Re: [PATCH v8 10/12] drm/i915: add oa_event_min_timer_exponent sysctl

[sourab gupta <sourab.gupta@intel.com>]

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@sixbynine.org>
> Reviewed-by: Matthew Auld <matthew.auld@intel.com>
> ---
>  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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