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Subject: Re: [PATCH v3 05/10] pwm: add PWM mode to pwm_config()
To:     Daniel Thompson <daniel.thompson@linaro.org>
CC:     Jani Nikula <jani.nikula@linux.intel.com>,
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References: <1519300881-8136-1-git-send-email-claudiu.beznea@microchip.com>
 <1519300881-8136-6-git-send-email-claudiu.beznea@microchip.com>
 <20180222123308.mypx2r7n6o63mj5z@oak.lan> <87po4s2hve.fsf@intel.com>
 <3a70b89c-b470-3723-760c-5294d0a75230@microchip.com>
 <20180227105444.lo4pee7vh4we3foq@oak.lan>
From:   Claudiu Beznea <Claudiu.Beznea@microchip.com>
Message-ID: <8e1d3b30-3543-56fd-7be6-7fe6edcb40d9@microchip.com>
Date:   Tue, 27 Feb 2018 13:40:58 +0200
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On 27.02.2018 12:54, Daniel Thompson wrote:
> On Mon, Feb 26, 2018 at 04:24:15PM +0200, Claudiu Beznea wrote:
>> On 26.02.2018 11:57, Jani Nikula wrote:
>>> On Thu, 22 Feb 2018, Daniel Thompson <daniel.thompson@linaro.org> wrote:
>>>> On Thu, Feb 22, 2018 at 02:01:16PM +0200, Claudiu Beznea wrote:
>>>>> Add PWM mode to pwm_config() function. The drivers which uses pwm_config()
>>>>> were adapted to this change.
>>>>>
>>>>> Signed-off-by: Claudiu Beznea <claudiu.beznea@microchip.com>
>>>>> ---
>>>>>  arch/arm/mach-s3c24xx/mach-rx1950.c  | 11 +++++++++--
>>>>>  drivers/bus/ts-nbus.c                |  2 +-
>>>>>  drivers/clk/clk-pwm.c                |  3 ++-
>>>>>  drivers/gpu/drm/i915/intel_panel.c   | 17 ++++++++++++++---
>>>>>  drivers/hwmon/pwm-fan.c              |  2 +-
>>>>>  drivers/input/misc/max77693-haptic.c |  2 +-
>>>>>  drivers/input/misc/max8997_haptic.c  |  6 +++++-
>>>>>  drivers/leds/leds-pwm.c              |  5 ++++-
>>>>>  drivers/media/rc/ir-rx51.c           |  5 ++++-
>>>>>  drivers/media/rc/pwm-ir-tx.c         |  5 ++++-
>>>>>  drivers/video/backlight/lm3630a_bl.c |  4 +++-
>>>>>  drivers/video/backlight/lp855x_bl.c  |  4 +++-
>>>>>  drivers/video/backlight/lp8788_bl.c  |  5 ++++-
>>>>>  drivers/video/backlight/pwm_bl.c     | 11 +++++++++--
>>>>>  drivers/video/fbdev/ssd1307fb.c      |  3 ++-
>>>>>  include/linux/pwm.h                  |  6 ++++--
>>>>>  16 files changed, 70 insertions(+), 21 deletions(-)
>>>>>
>>>>> diff --git a/drivers/video/backlight/lm3630a_bl.c b/drivers/video/backlight/lm3630a_bl.c
>>>>> index 2030a6b77a09..696fa25dafd2 100644
>>>>> --- a/drivers/video/backlight/lm3630a_bl.c
>>>>> +++ b/drivers/video/backlight/lm3630a_bl.c
>>>>> @@ -165,8 +165,10 @@ static void lm3630a_pwm_ctrl(struct lm3630a_chip *pchip, int br, int br_max)
>>>>>  {
>>>>>  	unsigned int period = pchip->pdata->pwm_period;
>>>>>  	unsigned int duty = br * period / br_max;
>>>>> +	struct pwm_caps caps = { };
>>>>>  
>>>>> -	pwm_config(pchip->pwmd, duty, period);
>>>>> +	pwm_get_caps(pchip->pwmd->chip, pchip->pwmd, &caps);
>>>>> +	pwm_config(pchip->pwmd, duty, period, BIT(ffs(caps.modes) - 1));
>>>>
>>>> Well... I admit I've only really looked at the patches that impact 
>>>> backlight but dispersing this really odd looking bit twiddling 
>>>> throughout the kernel doesn't strike me a great API design.
>>>>
>>>> IMHO callers should not be required to find the first set bit in
>>>> some specially crafted set of capability bits simply to get sane 
>>>> default behaviour.
>>>
>>> Agreed. IMHO the regular use case becomes rather tedious, ugly, and
>>> error prone.
>>
>> Using simply PWM_MODE(NORMAL) instead of BIT(ffs(caps.modes) - 1) would be OK
>> from your side?
>>
>> Or, what about using a function like pwm_mode_first() to get the first supported
>> mode by PWM channel?
>>
>> Or, would you prefer to solve this inside pwm_config() function, let's say, in
>> case an invalid mode is passed as argument, to let pwm_config() to choose the
>> first available PWM mode for PWM channel passed as argument?
> 
> What is it that actually needs solving?
> 
> If a driver requests normal mode and the PWM driver cannot support it
> why not just return an error an move on.
Because, simply, I wasn't aware of what these PWM client drivers needs for.

> 
> Put another way, what is the use case for secretly adopting a mode the
> caller didn't want? Under what circumstances is this a good thing?
No one... But I wasn't aware of what the PWM clients needs for from their PWM
controllers. At this moment having BIT(ffs(caps.modes)) instead of
PWM_MODE(NORMAL) is mostly the same since all the driver that has not explicitly
registered PWM caps will use PWM normal mode.

I will use PWM_MODE(NORMAL) instead of this in all the cases if this is OK from
your side.

Thank you,
Claudiu Beznea
> 
> 
> Daniel.
>