Re: [Intel-gfx] [PATCH] drm/i915/slpc: Let's fix the PCODE min freq table setup for SLPC

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

On Wed, 31 Aug 2022 14:45:38 -0700, Rodrigo Vivi wrote:
>

Hi Rodrigo,

> We need to inform PCODE of a desired ring frequencies so PCODE update
> the memory frequencies to us. rps->min_freq and rps->max_freq are the
> frequencies used in that request. However they were unset when SLPC was
> enabled and PCODE never updated the memory freq.
>
> v2 (as Suggested by Ashutosh): if SLPC is in use, let's pick the right
>    frequencies from the get_ia_constants instead of the fake init of
>    rps' min and max.
>
> v3: don't forget the max <= min return
>
> v4: Move all the freq conversion to intel_rps.c. And the max <= min
>     check to where it belongs.
>
> v5: (Ashutosh) Fix old comment s/50 HZ/50 MHz and add a doc explaining
>     the "raw format"

I think we both agree that mostly the way this patch is written it is to
add SLPC but not risk disturbing host turbo, specially old platforms
(CHV/VLV/ILK and pre-Gen 6). Also these freq units (sometimes 16.67 MHz
units, sometimes 50 MHz, sometime MHz) in different places in the driver
and different product generations is hugely confusing to say the least. For
old platform we don't really know what units the freq's are in, we only
know intel_gpu_freq will magically convert freq's to MHz. In any case let's
work with what we have.

> @@ -130,6 +123,12 @@ static void gen6_update_ring_freq(struct intel_llc *llc)
>	if (!get_ia_constants(llc, &consts))
>		return;
>
> +	/*
> +	 * Although this is unlikely on any platform during initialization,
> +	 * let's ensure we don't get accidentally into infinite loop
> +	 */
> +	if (consts.max_gpu_freq <= consts.min_gpu_freq)
> +		return;

As I said I would remove reference to "infinite loop", I am not seeing any
infinite loop, maybe just delete the comment.

Also as I said I see the check above should be completely removed (so it is
actually a pre-existing bug in the code). However since you want to carry
it forward in order not to risk disturbing legacy behavior that's fine.

Rest LGTM:

Reviewed-by: Ashutosh Dixit <ashutosh.dixit@intel.com>