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. Maxime
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