linux-kernel - Re: [PATCH 0/2] Change PWM-controlled LED pin active mode and algorithm

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Message-Id: <95F1EAA0-D8D6-4F8A-8049-5E7BFDE4C06C@jrtc27.com>
Date:   Fri, 13 Jan 2023 19:24:56 +0000
From:   Jessica Clarke <jrtc27@...c27.com>
To:     Conor Dooley <conor@...nel.org>
Cc:     Nylon Chen <nylon.chen@...ive.com>,
        Paul Walmsley <paul.walmsley@...ive.com>,
        Palmer Dabbelt <palmer@...belt.com>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        linux-riscv <linux-riscv@...ts.infradead.org>,
        nylon7717@...il.com, zong.li@...ive.com, greentime.hu@...ive.com,
        vincent.chen@...ive.com, Thierry Reding <thierry.reding@...il.com>,
        Uwe Kleine-König <u.kleine-koenig@...gutronix.de>,
        linux-pwm@...r.kernel.org
Subject: Re: [PATCH 0/2] Change PWM-controlled LED pin active mode and
 algorithm

On 13 Jan 2023, at 18:32, Conor Dooley <conor@...nel.org> wrote:
> 
> +CC Uwe, Thierry, linux-pwm
> 
> Hey Nylon,
> 
> Please run scripts/get_maintainer.pl before sending patches, you missed
> both me & the PWM maintainers unfortunately!
> AFAIK, the PWM maintainers use patchwork, so you will probably have to
> resend this patchset so that it is on their radar.
> I've marked the series as "Changes Requested" on the RISC-V one.
> 
> On Fri, Jan 13, 2023 at 04:31:13PM +0800, Nylon Chen wrote:
> 
>> According to the circuit diagram of User LEDs - RGB described in the
>> manual hifive-unmatched-schematics-v3.pdf[0].
>> The behavior of PWM is acitve-high.
>> 
>> According to the descriptionof PWM for pwmcmp in SiFive FU740-C000
>> Manual[1].
>> The pwm algorithm is (PW) pulse active time  = (D) duty * (T) period[2].
>> The `frac` variable is pulse "inactive" time so we need to invert it.
>> 
>> So this patchset removes active-low in DTS and adds reverse logic to
>> the driver.
>> 
>> [0]:https://sifive-china.oss-cn-zhangjiakou.aliyuncs.com/HiFIve%20Unmatched/hifive-unmatched-schematics-v3.pdf
>> [1]:https://sifive-china.oss-cn-zhangjiakou.aliyuncs.com/HiFIve%20Unmatched/fu740-c000-manual-v1p2.pdf
>> [2]:https://en.wikipedia.org/wiki/Duty_cycle
> 
> Please delete link 2, convert the other two to standard Link: tags and
> put this information in the dts patch. Possibly into the PWM patch too,
> depending on what the PWM maintainers think.
> This info should be in the commit history IMO and the commit message for
> the dts patch says what's obvious from the diff without any explanation
> as to why.
> 
> I did a bit of looking around on lore, to see if I could figure out
> why it was done like this in the first place, and I found:
> https://lore.kernel.org/linux-pwm/CAJ2_jOG2M03aLBgUOgGjWH9CUxq2aTG97eSX70=UaSbGCMMF_g@mail.gmail.com/

That DTS documentation makes no sense to me, why does what the LED is
wired to matter? Whether you have your transistor next to ground or
next to Vdd doesn’t matter, what matters is whether the transistor is
on or off. Maybe what they mean is whether the *PWM's output* / *the
transistor's input* is pulled to ground or Vdd? In which case the
property would indeed not apply here.

Unless that’s written assuming the LED is wired directly to the PWM, in
which case it would make sense, but that’s a very narrow-minded view of
what the PWM output is (directly) driving.

Jess

> That doesn't explain the driver, but it does explain the dts being that
> way. Perhaps a Fixes tag is also in order? But only if both patches get
> one, otherwise backporting would lead to breakage.
> 
> The min() construct appears to have been there since the RFC driver was
> first posted.
> 
> Thanks,
> Conor.
> 
>> 
>> Nylon Chen (2):
>>  riscv: dts: sifive unmatched: Remove PWM controlled LED's active-low
> 
> nit: s/sifive unmatched:/sifive: unmatched:/
> 
>>    properties
>>  pwm: sifive: change the PWM controlled LED algorithm
>> 
>> arch/riscv/boot/dts/sifive/hifive-unmatched-a00.dts | 4 ----
>> drivers/pwm/pwm-sifive.c                            | 1 +
>> 2 files changed, 1 insertion(+), 4 deletions(-)
>> 
>> -- 
>> 2.36.1
>> 
>> 
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