Messages in this thread | | | Date | Tue, 8 Dec 2020 10:10:33 +0100 | From | Uwe Kleine-König <> | Subject | Re: [PATCH v4 1/4] pwm: pca9685: Switch to atomic API |
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Hello Clemens,
On Tue, Dec 08, 2020 at 12:13:44AM +0100, Clemens Gruber wrote: > On Mon, Dec 07, 2020 at 11:00:25PM +0100, Uwe Kleine-König wrote: > > On Mon, Dec 07, 2020 at 08:36:27PM +0100, Clemens Gruber wrote: > > > The hardware readout may return slightly different values than those > > > that were set in apply due to the limited range of possible prescale and > > > counter register values. If one channel is reconfigured with new duty > > > cycle and period, the others will keep the same relative duty cycle to > > > period ratio as they had before, even though the per-chip / global > > > frequency changed. (The PCA9685 has only one prescaler!) > > > > 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.) > > Good idea, but is it OK to potentially break users relying on the old > behavior ("the last one who changes the period wins") ?
If this is already in the old code, this probably warrants a separate fix, and yes, I consider this a severe bug. (Consider one channel driving a motor and reconfiguring an LED modifies the motor's speed.)
Best regards Uwe
-- Pengutronix e.K. | Uwe Kleine-König | Industrial Linux Solutions | https://www.pengutronix.de/ | [unhandled content-type:application/pgp-signature] | |