linux-kernel - Re: [PATCH v6 4/4] media: pwm-ir-tx: trigger edges from hrtimer interrupt context

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Message-ID: <ZXNEg3ax4MChSJ5A@orome.fritz.box>
Date:   Fri, 8 Dec 2023 17:29:55 +0100
From:   Thierry Reding <thierry.reding@...il.com>
To:     Sean Young <sean@...s.org>
Cc:     linux-media@...r.kernel.org, linux-pwm@...r.kernel.org,
        Ivaylo Dimitrov <ivo.g.dimitrov.75@...il.com>,
        Uwe Kleine-König 
        <u.kleine-koenig@...gutronix.de>,
        Mauro Carvalho Chehab <mchehab@...nel.org>,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH v6 4/4] media: pwm-ir-tx: trigger edges from hrtimer
 interrupt context

On Wed, Nov 29, 2023 at 09:13:37AM +0000, Sean Young wrote:
> This makes the generated IR much more precise. Before this change, the
> driver is unreliable and many users opted to use gpio-ir-tx instead.
> 
> Signed-off-by: Sean Young <sean@...s.org>
> ---
>  drivers/media/rc/pwm-ir-tx.c | 79 ++++++++++++++++++++++++++++++++++--
>  1 file changed, 76 insertions(+), 3 deletions(-)
> 
> diff --git a/drivers/media/rc/pwm-ir-tx.c b/drivers/media/rc/pwm-ir-tx.c
> index cf51e2760975..8575c4596d7b 100644
> --- a/drivers/media/rc/pwm-ir-tx.c
> +++ b/drivers/media/rc/pwm-ir-tx.c
> @@ -10,6 +10,8 @@
>  #include <linux/slab.h>
>  #include <linux/of.h>
>  #include <linux/platform_device.h>
> +#include <linux/hrtimer.h>
> +#include <linux/completion.h>
>  #include <media/rc-core.h>
>  
>  #define DRIVER_NAME	"pwm-ir-tx"
> @@ -17,8 +19,14 @@
>  
>  struct pwm_ir {
>  	struct pwm_device *pwm;
> -	unsigned int carrier;
> -	unsigned int duty_cycle;
> +	struct hrtimer timer;
> +	struct completion tx_done;
> +	struct pwm_state *state;
> +	u32 carrier;
> +	u32 duty_cycle;
> +	uint *txbuf;

Maybe mark this as const to signal that it's not going to get modified?

> +	uint txbuf_len;
> +	uint txbuf_index;

uint is rather rare. Or so I thought. There seem to be quite a few
occurrences throughout the kernel. I'd still prefer unsigned int over
this abbreviated form, but ultimately up to you and Mauro to decide.

>  };
>  
>  static const struct of_device_id pwm_ir_of_match[] = {
> @@ -82,6 +90,62 @@ static int pwm_ir_tx(struct rc_dev *dev, unsigned int *txbuf,
>  	return count;
>  }
>  
> +static int pwm_ir_tx_atomic(struct rc_dev *dev, unsigned int *txbuf,
> +			    unsigned int count)
> +{
> +	struct pwm_ir *pwm_ir = dev->priv;
> +	struct pwm_device *pwm = pwm_ir->pwm;
> +	struct pwm_state state;
> +
> +	pwm_init_state(pwm, &state);
> +
> +	state.period = DIV_ROUND_CLOSEST(NSEC_PER_SEC, pwm_ir->carrier);
> +	pwm_set_relative_duty_cycle(&state, pwm_ir->duty_cycle, 100);
> +
> +	pwm_ir->txbuf = txbuf;
> +	pwm_ir->txbuf_len = count;
> +	pwm_ir->txbuf_index = 0;
> +	pwm_ir->state = &state;
> +
> +	hrtimer_start(&pwm_ir->timer, 0, HRTIMER_MODE_REL);
> +
> +	wait_for_completion(&pwm_ir->tx_done);
> +
> +	return count;
> +}
> +
> +static enum hrtimer_restart pwm_ir_timer(struct hrtimer *timer)
> +{
> +	struct pwm_ir *pwm_ir = container_of(timer, struct pwm_ir, timer);
> +	ktime_t now;
> +
> +	/*
> +	 * If we happen to hit an odd latency spike, loop through the
> +	 * pulses until we catch up.
> +	 */
> +	do {
> +		u64 ns;
> +
> +		pwm_ir->state->enabled = !(pwm_ir->txbuf_index % 2);
> +		pwm_apply_atomic(pwm_ir->pwm, pwm_ir->state);
> +
> +		if (pwm_ir->txbuf_index >= pwm_ir->txbuf_len) {
> +			complete(&pwm_ir->tx_done);
> +
> +			return HRTIMER_NORESTART;
> +		}
> +
> +		ns = US_TO_NS(pwm_ir->txbuf[pwm_ir->txbuf_index]);
> +		hrtimer_add_expires_ns(timer, ns);
> +
> +		pwm_ir->txbuf_index++;
> +
> +		now = timer->base->get_time();
> +	} while (hrtimer_get_expires_tv64(timer) < now);
> +
> +	return HRTIMER_RESTART;
> +}
> +
>  static int pwm_ir_probe(struct platform_device *pdev)
>  {
>  	struct pwm_ir *pwm_ir;
> @@ -103,10 +167,19 @@ static int pwm_ir_probe(struct platform_device *pdev)
>  	if (!rcdev)
>  		return -ENOMEM;
>  
> +	if (pwm_is_atomic(pwm_ir->pwm)) {
> +		init_completion(&pwm_ir->tx_done);
> +		hrtimer_init(&pwm_ir->timer, CLOCK_MONOTONIC, HRTIMER_MODE_REL);
> +		pwm_ir->timer.function = pwm_ir_timer;
> +		rcdev->tx_ir = pwm_ir_tx_atomic;
> +	} else {
> +		dev_info(&pdev->dev, "tx will not be accurate as pwm device does not support atomic mode");

s/tx/TX and s/pwm/PWM/? Also, I'm a bit unhappy about "atomic mode" here
because the term is overloaded in PWM. If you call pwm_appy_*() then by
definition it's going to be "atomic" in the "atomic state" sense. So
maybe switch to something like:

	"TX will not be accurate as PWM device might sleep"

?

Thierry

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