[Intel-gfx] [PATCH] drm/i915: initialize ring frequency scaling table on SNB

[Jesse Barnes]

Chris Wilson <chris at chris-wilson.co.uk> wrote:

>On Tue, 21 Jun 2011 15:24:24 -0700, Jesse Barnes
><jbarnes at virtuousgeek.org> wrote:
>> The ring frequency scaling table tells the PCU to treat certain GPU
>> frequencies as if they were a given CPU frequency for purposes of
>> scaling the ring frequency.  Normally the PCU will scale the ring
>> frequency based on the CPU P-state, but with the table present, it
>will
>> also take the GPU frequency into account.  The scaling_factor used in
>> this patch may not be ideal, but is enough to increase performance in
>> nexuiz on a 1366x768 panel by about 20%.
>
>Am I right in thinking that the improvement offered by these new
>defaults
>is dependent upon the previous values programmed by the BIOS? On my
>desktop SNB, I only see a marginal improvement (<~1%) for games and
>similarly small differences for cairo-gl/xlib.
>
>Plus they also revealed my confusion over dev_priv->min_delay is
>actually
>min_freq on SNB. Evil. :-p
>-Chris
>
>-- 
>Chris Wilson, Intel Open Source Technology Centre

Yes, though on my machines the BIOS programs these values to 0, indicating no minimum ring freq.  The perf increase will be proportional to the bandwidth requirements not met by the existing frequency.  So for your tests, it's possible that the CPU is being kept busy enough to kee the ring freq up anyway (iow this patch is redundant for those loads) or that your workloads don't have high memory bw requirements to begin with, so the higher ring freq isn't a benefit.
-- 
Jesse Barnes, Intel Open Source Technology Center