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

[Robert Bragg]

On Wed, Nov 2, 2016 at 6:29 AM, sourab gupta <sourab.gupta at intel.com> wrote:

> 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 at sixbynine.org>
> > Reviewed-by: Matthew Auld <matthew.auld at 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?
>

actually it's bothered me a bit that I've been lazy with not having this
adapt for gen9+ in later patches

I think it would probably be better to make this a Hz based threshold for
userspace, otherwise any userspace policy here needs to be adapted for each
system with a different timestamp frequency which isn't great.

I've updated the patch locally to make this an oa_max_sample_rate parameter
in Hz, which I'll aim to test on haswell tomorrow and send out.

Thanks,
- Robert
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