Re: [PATCH v4 05/14] soc: mediatek: mtk-svs: use svs clk control APIs

[Matthias Brugger]

你好，

On 01/02/2023 13:28, Roger Lu (陸瑞傑) wrote:
> Hi Matthias Sir,
>
> On Tue, 2023-01-31 at 14:19 +0100, Matthias Brugger wrote:
> > On 11/01/2023 08:45, Roger Lu wrote:
> > > In MediaTek HW design, svs and thermal both use the same clk source.
> > > It means that svs clk reference count from CCF includes thermal control
> > > counts. That makes svs driver confuse whether it disabled svs's main clk
> > > or not from CCF's perspective and lead to turn off their shared clk
> > > unexpectedly. Therefore, we add svs clk control APIs to make sure svs's
> > > main clk is controlled well by svs driver itself.
> > >
> > > Here is a NG example. Rely on CCF's reference count and cause problem.
> > >
> > > thermal probe (clk ref = 1)
> > > -> svs probe (clk ref = 2)
> > >      -> svs suspend (clk ref = 1)
> > >         -> thermal suspend (clk ref = 0)
> > >         -> thermal resume (clk ref = 1)
> > >      -> svs resume (encounter error, clk ref = 1)
> > >      -> svs suspend (clk ref = 0)
> > >         -> thermal suspend (Fail here, thermal HW control w/o clk)
> > >
> > > Fixes: a825d72f74a3 ("soc: mediatek: fix missing clk_disable_unprepare() on
> > > err in svs_resume()")
> > > Signed-off-by: Roger Lu <roger.lu@xxxxxxxxxxxx>
> >
> > That looks wrong. Although I don't out of my mind, there should be a way to
> > tell
> > the clock framework that this clock is shared between several devices.
> >
> > I wonder if using clk_enable and clk_disable in svs_resume/suspend wouldn't
> > be
> > enough.
>
> Oh yes, Common Clock Framework (CCF) knows the clock shared between several
> devices and maintains clock "on/off" by reference count.

The thing is if you use clk_prepare_enable then the clock framework check's if 
the clock is already prepared, which could happen like you described in the 
svs_resume (encount error) case in the commit message. The question is, can't we 
just use clk_enable and clk_disable in resume/suspend and only prepare the clock 
in the probe function?

> We concern how to stop running svs_suspend() when svs clk is already disabled by
> svs_resume(). Take an example as below, if we refers to __clk_is_enabled()
> result for knowing svs clk status, it will return "true" all the time because
> thermal clk is still on. This causes the problem mentioned in commit message.

I would expect that the kernel takes care that we can't enter a resume path for 
a device before the suspend path has finished. Honestly I don't really 
understand the problem here. It seems something different then what you 
described in the commit message.

Please help me understand better.

谢谢，再见

Matthias

> static int svs_suspend(struct device *dev)
> {
> ...
> 	if (!__clk_is_enabled(svsp->main_clk)) //always get `true`
> 		return 0;
> ...
> }
>
> > Regards,
> > Matthias
>
> ... [snip] ...