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Date:   Mon, 26 Jul 2021 05:14:09 +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

Hi Uwe

On 2021/7/24, 4:13 AM, "Uwe Kleine-König" <u.kleine-koenig@...gutronix.de> wrote:

    >   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).

Our hardware is the same as this description. 
So I didn't need to add the limitations about the duty cycle, right?
Or I need to note that the duty cycle 0% is just the inactive output that doesn't have the period concept.

    >   I hope this answered your remaining questions.

Thanks.

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