sunxi-ng clocks: leaving certain clocks alone?

[andre.przywara@arm.com (André Przywara)]

Hi,

I was observing that the new sunxi-ng clock code apparently explicitly
turns off _all_ clocks that are not used or needed. I find this rather
unfortunate, as I wanted to use the THS temperature sensor from ARM
Trusted Firmware to implement emergency shutdown or DVFS throttling.
That works until the clock framework finds the clock (as enumerated in
ccu-sun50i-a64.c) and obviously explicitly clears bit 31 in the THS mod
clock register and bit 8 in the respective clock gate register.
Turning them manually back on via /dev/mem or removing the THS clock
from the sunxi-ng driver fixes this for me.

This was not happening with the old Allwinner clocks, since the kernel
wouldn't even know about it if there was no driver and the clock wasn't
mentioned in the DT.

Now with sunxi-ng even though the THS clock is not actually referenced
or used in the DT, the kernel turns it off. I would expect that upon
entering the kernel all unneeded clocks are turned off anyway, so there
is no need to mess with clocks that have no user, but are enumerated in
the ccu driver.

I wonder if this kills simplefb as well, for instance, since I believe
that U-Boot needs to turn on certain clocks and relies on them staying up.

So my questions:
1) Is this expected behaviour?
2) If yes, should it really be?
3) If yes, shouldn't there be way to explicitly tell Linux to leave that
clock alone, preferably via DT? Although the sunxi-ng clocks take
control over the whole CCU unit, I wonder if it should really mess with
clocks there are not referenced in the DT.

Maybe there is some way to reference those clocks via some dummy driver
or DT node to avoid this behaviour? Is there any prior art in this respect?

I think this issue will affect more future users (thinking of EFI RTC,
EFI graphics, etc.), so I wanted to start a discussion on this.

Any input welcome.

Cheers,
Andre.

P.S. For reference my use case: ARM Trusted Firmware (ATF) enables the
temperature sensor (THS) and programs a shutdown value. It programs the
respective interrupt as secure and registers an IRQ handler in ATF to
shutdown the system or take other appropriate matters to avoid
overheating. Additionally the sensor is exported via the generic SCPI
interface, so the existing scpi-hwmon driver picks it up and reports it
to whoever is interested in Linux land via the normal interfaces.