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

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Message-ID: <20210723201250.x4ki5ackfznmn4aw@pengutronix.de>
Date:   Fri, 23 Jul 2021 22:12:50 +0200
From:   Uwe Kleine-König <u.kleine-koenig@...gutronix.de>
To:     Billy Tsai <billy_tsai@...eedtech.com>
Cc:     "lee.jones@...aro.org" <lee.jones@...aro.org>,
        "robh+dt@...nel.org" <robh+dt@...nel.org>,
        "joel@....id.au" <joel@....id.au>,
        "andrew@...id.au" <andrew@...id.au>,
        "thierry.reding@...il.com" <thierry.reding@...il.com>,
        "p.zabel@...gutronix.de" <p.zabel@...gutronix.de>,
        "devicetree@...r.kernel.org" <devicetree@...r.kernel.org>,
        "linux-arm-kernel@...ts.infradead.org" 
        <linux-arm-kernel@...ts.infradead.org>,
        "linux-aspeed@...ts.ozlabs.org" <linux-aspeed@...ts.ozlabs.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "linux-pwm@...r.kernel.org" <linux-pwm@...r.kernel.org>,
        BMC-SW <BMC-SW@...eedtech.com>
Subject: Re: [v9 2/2] pwm: Add Aspeed ast2600 PWM support

On Fri, Jul 23, 2021 at 04:23:23AM +0000, Billy Tsai wrote:
> On 2021/7/23, 3:17 AM, "Uwe Kleine-König" <u.kleine-koenig@...gutronix.de> wrote:
> 
>     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@...gutronix.de> 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@...gutronix.de> 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@...gutronix.de>> 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.
> 
> Thanks for your quick reply.
> When duty_cycle = 0 is requested my driver currently will emit the inactive level.
> So, the next patch I need to do is to add the comment about this?

Not sure I got the complete picture now. The things I consider important
are:

 - If your hardware cannot emit a 100% or 0% relative duty cycle, note
   this in the Limitations section

 - Assuming your PWM emits the inactive level when disabled (that is 0
   for PWM_POLARITY_NORMAL and 1 for PWM_POLARITY_INVERSED) this is the
   best that can be done when a 0% relative duty cycle is requested
   (assuming the hardware cannot implement that in a normal way).

I hope this answered your remaining questions.

Best regards
Uwe

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

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