[Intel-gfx] [RFC] drm/i915: downclock support

[ykzhao]

On Thu, 2009-08-27 at 23:24 +0800, Keith Packard wrote:
> On Thu, 2009-08-27 at 15:16 +0800, Fu Michael wrote:
> >   
> > I don't see why this change is needed.  Are there any bug using the old 
> > code?
> 
> No, not if you wanted only a single clock. However, downclocking can
> only modify the P parameter, not the M or N values. So, choosing a PLL
> programming that has the largest possible P means being able to
> downclock successfully.
According to the spec the mutiply of P1 and P2 will be P. And the value
of P2 is fixed. Then only P1 can be changed to change the value of P.

If we only changes P1, maybe the calculated VCO will be beyond the VCO
limit.

So IMO it is inappropriate to downclock by only changing the P value.

thanks.
> 
>