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

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On Tue, Nov 8, 2016 at 6:19 AM, sourab gupta <sourab.gupta@xxxxxxxxx> wrote:
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@xxxxxxxxxxxxx>
> ---
>  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.

It's in the struct ctl_table oa_table[] declaration of the "oa_max_sample_rate" paramater, assigned to .extra2 which is referenced by the proc_dointvec_minmax validation handler for the parameter.
 

> +
> +/* 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.

Yeah, this code will need adapting for gen9+. I guess we'll change it to work in terms of ((2<<exp) * NSEC_PER_SEC) / timestamp_frequency.
 

> +
> +                     /* 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 = ""> > +      .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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