* Sebastian Reichel <s...@kernel.org> [180504 13:40]: > Hi, > > On Fri, May 04, 2018 at 02:04:15PM +0200, H. Nikolaus Schaller wrote: > > > Am 04.05.2018 um 13:42 schrieb Sebastian Reichel <s...@kernel.org>: > > >> I think it does not need much more (if at all) than a gpio controller on > > >> the OMAP3 chip (I think the clocks are active anyways for use by the > > >> other > > >> UARTs). > > >> > > >> We had proposed years ago to reprogram the UART RX pin by pinmux-states > > >> into an interrupt gpio but that was rejected because it was not general > > >> enough and ugly in the device tree (an rx-gpios record where the rx-line > > >> is already connected to the UART-rx). > > >> > > >> Then we did experiment with tapping the UART driver and finally the > > >> serdev API was developed to solve this problem. Hence we use it now this > > >> way. > > > > > > Having any UART active on OMAP results in the SoC not entering > > > idle/off wasting energy. For normal (i.e. not connected to a peripheral) > > > TTYs you can enable runtime autosuspend and configure the RX pin as > > > wakeup interrupt. This will wakeup the TTY on incoming traffic, but you > > > will lose the first few characters (since the serial device needs some > > > time to wakeup). This is for example supported by the N900 uart3 > > > (debug uart): > > > > > > $ grep -A4 "&uart3 {" arch/arm/boot/dts/omap3-n900.dts > > > &uart3 { > > > interrupts-extended = <&intc 74 &omap3_pmx_core OMAP3_UART3_RX>; > > > pinctrl-names = "default"; > > > pinctrl-0 = <&uart3_pins>; > > > }; > > > > > > To get it working, you also need to enable autosuspend for the tty > > > in userspace (echo 3000 > > > /sys/class/tty/ttyS2/device/power/autosuspend_delay_ms). > > > This is not enabled by default due to the character loss characteristic > > > during wakeup. > > > > > > Having said all of this, serdev does not yet support runtime PM (at > > > all). Tony is currently looking into it. Fortunately serdev allows > > > us to enable runtime PM by default (once implemented), since we know > > > the remote side and can (hopefully) avoid losing characters (i.e. > > > with sideband wakeup gpios). > > > > thanks for explaining this! We originally had similar thoughts when > > proposing a w2sg0004 driver for the first time (years ago), but we can > > not accept loosing characters... > > > > Now, I could imagine a potential solution. The key situation why we keep > > the serdev open and listening is if the driver did try to turn the module > > off, but in fact did turn it on (because it was not in sync). > > > > It should be possible to cover this by a timer that is started > > in this case. If there is serdev data received after assuming the module > > is turned off, the driver has detected the wrong case - and can safely > > close the serdev until we want to have it powered on again. > > > > If there is no response after turing off, the module power state is already > > in sync and we can close the serdev as well - after the timeout (let's say > > 30 seconds). Then, the serdev UART can idle. We should open the serdev > > and start this timer also in the probe function to catch an initially wrong > > state. > > That sounds like a good plan.
I don't know the details, but sounds like you should might be able to just the timer that comes with pm_runtime_use_autosuspend() :) Regards, Tony