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

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Date:   Wed, 5 Jun 2019 15:53:56 +0300
From:   Mika Westerberg <mika.westerberg@...ux.intel.com>
To:     Sven Van Asbroeck <thesven73@...il.com>
Cc:     Thierry Reding <thierry.reding@...il.com>,
        linux-pwm@...r.kernel.org, linux-kernel@...r.kernel.org,
        Uwe Kleine-König 
        <u.kleine-koenig@...gutronix.de>,
        YueHaibing <yuehaibing@...wei.com>
Subject: Re: [PATCH v2] pwm: pca9685: fix pwm/gpio inter-operation

On Tue, Jun 04, 2019 at 02:13:45PM -0400, Sven Van Asbroeck wrote:
> 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>

Reviewed-by: Mika Westerberg <mika.westerberg@...ux.intel.com>