linux-kernel - Re: [v2 2/2] pwm: Add Aspeed ast2600 PWM support

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Message-ID: <20210503062338.meohvoth7cyfppvu@pengutronix.de>
Date:   Mon, 3 May 2021 08:23:38 +0200
From:   Uwe Kleine-König <u.kleine-koenig@...gutronix.de>
To:     Billy Tsai <billy_tsai@...eedtech.com>
Cc:     "devicetree@...r.kernel.org" <devicetree@...r.kernel.org>,
        "kernel@...gutronix.de" <kernel@...gutronix.de>,
        "linux-aspeed@...ts.ozlabs.org" <linux-aspeed@...ts.ozlabs.org>,
        "linux-pwm@...r.kernel.org" <linux-pwm@...r.kernel.org>,
        "andrew@...id.au" <andrew@...id.au>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "robh+dt@...nel.org" <robh+dt@...nel.org>,
        "thierry.reding@...il.com" <thierry.reding@...il.com>,
        "joel@....id.au" <joel@....id.au>,
        "p.zabel@...gutronix.de" <p.zabel@...gutronix.de>,
        BMC-SW <BMC-SW@...eedtech.com>,
        "lee.jones@...aro.org" <lee.jones@...aro.org>,
        "linux-arm-kernel@...ts.infradead.org" 
        <linux-arm-kernel@...ts.infradead.org>
Subject: Re: [v2 2/2] pwm: Add Aspeed ast2600 PWM support

Hello,

your second reply is nearly identical to the first. It would be helpful
to only write new stuff in new mail. I think there is only a single new
paragraph that I will reply to here.

On Mon, May 03, 2021 at 05:57:23AM +0000, Billy Tsai wrote:
> On 2021/4/27, 4:44 AM,Uwe Kleine-Königwrote:
>     > Also please round down in the division to never provide a duty_cycle
>     > bigger than the requested vaule. Also you have to use the actually used
>     > period as divider, not state->period.
> 
> I don’t think that I should use the actually used period as divider. 
> The state->duty_cycle is relative with state->period, not the actual period
> if I use the actual period the precision of the duty cycle may lose.

The strategy you should implement in .apply() is: Pick the biggest
period that is not bigger than the requested period. With that period
pick the biggest duty_cycle that is not bigger than the requested
duty_cycle.

As the actual period might be smaller than state->period, dividing by
the latter yields a result that might be too small.

See commit 8035e6c66a5e98f098edf7441667de74affb4e78 (currently in next)
for a similar example.

Best regards
Uwe

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

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