Re: [PATCH v8 1/6] pwm: extend PWM framework with PWM modes

[Claudiu.Beznea]

Hi Uwe,

On 08.01.2019 00:10, Uwe Kleine-KÃnig wrote:
> Hello Claudiu,
> 
> On Mon, Jan 07, 2019 at 09:30:55AM +0000, Claudiu.Beznea@xxxxxxxxxxxxx wrote:
>> On 05.01.2019 23:05, Uwe Kleine-KÃnig wrote:
>>> On Thu, Jan 03, 2019 at 01:29:44PM +0000, Claudiu.Beznea@xxxxxxxxxxxxx wrote:
>>>> From: Claudiu Beznea <claudiu.beznea@xxxxxxxxxxxxx>
>>>>
>>>> Add basic PWM modes: normal and complementary. These modes should
>>>> differentiate the single output PWM channels from two outputs PWM
>>>> channels. These modes could be set as follow:
>>>> 1. PWM channels with one output per channel:
>>>> - normal mode
>>>> 2. PWM channels with two outputs per channel:
>>>> - normal mode
>>>> - complementary mode
>>>> Since users could use a PWM channel with two output as one output PWM
>>>> channel, the PWM normal mode is allowed to be set for PWM channels with
>>>> two outputs; in fact PWM normal mode should be supported by all PWMs.
>>>
>>> I still think that my suggestion that I sent in reply to your v5 using
>>> .alt_duty_cycle and .alt_offset is the better one as it is more generic.
>>
>> I like it better my way, I explained myself why.
> 
> I couldn't really follow your argument though. You seemed to acknowledge
> that using .alt_duty_cycle and .alt_offset is more generic.

True it is more generic in the way that it gives the possibility to
configure all kind of waveforms. But not all controllers supports this.
The use case of this would be to have dead-times with any values, right?

> Then you
> wrote that the push-pull mode is hardware generated on Atmel with some
> implementation details.

Yes, I wanted to say that Atmel PWM generates only a set of waveforms.

> IMHO these implementation details shouldn't be
> part of the PWM API and atmel's .apply should look as follows:
> 
> 	if (state->alt_duty_cycle == 0) {
> 
> 		... configure for normal mode ...
> 
> 	} else if (state->duty_cycle == state->alt_duty_cycle &&
> 	           state->alt_offset == state->period / 2) {
> 
> 		... configure for push pull mode ...
> 
> 	} else if (state->duty_cycle + state->alt_duty_cycle == state->period &&
> 		   state->alt_offset == state->duty_cycle) {
> 
> 		... configure for complementary mode ...
> 
> 	} else {
> 		return -EINVAL;
> 	}

If so, the user should have been taken care of the relations b/w these
values in order to configure a proper working mode.

It would be good to have Thierry's inputs on these.

> 
> If it turns out to be a common pattern, we can add helper functions à la
> pwm_is_complementary_mode(state) and
> pwm_set_complementary_mode(state, period, duty_cycle). This allows to
> have a generic way to describe a wide range of wave forms in a uniform
> way in the API (which is good) and each driver implements the parts of
> this range that it can support.
> 
>>> I don't repeat what I wrote there assuming you still remember or are
>>> willing to look it up at
>>> e.g. https://www.spinics.net/lists/linux-pwm/msg08174.html (in the 2nd half
>>> of my mail).
>>
>> Yes, I remember it.
> 
> I expected that, my words were more directed to Thierry than you.
>  
>>> Also I think that if the capabilities function is the way forward adding
>>> support to detect availability of polarity inversion should be
>>> considered. 
>>
>> Yep, why not. But it should be done in a different patch. It is not related
>> to this series.
> 
> Yes, given that polarity already exists, this would be a good
> opportunity to introduce the capability function for that and only
> afterwards add the new use case with modes. (But having said this, read
> further as I think that this capability function is a bad idea.)
> 
>>> This would also be an opportunity to split the introduction
>>> of the capabilities function and the introduction of complementary mode.
>>> (But my personal preference would be to just let .apply fail when an
>>> unsupported configuration is requested.)
>>
>> .apply fails when something wrong is requested.
> 
> If my controller doesn't support a second output is it "wrong" to
> request complementary mode? I'd say yes. So you have to catch that in
> .apply anyhow and there is little benefit to be able to ask the
> controller if it supports it beforehand.

This is checked by pwm_supports_mode() called in pwm_apply_state(). If
controller doesn't support the mode, the apply will fail.

> 
> I don't have a provable statistic at hand, but my feeling is that quite
> some users of the i2c frame work get it wrong to first check the
> capabilities and only then try to use them. This is at least error prone
> and harder to use than the apply function returning an error code.
> And on the driver side the upside is to have all stuff related to which
> wave form can be generated and which cannot is a single place. (Just
> consider "inverted complementary mode". Theoretically this should work
> if your controller supports complementary mode and inverted mode. If you
> now have a driver for a controller that can do both, but not at the same
> time, the separation gets ugly. OK, this is a constructed example, but
> in my experience something like that happens earlier or later.)
> 
>>>> [...]
>>>> @@ -53,12 +75,14 @@ enum {
>>>>   * @period: PWM period (in nanoseconds)
>>>>   * @duty_cycle: PWM duty cycle (in nanoseconds)
>>>>   * @polarity: PWM polarity
>>>> + * @modebit: PWM mode bit
>>>>   * @enabled: PWM enabled status
>>>>   */
>>>>  struct pwm_state {
>>>>  	unsigned int period;
>>>>  	unsigned int duty_cycle;
>>>>  	enum pwm_polarity polarity;
>>>> +	unsigned long modebit;
>>>
>>> I fail to see the upside of storing the mode as 2^mode instead of a
>>> plain enum pwm_mode. Given that struct pwm_state is visible for pwm
>>> users a plain pwm_mode would at least be more intuitive.
>>
>> To have all modes supported by a controller grouped in pwm_caps::modes_msk.
> 
> My question was not about struct pwm_caps::modes_msk but about
> struct pwm_state::modebit. As struct pwm_state has visibility even
> outside of the pwm API (i.e. it is used by consumers) it is beneficial
> to keep that simple. Letting a consumer pass in the mode he wants is
> easier to explain than setting a single bit. Also error checking with a
> plain enum is easier because you just do:
> 
> 	if (mode >= MODE_CNT)
> 		error()
> 
> which is easy to grasp. Compare that to
> 
> 	if (!is_power_of_two(modebit) || modebit >= PWM_MODE_BIT(CNT))
> 		error()
> 
> (modulo syntactical correctness).

The reason I choose to have it as bit was the memcmp() at the beginning of
pwm_apply_state() and to avoid starting enum pwm_mode from 1 and to avoid
having bit 0 of pwm_caps::modes_msk unused (in the driver I'm using
PWM_MODE_BIT() macro to fill in the driver's supported modes).

Thank you,
Claudiu Beznea

> 
> Best regards
> Uwe
>