Date:	Fri, 10 Apr 2015 06:09:12 -0700
From:	Guenter Roeck <linux@...ck-us.net>
To:	Anand Moon <linux.amoon@...il.com>,
	Sjoerd Simons <sjoerd.simons@...labora.co.uk>
CC:	Thierry Reding <thierry.reding@...il.com>,
	Lukasz Majewski <l.majewski@...sung.com>,
	Eduardo Valentin <edubezval@...il.com>,
	Russell King <linux@....linux.org.uk>,
	Kukjin Kim <kgene@...nel.org>, devicetree@...r.kernel.org,
	"linux-samsung-soc@...r.kernel.org" 
	<linux-samsung-soc@...r.kernel.org>, linux-pwm@...r.kernel.org,
	linux-kernel@...r.kernel.org, linux-arm-kernel@...ts.infradead.org,
	Markus Reichl <m.reichl@...etechno.de>
Subject: Re: [PATCH 6/6] hwmon: pwm-fan: Update the duty cycle inorder to
 control the pwm-fan

On 04/10/2015 05:59 AM, Anand Moon wrote:
> Hi Sjoerd,
>
> I don't much advance knowledge on internal signaling of pwm-samsung module.
>
> So do I need to send this patch again ?
>

 From the context, it seems that the fix in hwmon would only paint
over a problem in the actual pwm driver, correct ?

If you resubmit the patch I would expect you to explain this in the
commit log.

Guenter

> -Anand Moon
>
>
> On 10 April 2015 at 17:30, Sjoerd Simons <sjoerd.simons@...labora.co.uk> wrote:
>> Hey Anand,
>>
>> On Fri, 2015-04-10 at 16:58 +0530, Anand Moon wrote:
>>> Hi Guenter/Lukasz,
>>>
>>> Earlier I send v2 version of the patch spiking this one.
>>>
>>> Markus Riechl came back to me with below mail.
>>> So This patch confirms fixes the bug.
>>>
>>> I will send v3 version of the patch. Earlier I was in delima about the bug.
>>>
>>> -Anand Moon
>>> -------------------------------------------
>>> Hi Anand,
>>>
>>> I tested your patch.
>>>
>>> After booting the fan is spinning despite only 44°C.
>>>
>>> /sys/class/thermal/cooling_device0/curstate is 0.
>>> /sys/class/hwmon/hwmon4/pwm1 is 0
>>>
>>> when I echo 1 > cur_state and then echo 0 > cur_state again,
>>> the fan switches to off and behaves as expected.
>>>
>>> It looks like there is a bug in initializing the pwm output
>>> immediately after booting.
>>
>> The problem here will be that at boot the PWM runs at full duty. With
>> the current exynos PWM drive if you disable the PWM it will stop pulsing
>> but remain high if it was at 100% duty. My patch on which you depend
>> upon fixed a race  where disabling the pwm right after changing the duty
>> cycle (e.g. to 0%) also kept the signal high.
>>
>>  From looking at other PWM users at the time it seemed that most if not
>> all always first set to duty to 0% and then disable the pwm. Which
>> should work fine on exynos now. However iirc Thierry recently clarified
>> that the expected result of pwm_disable is not just that the modulation
>> stops but also that the output signal goes low, although that's not very
>> explicit in the current pwm documentation.. The exynos PWM driver will
>> need another fix tweak to make that true.
>>
>>
>>
>>> Best Regards,
>>
>>
>>
>>> --
>>> Markus Reichl
>>>
>>> On 8 April 2015 at 23:19, Anand Moon <linux.amoon@...il.com> wrote:
>>>> Hi Guenter,
>>>>
>>>> Sorry my blunder mistake. Sorry for the noise.
>>>>
>>>> I just tested with spiking this patch and my observation and testing
>>>> were wrong we can skip this patch.
>>>>
>>>> I will send an v2 patch series removing the patch 5 and patch 6.
>>>>
>>>> With correct dts changes.
>>>>
>>>> Thanks for pointing my mistake.
>>>>
>>>> -Anand Moon
>>>>
>>>> On 8 April 2015 at 22:23, Guenter Roeck <linux@...ck-us.net> wrote:
>>>>> On Wed, Apr 08, 2015 at 09:32:05PM +0530, Anand Moon wrote:
>>>>>> Hi Guenter,
>>>>>>
>>>>>> Initially the board bootup the cooling level state is 0.
>>>>>> So update the duty cycle and this power off the fan.
>>>>>> As their is no state change the fan will not spin.
>>>>>>
>>>>>> Once the temperature sensor is reached to alert temperature it changes state.
>>>>>> With the state change the fan cools the CPU and then stop's
>>>>>>
>>>>>> I have observed this state change with tmon utility in linux/tools/thermal/tmon/
>>>>>>
>>>>> Sorry, I am missing something. I still don't see what problem you are fixing
>>>>> with this patch. What behavior is wrong with the current code, and how does your
>>>>> patch fix it ?
>>>>>
>>>>> Guenter
>>>>>
>>>>>> -Anand Moon
>>>>>>
>>>>>> On 8 April 2015 at 21:02, Guenter Roeck <linux@...ck-us.net> wrote:
>>>>>>> On Wed, Apr 08, 2015 at 10:44:15AM +0200, Lukasz Majewski wrote:
>>>>>>>> Hi Anand,
>>>>>>>>
>>>>>>>>> Below changes depend on following patch.
>>>>>>>>> https://patchwork.kernel.org/patch/5944061/
>>>>>>>>>
>>>>>>>>> Update the pwm_config with duty then update the pwm_disable
>>>>>>>>> to poweroff the cpu fan.
>>>>>>>>>
>>>>>>>
>>>>>>> Unfortunately, the patch does not include an explanation why it is needed.
>>>>>>>
>>>>>>> The original code presumably did not update the duty cycle because
>>>>>>> pwm was about to be disabled anyway. That kind of made sense to me.
>>>>>>> Updating the duty cycle to 0 just to disable the pwm channel right
>>>>>>> afterwards does not immediately make sense.
>>>>>>>
>>>>>>> Given that, I would expect to see a rationale here. Why is this patch needed ?
>>>>>>> Does it fix a bug ? If yes, pelase describe the bug. If not, what is the
>>>>>>> purpose of this patch ?
>>>>>>>
>>>>>>> Maybe that is all explained in patch 0/6, which I was not copied on. Even
>>>>>>> if so, the reationale will be needed in the changelog to explain to future
>>>>>>> developers why this change was made.
>>>>>>>
>>>>>>> Thanks,
>>>>>>> Guenter
>>>>>>>
>>>>>>>>> Tested on OdroidXU3 board.
>>>>>>>>>
>>>>>>>>> Signed-off-by: Anand Moon <linux.amoon@...il.com>
>>>>>>>>> ---
>>>>>>>>>   drivers/hwmon/pwm-fan.c | 10 ++++------
>>>>>>>>>   1 file changed, 4 insertions(+), 6 deletions(-)
>>>>>>>>>
>>>>>>>>> diff --git a/drivers/hwmon/pwm-fan.c b/drivers/hwmon/pwm-fan.c
>>>>>>>>> index 7c83dc4..f25c841 100644
>>>>>>>>> --- a/drivers/hwmon/pwm-fan.c
>>>>>>>>> +++ b/drivers/hwmon/pwm-fan.c
>>>>>>>>> @@ -44,26 +44,24 @@ static int  __set_pwm(struct pwm_fan_ctx *ctx,
>>>>>>>>> unsigned long pwm) int ret = 0;
>>>>>>>>>
>>>>>>>>>      mutex_lock(&ctx->lock);
>>>>>>>>> +
>>>>>>>
>>>>>>> [ please refrain from unnecessary whitespace changes ]
>>>>>>>
>>>>>>>>>      if (ctx->pwm_value == pwm)
>>>>>>>>>              goto exit_set_pwm_err;
>>>>>>>>>
>>>>>>>>> -   if (pwm == 0) {
>>>>>>>>> -           pwm_disable(ctx->pwm);
>>>>>>>>> -           goto exit_set_pwm;
>>>>>>>>> -   }
>>>>>>>>> -
>>>>>>>>>      duty = DIV_ROUND_UP(pwm * (ctx->pwm->period - 1), MAX_PWM);
>>>>>>>>>      ret = pwm_config(ctx->pwm, duty, ctx->pwm->period);
>>>>>>>>>      if (ret)
>>>>>>>>>              goto exit_set_pwm_err;
>>>>>>>>>
>>>>>>>>> +   if (pwm == 0)
>>>>>>>>> +           pwm_disable(ctx->pwm);
>>>>>>>>> +
>>>>>>>>>      if (ctx->pwm_value == 0) {
>>>>>>>>>              ret = pwm_enable(ctx->pwm);
>>>>>>>>>              if (ret)
>>>>>>>>>                      goto exit_set_pwm_err;
>>>>>>>>>      }
>>>>>>>>>
>>>>>>>>> -exit_set_pwm:
>>>>>>>>>      ctx->pwm_value = pwm;
>>>>>>>>>   exit_set_pwm_err:
>>>>>>>>>      mutex_unlock(&ctx->lock);
>>>>>>>>
>>>>>>>> Reviewed-by: Lukasz Majewski <l.majewski@...sung.com>
>>>>>>>>
>>>>>>>> BTW: I've added Guenter to CC.
>>>>>>>>
>>>>>>>> --
>>>>>>>> Best regards,
>>>>>>>>
>>>>>>>> Lukasz Majewski
>>>>>>>>
>>>>>>>> Samsung R&D Institute Poland (SRPOL) | Linux Platform Group
>>
>>
>

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