[Intel-gfx] [PATCH 14/18] drm/i915: Don't enable IPS when pixel rate exceeds 95% of cdclk

[Runyan, Arthur J]

>> +       /*
>> +        * FIXME if we compare against max we should then
>> +        * increase the cdclk frequency when the current
>> +        * value is too low. The other option is to compare
>> +        * against the cdclk frqeuncy we're going have post
>> +        * modeset (ie. one we computed using other constraints).
>> +        * Need to measure whether using a lower cdclk w/o IPS
>> +        * is better or worse than a higher cdclk w/ IPS.
>> +        */
>> +       return ilk_pipe_pixel_rate(pipe_config) <=
>> +               dev_priv->max_cdclk_freq * 95 / 100;
>> +}
>
>Adding Art for insight into this question (lower cdclock or ips) or
>whether this just isn't worth the complexity.
>-Daniel

IPS enabled probably saves more power than lower cdclk, but it will be dependent on resolution, content, and other features, like FBC and PSR.  I don't have any real measurements.