Re: [PATCH 2/3] drm/i915/guc/slpc: Add waitboost functionality for SLPC

["Dixit, Ashutosh" <ashutosh.dixit@intel.com>]

On Sun, 31 Oct 2021 21:39:36 -0700, Belgaumkar, Vinay wrote:
>
> @@ -945,6 +960,17 @@ void intel_rps_boost(struct i915_request *rq)
>	if (!test_and_set_bit(I915_FENCE_FLAG_BOOST, &rq->fence.flags)) {
>		struct intel_rps *rps = &READ_ONCE(rq->engine)->gt->rps;
>
> +		if (rps_uses_slpc(rps)) {
> +			slpc = rps_to_slpc(rps);
> +
> +			/* Return if old value is non zero */
> +			if (atomic_fetch_inc(&slpc->num_waiters))
> +				return;
> +
> +			if (intel_rps_get_requested_frequency(rps) < slpc->boost_freq)

I think this check is not needed because:

a. The waitboost code only changes min_freq. i915 code should not depend on
   how GuC changes requested_freq in response to change in min_freq.

b. What is more worrisome is that when we "de-boost" we set min_freq to
   min_freq_softlimit. If GuC e.g. has a delay in bringing requested_freq
   down and intel_rps_boost() gets called meanwhile we will miss the one
   opportunity we have to boost the freq (when num_waiters goes from 0 to
   1. Asking GuC to boost when actual_freq is already boost_freq is
   harmless in comparison). So to avoid this risk of missing the chance to
   boost I think we should delete this check and replace the code above
   with something like:

                if (rps_uses_slpc(rps)) {
                        struct intel_guc_slpc *slpc = rps_to_slpc(rps);

                        if (slpc->boost_freq <= slpc->min_freq_softlimit)
                                return;

                        if (!atomic_fetch_inc(&slpc->num_waiters))
                                schedule_work(&slpc->boost_work);

                        return;
                }

Note that this check:

                if (slpc->boost_freq <= slpc->min_freq_softlimit)
                                return;

(which is basically a degenerate case in which we don't have to do
anything), can be probably be implemented when boost_freq is set in sysfs,
or may already be encompassed in "val < slpc->min_freq" in
intel_guc_slpc_set_boost_freq() in which case this check can also be
skipped from this function.

> +void intel_guc_slpc_dec_waiters(struct intel_guc_slpc *slpc)
> +{
> +	/* Return min back to the softlimit.
> +	 * This is called during request retire,
> +	 * so we don't need to fail that if the
> +	 * set_param fails.
> +	 */

nit: maybe follow kernel multi-line comment format.