On Thu, Mar 01, 2018 at 11:32:32AM +0000, Andre Przywara wrote:
> Hi,
> 
> On 01/03/18 10:32, Maxime Ripard wrote:
> > On Wed, Feb 28, 2018 at 11:19:11AM -0600, Samuel Holland wrote:
> >> Hi,
> >>
> >> On 02/28/18 02:32, Maxime Ripard wrote:
> >>> On Tue, Feb 27, 2018 at 08:27:14PM -0600, Samuel Holland wrote:
> >>>> +	/*
> >>>> +	 * The failure path should not disable the clock or assert the reset,
> >>>> +	 * because the PSCI implementation in firmware relies on this device
> >>>> +	 * being functional. Claiming the clock in this driver is required to
> >>>> +	 * prevent Linux from turning it off.
> >>>> +	 */
> >>>> +	ret = clk_prepare_enable(clk);
> >>>> +	if (ret) {
> >>>> +		dev_err(dev, "Failed to enable bus clock: %d\n", ret);
> >>>> +		return ret;
> >>>> +	}
> >>>
> >>> You don't need it to be always on though. You only need it to be
> >>> enabled when you access the registers (on both sides I guess?). So you
> >>> could very well enable the clock in your registers accessors in Linux,
> >>> and do the same in the ARISC firmware. That should work.
> >>
> >> It does need to always be on because the *PSCI* implementation (ATF) also uses
> >> SCPI concurrently with Linux (on a separate channel). Turning off the clock
> >> anywhere in Linux could turn it off in the middle of a PSCI SCPI call on a
> >> different CPU, causing ATF to hang in EL3 (which would be very bad).
> > 
> > Then the above code doesn't fix anything. You should mark the clock
> > critical, otherwise that clock will be turned off if the driver is
> > compiled as a module and not loaded (or loaded later), or if the
> > driver is not even compiled in.
> 
> ... or if the module gets unloaded for some reason. So yes, I agree.
> Actually I was hitting this problem before on several occasions:
> - If firmware (either ATF or on the ARISC) wants to check temperatures,
> it needs to rely on Linux not turning off the THS clock - which it does
> at the moment when there is no Linux driver.
> - If an EFI framebuffer needs to keep running in Linux, we should not
> turn off the HDMI and DE clocks. simplefb solves this, but efifb has no
> simple way of copying this approach.
> - If a type 1 hypervisor like Xen uses the serial console (and exports a
> virtual console to the guest), Linux turns off the now apparently unused
> UART0 gate clock, killing Xen's console. So booting Xen on Allwinner
> requires clk_ignore_unused at the moment.
> 
> There are ways to mitigate or workaround each one of these, but I was
> wondering if we should look at a more general approach.
> 
> The most flexible seems to have some "clock victim" DT node, somewhat
> mimicking the simplefb solution: One DT node which just references
> clocks that are used by other (firmware) components in the system and
> which can't be protected otherwise.
> Normally this node would be empty, but firmware can add clock references
> the moment it needs one clock. So ATF could add the THS clock, and Xen
> could add the UART0 gate clock. This could be done at runtime by the
> firmware or hypervisor. Xen for instance already amends the DT before
> passing it on to Dom0, so copying all the clock references from the UART
> to this node would be easy to implement.
> 
> Does that sound worthwhile to pursue? I could sketch up something if
> that makes sense.

This makes sense, but I remember how heated the simplefb discussion
has been to introduce the clocks and regulator properties, so I'm not
sure this would be nice to encourage you to do that :)

Maxime

-- 
Maxime Ripard, Bootlin (formerly Free Electrons)
Embedded Linux and Kernel engineering
https://bootlin.com