[RFC 6/6] drm/i915: add oa_event_min_timer_exponent sysctl

[Robert Bragg]

The minimal sampling period is now configurable via a
dev.i915.oa_event_min_timer_exponent sysctl parameter.

Following the precedent set by perf, the default is the minimum that
won't (on its own) exceed the default kernel.perf_event_max_sample_rate
default of 100000 samples/s.

Signed-off-by: Robert Bragg <robert at sixbynine.org>
---
 drivers/gpu/drm/i915/i915_perf.c | 37 ++++++++++++++++++++++++++++---------
 1 file changed, 28 insertions(+), 9 deletions(-)

diff --git a/drivers/gpu/drm/i915/i915_perf.c b/drivers/gpu/drm/i915/i915_perf.c
index ab82857..5ef7d92 100644
--- a/drivers/gpu/drm/i915/i915_perf.c
+++ b/drivers/gpu/drm/i915/i915_perf.c
@@ -42,6 +42,23 @@ static u32 i915_perf_event_paranoid = true;
 
 #define OA_EXPONENT_MAX 0x3f
 
+/* for sysctl proc_dointvec_minmax of i915_oa_event_min_timer_exponent */
+static int zero;
+static int oa_exponent_max = OA_EXPONENT_MAX;
+
+/* Theoretically we can program the OA unit to sample every 160ns but don't
+ * allow that by default unless root...
+ *
+ * The period is derived from the exponent as:
+ *
+ *   period = 80ns * 2^(exponent + 1)
+ *
+ * Referring to perf's kernel.perf_event_max_sample_rate for a precedent
+ * (100000 by default); with an OA exponent of 6 we get a period of 10.240
+ * microseconds - just under 100000Hz
+ */
+static u32 i915_oa_event_min_timer_exponent = 6;
+
 static struct i915_oa_format hsw_oa_formats[I915_OA_FORMAT_MAX] = {
 	[I915_OA_FORMAT_A13]	    = { 0, 64 },
 	[I915_OA_FORMAT_A29]	    = { 1, 128 },
@@ -674,15 +691,8 @@ static int i915_oa_event_init(struct i915_perf_event *event,
 		if (period_exponent > OA_EXPONENT_MAX)
 			return -EINVAL;
 
-		/* Theoretically we can program the OA unit to sample every
-		 * 160ns but don't allow that by default unless root...
-		 *
-		 * Referring to perf's kernel.perf_event_max_sample_rate for
-		 * a precedent (100000 by default); with an OA exponent of
-		 * 6 we get a period of 10.240 microseconds -just under
-		 * 100000Hz
-		 */
-		if (period_exponent < 6 && !capable(CAP_SYS_ADMIN)) {
+		if (period_exponent < i915_oa_event_min_timer_exponent &&
+		    !capable(CAP_SYS_ADMIN)) {
 			DRM_ERROR("Sampling period too high without root privileges\n");
 			return -EACCES;
 		}
@@ -1113,6 +1123,15 @@ static struct ctl_table oa_table[] = {
 	 .mode = 0644,
 	 .proc_handler = proc_dointvec,
 	 },
+	{
+	 .procname = "oa_event_min_timer_exponent",
+	 .data = &i915_oa_event_min_timer_exponent,
+	 .maxlen = sizeof(i915_oa_event_min_timer_exponent),
+	 .mode = 0644,
+	 .proc_handler = proc_dointvec_minmax,
+	 .extra1 = &zero,
+	 .extra2 = &oa_exponent_max,
+	 },
 	{}
 };
 
-- 
2.5.2