[PATCH] drm/etnaviv: add etnaviv cooling device

[linux@armlinux.org.uk (Russell King - ARM Linux)]

On Wed, Mar 15, 2017 at 02:03:09PM +0100, Lucas Stach wrote:
> Am Sonntag, den 12.03.2017, 19:00 +0000 schrieb Russell King:
> > Each Vivante GPU contains a clock divider which can divide the GPU clock
> > by 2^n, which can lower the power dissipation from the GPU.  It has been
> > suggested that the GC600 on Dove is responsible for 20-30% of the power
> > dissipation from the SoC, so lowering the GPU clock rate provides a way
> > to throttle the power dissiptation, and reduce the temperature when the
> > SoC gets hot.
> > 
> > This patch hooks the Etnaviv driver into the kernel's thermal management
> > to allow the GPUs to be throttled when necessary, allowing a reduction in
> > GPU clock rate from /1 to /64 in power of 2 steps.
> 
> Are those power of 2 steps a hardware limitation, or is it something you
> implemented this way to get a smaller number of steps, with a more
> meaningful difference in clock speed?
> My understanding was that the FSCALE value is just a regular divider
> with all steps values in the range of 1-64 being usable.

I don't share your understanding.  The Vivante GAL kernel driver only
ever sets power-of-two values.  I have no evidence to support your
suggestion.

There's evidence that says your understanding is incorrect however.
It isn't a divider.  A value of 0x40 gives the fastest clock rate,
a value of 0x01 gives the slowest.  If it were a binary divider,
a value of 0x7f would give the slowest rate - so why doesn't Vivante
use that in galcore when putting the GPU into idle/lower power - why
do they just 0x01.

This all leads me to believe that it's not a binary divider, but a
set of bits that select the clock from a set of divide-by-two stages,
and having more than one bit set is invalid.

However, without definitive information from Vivante, we'll never
really know.  We're unlikely to get that.

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