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

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Date:   Fri, 23 Jul 2021 04:23:23 +0000
From:   Billy Tsai <billy_tsai@...eedtech.com>
To:     Uwe Kleine-König 
        <u.kleine-koenig@...gutronix.de>
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 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?

Best Regards,
Billy Tsai