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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 PWMm 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 in commite926b12c61("pwm: Clear chip_data in pwm_put()"). 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 PWMs 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@huawei.com> Fixes:e926b12c61("pwm: Clear chip_data in pwm_put()") Cc: Mika Westerberg <mika.westerberg@linux.intel.com> Cc: Uwe Kleine-König <u.kleine-koenig@pengutronix.de> Cc: YueHaibing <yuehaibing@huawei.com> Link: https://lkml.org/lkml/2019/5/31/963 Signed-off-by: Sven Van Asbroeck <TheSven73@gmail.com> Reviewed-by: Mika Westerberg <mika.westerberg@linux.intel.com> [cg: Tested on an i.MX6Q board with two NXP PCA9685 chips] Tested-by: Clemens Gruber <clemens.gruber@pqgruber.com> Reviewed-by: Sven Van Asbroeck <TheSven73@gmail.com> # cg's rebase Link: https://lore.kernel.org/lkml/20200330160238.GD2817345@ulmo/ Signed-off-by: Thierry Reding <thierry.reding@gmail.com>
Linux kernel
============
There are several guides for kernel developers and users. These guides can
be rendered in a number of formats, like HTML and PDF. Please read
Documentation/admin-guide/README.rst first.
In order to build the documentation, use ``make htmldocs`` or
``make pdfdocs``. The formatted documentation can also be read online at:
https://www.kernel.org/doc/html/latest/
There are various text files in the Documentation/ subdirectory,
several of them using the Restructured Text markup notation.
Please read the Documentation/process/changes.rst file, as it contains the
requirements for building and running the kernel, and information about
the problems which may result by upgrading your kernel.