Re: [PATCH v3 4/7] pwm: rockchip: Eliminate potential race condition when probing

["Uwe Kleine-König" <u.kleine-koenig@pengutronix.de>]

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Hello,

On Wed, Dec 23, 2020 at 11:01:06AM -0500, Simon South wrote:
> Commit 48cf973cae33 ("pwm: rockchip: Avoid glitches on already running
> PWMs") introduced a potential race condition in rockchip_pwm_probe(): A
> consumer could enable an inactive PWM, or disable a running one, between
> rockchip_pwm_probe() registering the device via pwmchip_add() and checking
> whether it is enabled (to determine whether it was started by a
> bootloader). This could result in a device's PWM clock being either enabled
> once more than necessary, potentially causing it to continue running when
> no longer needed, or disabled once more than necessary, producing a warning
> from the kernel.
> 
> Eliminate these possibilities by modifying rockchip_pwm_probe() so it
> checks whether a device is enabled before registering it rather than after.
> 
> Also update the code that handles errors from pwmchip_add() to account for
> the fact a device's PWM clock may now be disabled and that its APB clock
> certainly is, and eliminate the "err_pclk" goto target as it is no longer
> of use.
> 
> Fixes: 48cf973cae33 ("pwm: rockchip: Avoid glitches on already running PWMs")
> Reported-by: Trent Piepho <tpiepho@gmail.com>
> Signed-off-by: Simon South <simon@simonsouth.net>
> ---
>  drivers/pwm/pwm-rockchip.c | 22 +++++++++++++---------
>  1 file changed, 13 insertions(+), 9 deletions(-)
> 
> diff --git a/drivers/pwm/pwm-rockchip.c b/drivers/pwm/pwm-rockchip.c
> index 3b1aa5daafff..d904a5d24885 100644
> --- a/drivers/pwm/pwm-rockchip.c
> +++ b/drivers/pwm/pwm-rockchip.c
> @@ -289,6 +289,7 @@ static int rockchip_pwm_probe(struct platform_device *pdev)
>  	struct rockchip_pwm_chip *pc;
>  	struct resource *r;
>  	u32 enable_conf, ctrl;
> +	bool enabled;
>  	int ret, count;
>  
>  	id = of_match_device(rockchip_pwm_dt_ids, &pdev->dev);
> @@ -351,24 +352,27 @@ static int rockchip_pwm_probe(struct platform_device *pdev)
>  		pc->chip.of_pwm_n_cells = 3;
>  	}
>  
> -	ret = pwmchip_add(&pc->chip);
> -	if (ret < 0) {
> -		dev_err(&pdev->dev, "pwmchip_add() failed: %d\n", ret);
> -		goto err_pclk;
> -	}
> -
>  	/* Keep the PWM clk enabled if the PWM appears to be up and running. */
>  	enable_conf = pc->data->enable_conf;
>  	ctrl = readl_relaxed(pc->base + pc->data->regs.ctrl);
> -	if ((ctrl & enable_conf) != enable_conf)
> +	enabled = ((ctrl & enable_conf) == enable_conf);

The outer parenthesis pair isn't necessary, please drop it.

> +	if (!enabled)
>  		clk_disable(pc->clk);
>  
>  	clk_disable(pc->pclk);
>  
> +	ret = pwmchip_add(&pc->chip);
> +	if (ret < 0) {
> +		dev_err(&pdev->dev, "pwmchip_add() failed: %d\n", ret);
> +		if (enabled)
> +			clk_disable(pc->clk);
> +		clk_unprepare(pc->clk);
> +		clk_unprepare(pc->pclk);
> +		return ret;
> +	}
> +

If you do the pwmchip_add before the clock disable, you can continue to
only have a single and simpler error path. The critical part is to check
for the enabled hardware before.

Best regards
Uwe

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

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