drm/i915: add dev.i915.oa_max_sample_rate sysctl

The maximum OA sampling frequency is now configurable via a
dev.i915.oa_max_sample_rate sysctl parameter.

Following the precedent set by perf's similar
kernel.perf_event_max_sample_rate the default maximum rate is 100000Hz

Signed-off-by: Robert Bragg <robert@sixbynine.org>
Reviewed-by: Matthew Auld <matthew.auld@intel.com>
Reviewed-by: Sourab Gupta <sourab.gupta@intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Link: http://patchwork.freedesktop.org/patch/msgid/20161107194957.3385-10-robert@sixbynine.org

diff --git a/drivers/gpu/drm/i915/i915_perf.c b/drivers/gpu/drm/i915/i915_perf.c
index e51c1d8..1a87fe9 100644 (file)
--- a/drivers/gpu/drm/i915/i915_perf.c
+++ b/drivers/gpu/drm/i915/i915_perf.c
@@ -82,6 +82,21 @@ static u32 i915_perf_stream_paranoid = true;
 #define INVALID_CTX_ID 0xffffffff
 
 
+/* For sysctl proc_dointvec_minmax of i915_oa_max_sample_rate
+ *
+ * 160ns is the smallest sampling period we can theoretically program the OA
+ * unit with on Haswell, corresponding to 6.25MHz.
+ */
+static int oa_sample_rate_hard_limit = 6250000;
+
+/* Theoretically we can program the OA unit to sample every 160ns but don't
+ * allow that by default unless root...
+ *
+ * The default threshold of 100000Hz is based on perf's similar
+ * kernel.perf_event_max_sample_rate sysctl parameter.
+ */
+static u32 i915_oa_max_sample_rate = 100000;
+
 /* XXX: beware if future OA HW adds new report formats that the current
  * code assumes all reports have a power-of-two size and ~(size - 1) can
  * be used as a mask to align the OA tail pointer.
@@ -1314,6 +1329,7 @@ static int read_properties_unlocked(struct drm_i915_private *dev_priv,
        }
 
        for (i = 0; i < n_props; i++) {
+               u64 oa_period, oa_freq_hz;
                u64 id, value;
                int ret;
 
@@ -1359,21 +1375,35 @@ static int read_properties_unlocked(struct drm_i915_private *dev_priv,
                                return -EINVAL;
                        }
 
-                       /* NB: The exponent represents a period as follows:
-                        *
-                        *   80ns * 2^(period_exponent + 1)
-                        *
-                        * Theoretically we can program the OA unit to sample
+                       /* 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
+                        * On Haswell the period is derived from the exponent
+                        * as:
+                        *
+                        *   period = 80ns * 2^(exponent + 1)
+                        */
+                       BUILD_BUG_ON(sizeof(oa_period) != 8);
+                       oa_period = 80ull * (2ull << value);
+
+                       /* This check is primarily to ensure that oa_period <=
+                        * UINT32_MAX (before passing to do_div which only
+                        * accepts a u32 denominator), but we can also skip
+                        * checking anything < 1Hz which implicitly can't be
+                        * limited via an integer oa_max_sample_rate.
                         */
-                       if (value < 6 && !capable(CAP_SYS_ADMIN)) {
-                               DRM_ERROR("Minimum OA sampling exponent is 6 without root privileges\n");
+                       if (oa_period <= NSEC_PER_SEC) {
+                               u64 tmp = NSEC_PER_SEC;
+                               do_div(tmp, oa_period);
+                               oa_freq_hz = tmp;
+                       } else
+                               oa_freq_hz = 0;
+
+                       if (oa_freq_hz > i915_oa_max_sample_rate &&
+                           !capable(CAP_SYS_ADMIN)) {
+                               DRM_ERROR("OA exponent would exceed the max sampling frequency (sysctl dev.i915.oa_max_sample_rate) %uHz without root privileges\n",
+                                         i915_oa_max_sample_rate);
                                return -EACCES;
                        }
 
@@ -1481,6 +1511,15 @@ static struct ctl_table oa_table[] = {
         .extra1 = &zero,
         .extra2 = &one,
         },
+       {
+        .procname = "oa_max_sample_rate",
+        .data = &i915_oa_max_sample_rate,
+        .maxlen = sizeof(i915_oa_max_sample_rate),
+        .mode = 0644,
+        .proc_handler = proc_dointvec_minmax,
+        .extra1 = &zero,
+        .extra2 = &oa_sample_rate_hard_limit,
+        },
        {}
 };