Re: [PATCH 1/4] dt-bindings: mailbox: qcom,apcs: Add separate node for clock-controller

[Stephan Gerhold <stephan.gerhold@linaro.org>]

On Sun, May 11, 2025 at 05:48:11PM -0500, Bjorn Andersson wrote:
> On Tue, May 06, 2025 at 03:10:08PM +0200, Stephan Gerhold wrote:
> > APCS "global" is sort of a "miscellaneous" hardware block that combines
> > multiple registers inside the application processor subsystem. Two distinct
> > use cases are currently stuffed together in a single device tree node:
> > 
> >  - Mailbox: to communicate with other remoteprocs in the system.
> >  - Clock: for controlling the CPU frequency.
> > 
> > These two use cases have unavoidable circular dependencies: the mailbox is
> > needed as early as possible during boot to start controlling shared
> > resources like clocks and power domains, while the clock controller needs
> > one of these shared clocks as its parent. Currently, there is no way to
> > distinguish these two use cases for generic mechanisms like fw_devlink.
> > 
> > This is currently blocking conversion of the deprecated custom "qcom,ipc"
> > properties to the standard "mboxes", see e.g. commit d92e9ea2f0f9
> > ("arm64: dts: qcom: msm8939: revert use of APCS mbox for RPM"):
> >   1. remoteproc &rpm needs mboxes = <&apcs1_mbox 8>;
> >   2. The clock controller inside &apcs1_mbox needs
> >      clocks = <&rpmcc RPM_SMD_XO_CLK_SRC>.
> >   3. &rpmcc is a child of remoteproc &rpm
> > 
> > The mailbox itself does not need any clocks and should probe early to
> > unblock the rest of the boot process. The "clocks" are only needed for the
> > separate clock controller. In Linux, these are already two separate drivers
> > that can probe independently.
> > 
> 
> Why does this circular dependency need to be broken in the DeviceTree
> representation?
> 
> As you describe, the mailbox probes and register the mailbox controller
> and it registers the clock controller. The mailbox device isn't affected
> by the clock controller failing to find rpmcc...
> 

That's right, but the problem is that the probe() function of the
mailbox driver won't be called at all. The device tree *looks* like the
mailbox depends on the clock, so fw_devlink tries to defer probing until
the clock is probed (which won't ever happen, because the mailbox is
needed to make the clock available).

I'm not sure why fw_devlink doesn't detect this cycle and tries to probe
them anyway, but fact is that we need to split this up in order to avoid
warnings and have the supplies/consumers set up properly. Those device
links are created based on the device tree and not the drivers.

Thanks,
Stephan