Messages in this thread | | | Date | Tue, 8 Dec 2020 10:17:55 +0100 | From | Uwe Kleine-König <> | Subject | Re: [PATCH v4 1/4] pwm: pca9685: Switch to atomic API |
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Hello Sven,
On Mon, Dec 07, 2020 at 05:34:58PM -0500, Sven Van Asbroeck wrote: > On Mon, Dec 7, 2020 at 5:00 PM Uwe Kleine-König > <u.kleine-koenig@pengutronix.de> wrote: > > > > This is not acceptable, if you have two PWM outputs and a consumer > > modifies one of them the other must change. So if this chip only > > supports a single period length of all channels, the first consumer > > enabling a channel defines the period to be used. All later consumers > > must live with that. (Also the first must be denied modifying the period > > if a second consumer has enabled its PWM.) > > That makes sense. However, a possible wrinkle: when more than one pwm channel > is requested, which one is able to change the period? > > Example: > 1. start with all pwms free > 2. pwm_request(0), pwm_apply(period=200Hz) > 3. pwm_request(1) > 4. pwm_apply(1, period=400Hz) fails?
Yes, pwm_apply_state is supposed to fail here (Sidenote: period is measured in ns, not Hz)
> 5. pwm_apply(0, period=400Hz) succeeds?
This succeeds iff channel 1 isn't enabled. (Unless changing might change the polarity of pwm #1 even if disabled.)
> And if (5) succeeds, then pwm_get_state(1) will still return period=200Hz, > because the pwm core doesn't realize anything has changed. Are you ok > with this behaviour?
"if (5) succeeds" implies channel 1 is disabled (it might otherwise have been enabled by the bootloader or a previous consumer).
With that sorted out, I'm ok that pwm_get_state() reports .period=200Hz (or whatever other value) because it also reports .enabled = false which makes every interpretation of the other values in pwm_state (apart from .polarity) moot.
Best regards Uwe
-- Pengutronix e.K. | Uwe Kleine-König | Industrial Linux Solutions | https://www.pengutronix.de/ | [unhandled content-type:application/pgp-signature] | |