Re: [Intel-gfx] [PATCH 16/19] drm/i915/perf: Apply Wa_18013179988

Umesh Nerlige Ramappa
2022-09-16T18:56:36Z

On Thu, Sep 15, 2022 at 10:16:30PM -0700, Dixit, Ashutosh wrote:
>On Tue, 23 Aug 2022 13:41:52 -0700, Umesh Nerlige Ramappa wrote:
>>
>
>Hi Umesh,
>
>> OA reports in the OA buffer contain an OA timestamp field that helps
>> user calculate delta between 2 OA reports. The calculation relies on the
>> CS timestamp frequency to convert the timestamp value to nanoseconds.
>> The CS timestamp frequency is a function of the CTC_SHIFT value in
>> RPM_CONFIG0.
>>
>> In DG2, OA unit assumes that the CTC_SHIFT is 3, instead of using the
>> actual value from RPM_CONFIG0. At the user level, this results in an
>> error in calculating delta between 2 OA reports since the OA timestamp
>> is not shifted in the same manner as CS timestamp.
>>
>> To resolve this, return actual OA timestamp frequency to the user in
>> i915_getparam_ioctl.
>
>Rather than exposing actual OA timestamp frequency to userspace (with the
>corresponding uapi change, specially if it's only DG2 and not all future
>products) questions about a couple of other options:
>
>Option 1. Can we set CTC_SHIFT in RPM_CONFIG0 to 3, so change GT freq to be the
> same as OA freq :-)
>
> The HSD seems to mention this:
> Is setting CTC SHIFT to 0b11 on driver init an acceptable W/A?
> Note: Changing the shift setting on live driver may break apps that are
> currently running (including desktop manager).
>
>Option 2. Is it possible to correct the timestamps in OA report headers to
> compensate for the difference between OA and GT frequencies (say when
> copying OA data to userspace)?
>
> Though not sure if this is preferable to having userspace do this.

It does affect other platforms too. There's no guarantee on what the
CTC_SHIFT value would be for different platforms, so user would have to
at least query that somehow (maybe from i915). It's simpler for user to
use the exported OA frequency since it is also backwards compatible.

https://patchwork.freedesktop.org/patch/498917/?series=107633&rev=3 is
consumed by GPUvis. That reminds me, I should include the UMD links for
the patches with uapi changes.

>
>A couple of minor optional nits on that patch below too.
>
>> +u32 i915_perf_oa_timestamp_frequency(struct drm_i915_private *i915)
>> +{
>> + /* Wa_18013179988:dg2 */
>> + if (IS_DG2(i915)) {
>> + intel_wakeref_t wakeref;
>> + u32 reg, shift;
>> +
>> + with_intel_runtime_pm(to_gt(i915)->uncore->rpm, wakeref)
>> + reg = intel_uncore_read(to_gt(i915)->uncore, RPM_CONFIG0);
>> +
>> + shift = (reg & GEN10_RPM_CONFIG0_CTC_SHIFT_PARAMETER_MASK) >>
>> + GEN10_RPM_CONFIG0_CTC_SHIFT_PARAMETER_SHIFT;
>
>This can be:
> shift = REG_FIELD_GET(GEN10_RPM_CONFIG0_CTC_SHIFT_PARAMETER_MASK, reg);

sure, will change

>
>> static u64 oa_exponent_to_ns(struct i915_perf *perf, int exponent)
>> {
>> - return intel_gt_clock_interval_to_ns(to_gt(perf->i915),
>> - 2ULL << exponent);
>> + u64 nom = (2ULL << exponent) * NSEC_PER_SEC;
>> + u32 den = i915_perf_oa_timestamp_frequency(perf->i915);
>> +
>> + return div_u64(nom + den - 1, den);
>
>div_u64_roundup?

true, but that is statically defined within intel_gt_clock_utils.c. I
didn't think there are enough users to export it outside.

Thanks,
Umesh

>
>Thanks.
>--
>Ashutosh