linux-kernel - Re: [PATCH v4] media: rc: pwm-ir-tx: Switch to atomic PWM API

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Date:   Thu, 28 Oct 2021 10:14:42 +0100
From:   Sean Young <sean@...s.org>
To:     Uwe Kleine-König 
        <u.kleine-koenig@...gutronix.de>
Cc:     Maíra Canal <maira.canal@....br>, lkp@...el.com,
        mchehab@...nel.org, thierry.reding@...il.com, lee.jones@...aro.org,
        llvm@...ts.linux.dev, kbuild-all@...ts.01.org,
        linux-media@...r.kernel.org, linux-kernel@...r.kernel.org,
        linux-pwm@...r.kernel.org
Subject: Re: [PATCH v4] media: rc: pwm-ir-tx: Switch to atomic PWM API

On Thu, Oct 28, 2021 at 08:45:13AM +0200, Uwe Kleine-König wrote:
> The conversion is right (I think),

We still have the problem that the pwm drivers calculate the period
incorrectly by rounding down (except pwm-bcm2835). So the period is not
as good as it could be in most cases, but this driver can't do anything
about that.

> note this could be optimized a bit
> further: state.period only depends on carrier which rarely changes, so
> the calculation could be done in pwm_ir_set_carrier(). Ditto for duty
> which only depends on state.period and pwm_ir->duty_cycle. (This is for
> a separate commit though.)

I'm not sure what caching this is much of a win. The calculation is a few
instructions, so you're not winning in the way of speed. On the flip side
you use more memory since pwm_state has to be kmalloc() rather than existing
just on the stack, and both ioctl handlers and the probe function need to
recalculate the period/duty cycle, so there is a slight increase in code size.

This change does not improve anything measurably and only increases code
complexity.

> Acked-by: Uwe Kleine-König <u.kleine-koenig@...gutronix.de>

Thanks for your review.

Sean