Re: [v9 2/2] pwm: Add Aspeed ast2600 PWM support

[Uwe Kleine-König]

On Wed, Jul 21, 2021 at 10:52:21AM +0000, Billy Tsai wrote:
> Hi Uwe,
> 
>     On 2021/7/16, 6:13 PM, "Uwe Kleine-König" <u.kleine-koenig@xxxxxxxxxxxxxx> wrote:
> 
>         On Fri, Jul 16, 2021 at 09:22:22AM +0000, Billy Tsai wrote:
>         >> On 2021/7/16, 3:10 PM, "Uwe Kleine-König" <u.kleine-koenig@xxxxxxxxxxxxxx> wrote:
>         >> 
>         >>     On Fri, Jul 16, 2021 at 01:48:20AM +0000, Billy Tsai wrote:
>         >>     >> On 2021/7/15, 11:06 PM, "Uwe Kleine-König" <u.kleine-koenig@xxxxxxxxxxxxxx>> wrote:
>         >>     >>     > Another is: The PWM doesn't support duty_cycle 0, on such a request the
>         >>     >>     > PWM is disabled which results in a constant inactive level.
>         >>     >> 
>         >>     >>     > (This is correct, is it? Or does it yield a constant 0 level?)
>         >>     >> 
>         >>     >> Our pwm can support duty_cycle 0 by unset CLK_ENABLE.
>         >> 
>         >>     > This has a slightly different semantic though. Some consumer might
>         >>     > expect that the following sequence:
>         >> 
>         >>     >	pwm_apply(mypwm, { .period = 10000, .duty_cycle = 10000, .enabled = true })
>         >>     >	pwm_apply(mypwm, { .period = 10000, .duty_cycle = 0, .enabled = true })
>         >>     >	pwm_apply(mypwm, { .period = 10000, .duty_cycle = 10000, .enabled = true })
>         >> 
>         >>     > results in the output being low for an integer multiple of 10 µs. This
>         >>     > isn't given with setting CLK_ENABLE to zero, is it? (I didn't recheck,
>         >>     > if the PWM doesn't complete periods on reconfiguration this doesn't
>         >>     > matter much though.)
>         >> Thanks for the explanation.
>         >> Our hardware actually can only support duty from 1/256 to 256/256.
>         >> For this situation I can do possible solution:
>         >> We can though change polarity to meet this requirement. Inverse the pin and use
>         >> duty_cycle 100. 
>         >> But I think this is not a good solution for this problem right?
> 
>         > If this doesn't result in more glitches that would be fine for me.
>         > (Assuming it is documented good enough in the code to be
>         > understandable.)
> 
>     > The polarity of our pwm controller will affect the duty cycle range:
>     > PWM_POLARITY_INVERSED : Support duty_cycle from 0% to 99%
>     > PWM_POLARITY_NORMAL: Support duty_cycle from 1% to 100%
>     > Dynamic change polarity will result in more glitches. Thus, this will become
>     > a trade-off between 100% and 0% duty_cycle support for user to use our pwm device.
>     > I will document it and send next patch.
> 
> For handling the situation that the user want to set the duty cycle to 0%, the driver can:
> 1. Just return the error.
> 2. Use the minimum duty cycle value.
> I don't know which solution will be the better way or others.
> I would be grateful if you can give me some suggestion about this problem.

I thought if you disable the PWM it emits the inactive level? Then this
is the best you can do if duty_cycle = 0 is requested.

Best regards
Uwe

-- 
Pengutronix e.K.                           | Uwe Kleine-König            |
Industrial Linux Solutions                 | https://www.pengutronix.de/ |

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