32 from below review
  https://patchwork.freedesktop.org/patch/330704/?series=66697&rev=1
v4: (Ashutosh, Lionel)
- Fix checkpatch errors
- Fix aging_timestamp initialization
- Check for only one valid landed report
- Fix check for unlanded report
v5: (Ashutosh)
- Fix bug in accurately determining landed report.
- Optimize the check for landed reports by going as far as the
  previously determined aged tail.
Signed-off-by: Lionel Landwerlin <lionel.g.landwerlin@intel.com>
Signed-off-by: Umesh Nerlige Ramappa <umesh.nerlige.ramappa@intel.com>
Reviewed-by: Ashutosh Dixit <ashutosh.dixit@intel.com>
Signed-off-by: Lionel Landwerlin <lionel.g.landwerlin@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20200324185457.14635-2-umesh.nerlige.ramappa@intel.com

drivers/gpu/drm/i915/i915_perf_types.h

@@ -273,21 +273,10 @@ struct i915_perf_stream {
 		spinlock_t ptr_lock;
 
 		/**
-		 * @tails: 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];
-
-		/**
-		 * @aged_tail_idx: Index for the aged tail ready to read() data up to.
+		 * @aging_tail: 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;
 
 		/**
 		 * @aging_timestamp: A monotonic timestamp for when the current aging tail pointer
@@ -303,6 +292,11 @@ struct i915_perf_stream {
 		 * OA buffer data to userspace.
 		 */
 		u32 head;
+
+		/**
+		 * @tail: The last verified tail that can be read by userspace.
+		 */
+		u32 tail;
 	} oa_buffer;
 
 	/**
@@ -420,6 +414,12 @@ struct i915_perf {
 	 */
 	struct ratelimit_state spurious_report_rs;
 
+	/**
+	 * For rate limiting any notifications of tail pointer
+	 * race.
+	 */
+	struct ratelimit_state tail_pointer_race;
+
 	u32 gen7_latched_oastatus1;
 	u32 ctx_oactxctrl_offset;
 	u32 ctx_flexeu0_offset;