linux-kernel - Re: [PATCH v2 REBASED] pwm: pca9685: fix pwm/gpio inter-operation

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Date:   Fri, 3 Apr 2020 21:44:25 +0200
From:   Thierry Reding <thierry.reding@...il.com>
To:     Clemens Gruber <clemens.gruber@...ruber.com>
Cc:     linux-pwm@...r.kernel.org,
        Andy Shevchenko <andy.shevchenko@...il.com>,
        Matthias Schiffer <matthias.schiffer@...tq-group.com>,
        Uwe Kleine-König 
        <u.kleine-koenig@...gutronix.de>, linux-kernel@...r.kernel.org,
        Sven Van Asbroeck <TheSven73@...il.com>,
        YueHaibing <yuehaibing@...wei.com>,
        Mika Westerberg <mika.westerberg@...ux.intel.com>
Subject: Re: [PATCH v2 REBASED] pwm: pca9685: fix pwm/gpio inter-operation

On Wed, Apr 01, 2020 at 07:01:06PM +0200, Clemens Gruber wrote:
> From: Sven Van Asbroeck <TheSven73@...il.com>
> 
> This driver allows pwms to be requested as gpios via gpiolib.
> Obviously, it should not be allowed to request a gpio when its
> corresponding pwm is already requested (and vice versa).
> So it requires some exclusion code.
> 
> Given that the pwm and gpio cores are not synchronized with
> respect to each other, this exclusion code will also require
> proper synchronization.
> 
> Such a mechanism was in place, but was inadvertently removed
> by Uwe's clean-up patch.
> 
> Upon revisiting the synchronization mechanism, we found that
> theoretically, it could allow two threads to successfully
> request conflicting pwms / gpios.
> 
> Replace with a bitmap which tracks pwm in-use, plus a mutex.
> As long as pwm and gpio's respective request/free functions
> modify the in-use bitmap while holding the mutex, proper
> synchronization will be guaranteed.
> 
> Reported-by: YueHaibing <yuehaibing@...wei.com>
> Fixes: e926b12c611c ("pwm: Clear chip_data in pwm_put()")
> Cc: Mika Westerberg <mika.westerberg@...ux.intel.com>
> Cc: Uwe Kleine-König <u.kleine-koenig@...gutronix.de>
> Cc: YueHaibing <yuehaibing@...wei.com>
> Link: https://lkml.org/lkml/2019/5/31/963
> Signed-off-by: Sven Van Asbroeck <TheSven73@...il.com>
> Reviewed-by: Mika Westerberg <mika.westerberg@...ux.intel.com>
> Tested-by: Clemens Gruber <clemens.gruber@...ruber.com>
> [cg: Tested on an i.MX6Q board with two NXP PCA9685 chips]
> 
> ---
>  drivers/pwm/pwm-pca9685.c | 85 ++++++++++++++++++++++-----------------
>  1 file changed, 48 insertions(+), 37 deletions(-)

Applied, thanks.

Thierry

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