[RFC PATCH 1/2] clk: add property for force to update clock setting

[Heiko Stübner]

Am Dienstag, 18. November 2014, 09:59:56 schrieb Doug Anderson:
> Hi,
> 
> On Mon, Nov 17, 2014 at 1:14 PM, Mike Turquette <mturquette at linaro.org> 
wrote:
> > Quoting Heiko Stübner (2014-11-14 10:06:47)
> > 
> >> Hi Mike,
> >> 
> >> Am Donnerstag, 13. November 2014, 17:41:02 schrieb Mike Turquette:
> >> > Quoting Doug Anderson (2014-11-13 15:27:32)
> >> 
> >> [...]
> >> 
> >> > All of the above is to say that perhaps the solution to this problem
> >> > belongs in the driver. In the end we're talking about details for
> >> > correctly programming hardware, which sounds an awful lot like what
> >> > drivers are supposed to do.
> >> > 
> >> > Let me know if the ->init() callback holds any promise for you. If not
> >> > we can figure something out.
> >> 
> >> From my theoretical musings, ->init() sounds like a nice idea - but of
> >> course it comes with a "but".
> >> 
> >> I guess the general idea would be to have the pll clk-type simply reset
> >> to the same rate but forcing it to use the parameters from its parameter
> >> table - when the rate params differ [0].
> >> 
> >> The only problem would be the apll supplying the cpu cores. After all
> >> clocks are registered, our armclk makes sure that the core clock gets
> >> reparented before changing the underlying apll [dpll is safe, as it is
> >> read-only currently].
> DPLL probably won't be read only forever...
> 
> >> At the moment the order would be
> >> clk_register(apll)
> >> apll->init()
> >> clk_register(armclk);
> > 
> > Sorry, but I don't understand the problem. The at registration-time,
> > apll is re-programmed to a correct value for its current rate. Then
> > armclk is registered which might change apll's rate. Any change to the
> > apll which is issued from armclk should insure that apll is programmed
> > correctly.
> 
> I think Heiko is worried that until the "armclk" is registered that
> nobody is there to reparent the ARM core to GPLL while APLL changes
> (that's armclk's job).  This is potentially unsafe.
> 
> NOTE: it actually might not be unsafe, just slow.  I think we'll
> actually swap the PLL into "slow" mode before changing it (24MHz) so
> we won't die we'll just run at 24MHz at boot time while we wait for
> APLL to re-lock.

that is correct, we switch the pll to slow-mode (which simply passes through 
the xin24m) before touching its params, which is also the behaviour described 
in the manual for this.


> One option would be to just add yet another per-pll parameter.  We'll
> only cleanup CPLL, GPLL, and NPLL.  If APLL (ARM clock) and DPLL
> (memory clock) are set differently by firmware then we're just SOL.
> Of course if firmware boots us on GPLL then I guess we're back to
> square one (would firmware really be that malicious?)

I was talking with Kever about the same thing today. My best idea would be to 
give our plls a flag property - like all the other clocks have (divider_flags, 
mux_flags, etc) - because who knows if later pll designs will need other 
smaller adjustments. Then adding a ROCKCHIP_PLL_FLAG_SYNC_RATE triggering the 
init function to check the rate params.


Heiko