[linux-sunxi] sunxi-ng clocks: leaving certain clocks alone?

[Andre Przywara]

Hi Chen-Yu,

thanks for the answer!

On 12/12/16 04:41, Chen-Yu Tsai wrote:
> On Mon, Dec 12, 2016 at 7:54 AM, André Przywara <andre.przywara at arm.com> wrote:
>> 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 can't say that's absolutely true (wink).
> 
>>
>> 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.
> 
> The simplefb bindings takes clocks and regulators expressly for the
> purpose of keeping them enabled.

Right, I should have checked this before ...

>>
>> So my questions:
>> 1) Is this expected behaviour?
> 
> Yes.
> 
>> 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.
> 
> As it owns the whole CCU unit, why not? And how would it know if some
> clock is referenced or not, unless going through the whole device tree?

I was hoping that it just provides clocks to any users (drivers) and
wouldn't bother with tinkering with every clock unless explicitly being
asked for (by a driver being pointed to a specific clock through DT).
So it would need to iterate through anything - neither the whole DT nor
it's own list of clocks.

> Furthermore, nothing prevents another device driver from referencing
> said clock and turning it off when it's not in use. Think THS driver
> with runtime PM.

I am totally OK with that: Any potential Linux THS driver can take over,
if the DT references this device and the clock.
My point is that atm there is no such driver and so the clocks framework
shouldn't turn that clock off.

> Are you also mapping the THS to secure only? Otherwise nothing would
> prevent Linux from also claiming it.

Unfortunately the THS is always unsecure. And even if it could be
switched, after a recent IRC discussion I came to believe that those
secure peripherals features only works when the secure boot feature is
used, which requires to blow an efuse and thus is not easily doable on
most boards and also irreversible.
Also I am not sure whether this security feature actually extends to the
mod clocks and the bus reset and clock gates bits.

But I was relying on that Linux doesn't touch hardware that's not
referenced in the DT, so if firmware uses the THS, any Linux THS node
would need to go - or the other way around: if there is a Linux THS
node, firmware backs off.

>> 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?
> 
> If you want a clock to not be disabled by anyone, adding CLK_IS_CRITICAL
> to its flags is the proper option. This is done in the clk driver though.

Yes, I was thinking about that, but it indeed sounds like a hack to
follow this.

> If you just don't want the clk subsystem to disable unused clks at boot
> time, you can add "clk_ignore_unused" to the kernel boot parameters.
> I think this is more of a hack and debugging tool though.

Good point, but indeed looks like a debug feature.

> About dummy drivers, simplefb comes to mind again. But simplefb disables
> them when it gets kicked out by the drm driver.

Which I am fine with. If people are desperate about a THS driver, this
could take over, although I would expect a firmware driving the THS
would discourage this - for instance by removing a THS node.

> Maybe there are other examples.

OK, thanks for the pointer, I will look into this direction.

Cheers,
Andre.

>> 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.
>>
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