Messages in this thread | | | Date | Mon, 7 Dec 2020 23:00:25 +0100 | From | Uwe Kleine-König <> | Subject | Re: [PATCH v4 1/4] pwm: pca9685: Switch to atomic API |
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On Mon, Dec 07, 2020 at 08:36:27PM +0100, Clemens Gruber wrote: > The switch to the atomic API goes hand in hand with a few fixes to > previously experienced issues: > - The duty cycle is no longer lost after disable/enable (previously the > OFF registers were cleared in disable and the user was required to > call config to restore the duty cycle settings) > - If one sets a period resulting in the same prescale register value, > the sleep and write to the register is now skipped > - The prescale register is now set to the default value in probe. On > systems without CONFIG_PM, the chip is woken up at probe time. > > 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.)
> Note that although the datasheet mentions 200 Hz as default frequency > when using the internal 25 MHz oscillator, the calculated period from > the default prescaler register setting of 30 is 5079040ns.
That means the datasheet is lax because 5079040ns corresponds to 196.88760080645162 Hz but it calls that 200 Hz, right?
I didn't look in the patch in detail, but get the impression it is more complicated than necessary. For example adding improved PM behaviour should probably go into a separate patch, also adding the .get_state callback should be split out.
Best regards Uwe
-- Pengutronix e.K. | Uwe Kleine-König | Industrial Linux Solutions | https://www.pengutronix.de/ | [unhandled content-type:application/pgp-signature] | |