Hello Sven, On Tue, Dec 08, 2020 at 01:15:10PM -0500, Sven Van Asbroeck wrote: > On Tue, Dec 8, 2020 at 11:57 AM Thierry Reding <thierry.red...@gmail.com> > wrote: > > > > Is this really that complicated? I sounds to me like the only thing that > > you need is to have some sort of usage count for the prescaler. Whenever > > you want to use the prescaler you check that usage count. If it is zero, > > then you can just set it to whatever you need. If it isn't zero, that > > means somebody else is already using it and you can't change it, which > > means you have to check if you're trying to request the value that's > > already set. If so, you can succeed, but otherwise you'll have to fail. > > +1 > I think your suggestion is an elegant solution to get the required behaviour. > > One possible complication is synchronization. The sysfs interface has a lock > protecting against concurrent pwm_apply() calls. But the in-kernel > API (e.g. pwm_apply_state()) doesn't seem to. This is not normally a problem > when pwm bits are strictly separated. But in this case we have shared state > (prescale value and use count), so we probably need to protect pwm_apply() > with a mutex?
Right, you need a lock. You can look at pwm-imx-tpm.c which has a similar limitation. > Not sure if it is currently possible *in practice* for two regulator consumer > drivers to call pwm_apply() from different threads. But Linux is slowly moving > towards asynchronous probing. You must assume that there is concurrent access to different channels of your hardware. Best regards Uwe -- Pengutronix e.K. | Uwe Kleine-König | Industrial Linux Solutions | https://www.pengutronix.de/ |
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