linux-kernel - Re: [PATCH] drivers: pwm: pwm-atmel: implement suspend/resume functions

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Message-ID: <2b061549-4710-eec2-fe91-f20ac92ee87a@microchip.com>
Date:   Tue, 11 Apr 2017 11:59:50 +0300
From:   m18063 <Claudiu.Beznea@...rochip.com>
To:     Boris Brezillon <boris.brezillon@...e-electrons.com>
CC:     Thierry Reding <thierry.reding@...il.com>,
        <linux-pwm@...r.kernel.org>, <linux-kernel@...r.kernel.org>,
        <linux-arm-kernel@...ts.infradead.org>,
        <alexandre.belloni@...e-electrons.com>,
        <nicolas.ferre@...rochip.com>
Subject: Re: [PATCH] drivers: pwm: pwm-atmel: implement suspend/resume
 functions



On 11.04.2017 11:50, Boris Brezillon wrote:
> On Tue, 11 Apr 2017 11:33:44 +0300
> m18063 <Claudiu.Beznea@...rochip.com> wrote:
> 
>> On 10.04.2017 19:27, Boris Brezillon wrote:
>>> On Mon, 10 Apr 2017 18:01:37 +0200
>>> Boris Brezillon <boris.brezillon@...e-electrons.com> wrote:
>>>   
>>>> On Mon, 10 Apr 2017 17:10:11 +0200
>>>> Thierry Reding <thierry.reding@...il.com> wrote:
>>>>  
>>>>> On Mon, Apr 10, 2017 at 04:35:58PM +0200, Boris Brezillon wrote:    
>>>>>> On Mon, 10 Apr 2017 17:20:20 +0300
>>>>>> Claudiu Beznea <claudiu.beznea@...rochip.com> wrote:
>>>>>>       
>>>>>>> Implement suspend and resume power management specific
>>>>>>> function to allow PWM controller to correctly suspend
>>>>>>> and resume.
>>>>>>>
>>>>>>> Signed-off-by: Claudiu Beznea <claudiu.beznea@...rochip.com>
>>>>>>> ---
>>>>>>>  drivers/pwm/pwm-atmel.c | 81 +++++++++++++++++++++++++++++++++++++++++++++++++
>>>>>>>  1 file changed, 81 insertions(+)
>>>>>>>
>>>>>>> diff --git a/drivers/pwm/pwm-atmel.c b/drivers/pwm/pwm-atmel.c
>>>>>>> index 530d7dc..75177c6 100644
>>>>>>> --- a/drivers/pwm/pwm-atmel.c
>>>>>>> +++ b/drivers/pwm/pwm-atmel.c
>>>>>>> @@ -58,6 +58,8 @@
>>>>>>>  #define PWM_MAX_PRD		0xFFFF
>>>>>>>  #define PRD_MAX_PRES		10
>>>>>>>  
>>>>>>> +#define PWM_MAX_CH_NUM		(4)
>>>>>>> +
>>>>>>>  struct atmel_pwm_registers {
>>>>>>>  	u8 period;
>>>>>>>  	u8 period_upd;
>>>>>>> @@ -65,11 +67,18 @@ struct atmel_pwm_registers {
>>>>>>>  	u8 duty_upd;
>>>>>>>  };
>>>>>>>  
>>>>>>> +struct atmel_pwm_pm_ctx {
>>>>>>> +	u32 cmr;
>>>>>>> +	u32 cdty;
>>>>>>> +	u32 cprd;
>>>>>>> +};
>>>>>>> +
>>>>>>>  struct atmel_pwm_chip {
>>>>>>>  	struct pwm_chip chip;
>>>>>>>  	struct clk *clk;
>>>>>>>  	void __iomem *base;
>>>>>>>  	const struct atmel_pwm_registers *regs;
>>>>>>> +	struct atmel_pwm_pm_ctx ctx[PWM_MAX_CH_NUM];      
>>>>>>
>>>>>> Hm, I'm pretty sure you can rely on the current PWM state and call
>>>>>> atmel_pwm_apply() at resume time instead of doing that. See what I did
>>>>>> here [1].
>>>>>>
>>>>>> Thierry, maybe it's time to start thinking about a generic solution to
>>>>>> save/restore PWM states.      
>>>>>
>>>>> Generally speaking I think applying the states are the right way to go.
>>>>> Ideally the PWM core could simply resume all of the PWM channels that a
>>>>> device exports and the ->apply() callback would be enough to restore
>>>>> that. I'm not sure if that's going to work with current implementations,
>>>>> though. Your pwm-atmel-hlcdc patch certainly indicates that we're not
>>>>> quite there yet.
>>>>>
>>>>> On the other hand, I'm beginning to think that maybe PWMs are too low-
>>>>> level for this kind of suspend/resume. For example if you use the PWM to
>>>>> control a backlight brightness, restoring it via the driver core's
>>>>> resume hook is potentially going to turn it back on at the wrong time. I
>>>>> have a feeling that we might be better off just pushing this up to the
>>>>> PWM users. A slight special case might be sysfs, for which no external
>>>>> user driver exists. But we already have separate data structures to keep
>>>>> track of sysfs-related context, so suspend/resume support could be added
>>>>> there.    
>>>>
>>>> Yep, you're probably right, we should let the PWM user take care of
>>>> re-applying the PWM state, because it's the only one having enough
>>>> knowledge about what the PWM is really driving to take a wise decision.  
>>>
>>> Note that we need drivers to implement both ->apply() and ->get_state()
>>> for this approach to work correctly, and we also need some help from
>>> the core to reset the PWM states at resume time, otherwise
>>> pwm_apply_state() will just compare the old state to the new one, see
>>> that they match and never call the ->apply() method.
>>>
>>> Another solution would be to remove the memcmp here [1] and
>>> unconditionally call ->apply().  
>> There are drivers which checks, in ->apply() hooks, the current PWM state
>> before applying the new state or take actions based on differences
>> b/w current and new PWM states. Removing memcmp without resetting
>> the PWM state would lead to wrong states in those drivers.
> 
> Indeed. So it just leaves the solution where we implement ->get_state().
> Honestly, it shouldn't be too hard to do that in the atmel driver.
I agree.
> 
> Note that for drivers that do not implement ->get_state(), the first
> pwm_apply_state() after the system has resumed should be harmless,
> because the current PWM should exactly match the one the PWM user is
> re-applying.
I agree.

>