On Thu, 2016-10-27 at 19:14 -0700, Robert Bragg wrote:
> The minimal sampling period is now configurable via a
> dev.i915.oa_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@xxxxxxxxxxxxx>
> Reviewed-by: Matthew Auld <matthew.auld@xxxxxxxxx>
> ---
> drivers/gpu/drm/i915/i915_perf.c | 42 ++++++++++++++++++++++++++++------------
> 1 file changed, 30 insertions(+), 12 deletions(-)
>
> diff --git a/drivers/gpu/drm/i915/i915_perf.c b/drivers/gpu/drm/i915/i915_perf.c
> index 4e42073..e3c6f51 100644
> --- a/drivers/gpu/drm/i915/i915_perf.c
> +++ b/drivers/gpu/drm/i915/i915_perf.c
> @@ -82,6 +82,22 @@ static u32 i915_perf_stream_paranoid = true;
> #define INVALID_CTX_ID 0xffffffff
>
>
> +/* for sysctl proc_dointvec_minmax of i915_oa_min_timer_exponent */
> +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_min_timer_exponent = 6;
For HSW, the timestamp period is 80ns, so the exponent of 6 translates
to sampling rate of ~100000Hz. But the timestamp period may change for
other platforms, leading to different values of oa_min_timer_exponent
corresponding to sampling rate of ~100000Hz. Do we plan to have this
value platform specific subsequently, or the guidance value of ~100000Hz
min sampling rate needn't be strictly followed?
> +
> /* 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.
> @@ -1353,21 +1369,14 @@ 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
> */
> - if (value < 6 && !capable(CAP_SYS_ADMIN)) {
> - DRM_ERROR("Minimum OA sampling exponent is 6 without root privileges\n");
> + if (value < i915_oa_min_timer_exponent &&
> + !capable(CAP_SYS_ADMIN)) {
> + DRM_ERROR("Minimum OA sampling exponent (sysctl dev.i915.oa_min_timer_exponent) is %u without root privileges\n",
> + i915_oa_min_timer_exponent);
> return -EACCES;
> }
>
> @@ -1475,6 +1484,15 @@ static struct ctl_table oa_table[] = {
> .extra1 = &zero,
> .extra2 = &one,
> },
> + {
> + .procname = "oa_min_timer_exponent",
> + .data = &i915_oa_min_timer_exponent,
> + .maxlen = sizeof(i915_oa_min_timer_exponent),
> + .mode = 0644,
> + .proc_handler = proc_dointvec_minmax,
> + .extra1 = &zero,
> + .extra2 = &oa_exponent_max,
> + },
> {}
> };
>
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