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 at sixbynine.org>
---
drivers/gpu/drm/i915/i915_perf.c | 61 ++++++++++++++++++++++++++++++++--------
1 file changed, 50 insertions(+), 11 deletions(-)
diff --git a/drivers/gpu/drm/i915/i915_perf.c b/drivers/gpu/drm/i915/i915_perf.c
index e51c1d8..1a87fe9 100644
--- 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,
+ },
{}
};
--
2.10.1
