Re: [PATCH v9 09/11] drm/i915: add dev.i915.oa_max_sample_rate sysctl

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

On Mon, 2016-11-07 at 11:49 -0800, Robert Bragg wrote:
> 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>
> ---
>  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;
There's no check for 'oa_sample_rate_hard_limit' anywhere below.

> +
> +/* 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);
I assume now that there'll be a platform specific check for 80ull, while
programming oa_period, for subquent Gen8+ platforms, which should be
fine.

> +
> +			/* 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,
> +	 },
>  	{}
>  };
>  


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