Hi Maxime, On Tue, 2020-02-25 at 10:54 +0100, Maxime Ripard wrote: > Hi Stefan, > > On Mon, Feb 24, 2020 at 08:25:46PM +0100, Stefan Wahren wrote: > > Hi Maxime, > > > > Am 24.02.20 um 10:06 schrieb Maxime Ripard: > > > The driver has really only supported one clock so far and has hardcoded > > > the > > > ID used in communications with the firmware in all the functions > > > implementing the clock framework hooks. Let's store that in the clock data > > > structure so that we can support more clocks later on. > > > > thank you for this series. I looked through it but i couldn't find an > > explanation why we need to expose firmware clocks via DT instead of > > extending clk-bcm2835. The whole pllb / clk-raspberrypi stuff was an > > exception to get cpufreq working. I prefer to keep it an exception. > > Thanks for pointing this out, I indeed forgot to address it in my > cover letter or my commit log. > > I'm not quite sure what the situation was with the previous > RaspberryPi, but the RPi4 firmware does a bunch of things under the > hood to make sure that everything works as expected: > > - The HSM (and V3D) clocks will be reparented to multiple PLLs > depending on the rate being asked for. > - Still depending on the rate, the firmware will adjust the voltage > of the various PLLs. > - Depending on the temperature of the CPU and GPU, the firmware will > change the rate of clocks to throttle in case of the cores > overheating, with all the fallout that might happen to clocks > deriving from it. > - No matter what we choose to do in Linux, this will happen so > whether or not we want to do it, so doing it behind the firmware's > back (or the firmware doing it behind Linux's back) will only > result in troubles, with voltages too low, or the firmware trying > to access the same register at the same time than the Linux driver > would, etc. > > So all in all, it just seems much easier and safer to use the firmware > clocks. I agree with your assesment. Both DVFS and overheating/overvoltage protections will cause trouble, if not, make a Linux solution impossible while using the Foundation's firmware. Please note that, as Stefan says, it'd be nice to keep track of those arguments somewhere in the commit messages. Regards, Nicolas
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