[Intel-gfx] [PATCH v2] drm/i915: set minimum CD clock to twice the BCLK.

[Ville Syrjälä]

On Tue, Apr 10, 2018 at 08:00:10PM -0700, Kumar, Abhay wrote:
> 
> 
> On 4/10/2018 1:49 AM, Nikula, Jani wrote:
> > On Tue, 10 Apr 2018, Jani Nikula <jani.nikula at intel.com> wrote:
> >> On Mon, 09 Apr 2018, "Kumar, Abhay" <abhay.kumar at intel.com> wrote:
> >>> Dynamic cdclk is disabled in BIOS/GOP hence gop makes it to highest
> >>> clock i.e 316.8. Will attach logs with drm debug enabled in bug.
> >>> I am also inclined towards 192 which will make max cdclk..
> >> Please also attach /sys/kernel/debug/dri/0/i915_vbt to the bug.
> >>
> >> It doesn't look like we look at the VBT dynamic cdclk field. Does
> >> dynamic debug disabled mean we should go for max?
> currently in kernel we don't honor this field. GOP does and by disabling 
> it cdclk will run at max speed.

I don't particularly like the idea of using that vbt field. I'm not aware
of any cases where changing cdclk would be totally busted in the hw, so
I don't really see why this thing should even exist in the vbt, apart
from some questionable marketing purposes. I think there's plenty of
other stuff like that in the vbt which we happily ignore as well.
Depending on how common it is to have it set in the vbt it could also
drastically reduce our testing coverage of the cdclk code.

-- 
Ville Syrjälä
Intel OTC