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

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Message-ID: <3F12A498-DF5C-4954-8BCE-8C0C66BC9734@aspeedtech.com>
Date:   Fri, 16 Jul 2021 10:56:24 +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 2021/7/16, 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.

Thanks

Best Regards,
Billy Tsai