Am Mittwoch, den 15.03.2017, 14:05 +0000 schrieb Russell King - ARM
> 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.
Yes, it seems your understanding is correct and my mental model of this
FSCALE thing was off.

I'll pick up this patch for the next round of etnaviv features.


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