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

[Robert Bragg]

On Tue, Nov 8, 2016 at 6:19 AM, sourab gupta <sourab.gupta at intel.com> 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 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;
> 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 = &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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