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

[Matthew Auld]

On 7 November 2016 at 19:49, Robert Bragg <robert at sixbynine.org> 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 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",
This line is getting a little too long.

Reviewed-by: Matthew Auld <matthew.auld at intel.com>