linux-kernel - Re: [PATCH v1] pwm: sifive: Always let the first pwm_apply

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Date:   Tue, 18 Oct 2022 15:29:21 +0200
From:   Uwe Kleine-König <u.kleine-koenig@...gutronix.de>
To:     Emil Renner Berthing <emil.renner.berthing@...onical.com>
Cc:     Thierry Reding <thierry.reding@...il.com>,
        Palmer Dabbelt <palmer@...belt.com>,
        Paul Walmsley <paul.walmsley@...ive.com>,
        Atish Patra <atishp@...shpatra.org>,
        "Wesley W. Terpstra" <wesley@...ive.com>,
        linux-pwm@...r.kernel.org, linux-riscv@...ts.infradead.org,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH v1] pwm: sifive: Always let the first pwm_apply_state
 succeed

Hello,

On Tue, Oct 18, 2022 at 11:13:16AM +0200, Emil Renner Berthing wrote:
> Commit 2cfe9bbec56ea579135cdd92409fff371841904f added support for the
> RGB and green PWM controlled LEDs on the HiFive Unmatched board
> managed by the leds-pwm-multicolor and leds-pwm drivers respectively.
> All three colours of the RGB LED and the green LED run from different
> lines of the same PWM, but with the same period so this works fine when
> the LED drivers are loaded one after the other.
> 
> Unfortunately it does expose a race in the PWM driver when both LED
> drivers are loaded at roughly the same time. Here is an example:
> 
>   |          Thread A           |          Thread B           |
>   |  led_pwm_mc_probe           |  led_pwm_probe              |
>   |    devm_fwnode_pwm_get      |                             |
>   |      pwm_sifive_request     |                             |
>   |        ddata->user_count++  |                             |
>   |                             |    devm_fwnode_pwm_get      |
>   |                             |      pwm_sifive_request     |
>   |                             |        ddata->user_count++  |
>   |         ...                 |          ...                |
>   |    pwm_state_apply          |    pwm_state_apply          |
>   |      pwm_sifive_apply       |      pwm_sifive_apply       |
> 
> Now both calls to pwm_sifive_apply will see that ddata->approx_period,
> initially 0, is different from the requested period and the clock needs
> to be updated. But since ddata->user_count >= 2 both calls will fail
> with -EBUSY, which will then cause both LED drivers to fail to probe.
> 
> Fix it by letting the first call to pwm_sifive_apply update the clock
> even when ddata->user_count != 1.
> 
> Fixes: 9e37a53eb051 ("pwm: sifive: Add a driver for SiFive SoC PWM")
> Signed-off-by: Emil Renner Berthing <emil.renner.berthing@...onical.com>
> ---
>  drivers/pwm/pwm-sifive.c | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
> 
> diff --git a/drivers/pwm/pwm-sifive.c b/drivers/pwm/pwm-sifive.c
> index 2d4fa5e5fdd4..ccdf92045f34 100644
> --- a/drivers/pwm/pwm-sifive.c
> +++ b/drivers/pwm/pwm-sifive.c
> @@ -159,7 +159,7 @@ static int pwm_sifive_apply(struct pwm_chip *chip, struct pwm_device *pwm,
>  
>  	mutex_lock(&ddata->lock);
>  	if (state->period != ddata->approx_period) {
> -		if (ddata->user_count != 1) {
> +		if (ddata->user_count != 1 && ddata->approx_period) {

IMHO this needs a code comment. It should among others mention that
approx_period is only zero if .apply() wasn't called before.

Let me note this is inconsistent. I didn't check the details, but let's
assume the PWM can implement .period = 500 and .period = 514 and nothing
in between. So if the the first PWM requests 512 ns it gets (I hope) 500
ns. Then when the second requests comes in requesting 511 it fails and
if it requests 512 is succeeds also getting 500 ns. Hmm.

Best regards
Uwe

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

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