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

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Date:   Thu, 2 Feb 2023 11:29:15 +0100
From:   Matthias Brugger <matthias.bgg@...il.com>
To:     Roger Lu (陸瑞傑) <Roger.Lu@...iatek.com>,
        "eballetbo@...il.com" <eballetbo@...il.com>,
        "khilman@...nel.org" <khilman@...nel.org>,
        "sboyd@...nel.org" <sboyd@...nel.org>,
        "drinkcat@...gle.com" <drinkcat@...gle.com>
Cc:     "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "linux-mediatek@...ts.infradead.org" 
        <linux-mediatek@...ts.infradead.org>,
        "devicetree@...r.kernel.org" <devicetree@...r.kernel.org>,
        "linux-pm@...r.kernel.org" <linux-pm@...r.kernel.org>,
        Project_Global_Chrome_Upstream_Group 
        <Project_Global_Chrome_Upstream_Group@...iatek.com>,
        "linux-arm-kernel@...ts.infradead.org" 
        <linux-arm-kernel@...ts.infradead.org>,
        Jia-wei Chang (張佳偉) 
        <Jia-wei.Chang@...iatek.com>,
        Fan Chen (陳凡) <fan.chen@...iatek.com>
Subject: Re: [PATCH v4 05/14] soc: mediatek: mtk-svs: use svs clk control APIs

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@...iatek.com>
>>
>> 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] ...