Messages in this thread |  | | Date | Mon, 24 Jun 2019 16:31:57 +0200 | From | Paul Cercueil <> | Subject | Re: [PATCH] backlight: pwm_bl: Set pin to sleep state when powered down |
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Le lun. 24 juin 2019 à 13:28, Daniel Thompson <daniel.thompson@linaro.org> a écrit : > On Fri, Jun 21, 2019 at 03:56:08PM +0200, Thierry Reding wrote: >> On Fri, Jun 21, 2019 at 01:41:45PM +0100, Daniel Thompson wrote: >> > On 22/05/2019 17:34, Paul Cercueil wrote: >> > > When the driver probes, the PWM pin is automatically configured >> to its >> > > default state, which should be the "pwm" function. >> > >> > At which point in the probe... and by who? >> >> The driver core will select the "default" state of a device right >> before >> calling the driver's probe, see: >> >> drivers/base/pinctrl.c: pinctrl_bind_pins() >> >> which is called from: >> >> drivers/base/dd.c: really_probe() >> > > Thanks. I assumed it would be something like that... although given > pwm-backlight is essentially a wrapper driver round a PWM I wondered > why > the pinctrl was on the backlight node (rather than the PWM node). > > Looking at the DTs in the upstream kernel it looks like ~20% of the > backlight drivers have pinctrl on the backlight node. Others > presumable > have none or have it on the PWM node (and it looks like support for > sleeping the pins is *very* rare amoung the PWM drivers).
If your PWM driver has more than one channel and has the pinctrl node, you cannot fine-tune the state of individual pins. They all share the same state.
>> > > However, at this >> > > point we don't know the actual level of the pin, which may be >> active or >> > > inactive. As a result, if the driver probes without enabling the >> > > backlight, the PWM pin might be active, and the backlight would >> be >> > > lit way before being officially enabled. >> > > >> > > To work around this, if the probe function doesn't enable the >> backlight, >> > > the pin is set to its sleep state instead of the default one, >> until the >> > > backlight is enabled. Whenk the backlight is disabled, the pin >> is reset >> > > to its sleep state. >> > Doesn't this workaround result in a backlight flash between >> whatever enables >> > it and the new code turning it off again? >> >> Yeah, I think it would. I guess if you're very careful on how you >> set up >> the device tree you might be able to work around it. Besides the >> default >> and idle standard pinctrl states, there's also the "init" state. The >> core will select that instead of the default state if available. >> However >> there's also pinctrl_init_done() which will try again to switch to >> the >> default state after probe has finished and the driver didn't switch >> away >> from the init state. >> >> So you could presumably set up the device tree such that you have >> three >> states defined: "default" would be the one where the PWM pin is >> active, >> "idle" would be used when backlight is off (PWM pin inactive) and >> then >> another "init" state that would be the same as "idle" to be used >> during >> probe. During probe the driver could then switch to the "idle" >> state so >> that the pin shouldn't glitch. >> >> I'm not sure this would actually work because I think the way that >> pinctrl handles states both "init" and "idle" would be the same >> pointer >> values and therefore pinctrl_init_done() would think the driver >> didn't >> change away from the "init" state because it is the same pointer >> value >> as the "idle" state that the driver selected. One way to work around >> that would be to duplicate the "idle" state definition and >> associate one >> instance of it with the "idle" state and the other with the "init" >> state. At that point both states should be different (different >> pointer >> values) and we'd get the init state selected automatically before >> probe, >> select "idle" during probe and then the core will leave it alone. >> That's >> of course ugly because we duplicate the pinctrl state in DT, but >> perhaps >> it's the least ugly solution. >> Adding Linus for visibility. Perhaps he can share some insight. > > To be honest I'm happy to summarize in my head as "if it flashes then > it's not > a pwm_bl.c's problem" ;-).
It does not flash. But the backlight lits way too early, so we have a 1-2 seconds of "white screen" before the panel driver starts.
-Paul
> > Daniel. > > >> >> On that note, I'm wondering if perhaps it'd make sense for pinctrl >> to >> support some mode where a device would start out in idle mode. That >> is, >> where pinctrl_bind_pins() would select the "idle" mode as the >> default >> before probe. With something like that we could easily support this >> use-case without glitching. >> >> I suppose yet another variant would be for the PWM backlight to not >> use >> any of the standard pinctrl states at all. Instead it could just >> define >> custom states, say "active" and "inactive". Looking at the code that >> would prevent pinctrl_bind_pins() from doing anything with pinctrl >> states and given the driver exact control over when each of the >> states >> will be selected. That's somewhat suboptimal because we can't make >> use >> of the pinctrl PM helpers and it'd require more boilerplate. >> >> Thierry >> >> > > Signed-off-by: Paul Cercueil <paul@crapouillou.net> > --- >> > > drivers/video/backlight/pwm_bl.c | 9 +++++++++ >> > > 1 file changed, 9 insertions(+) >> > > >> > > diff --git a/drivers/video/backlight/pwm_bl.c >> b/drivers/video/backlight/pwm_bl.c >> > > index fb45f866b923..422f7903b382 100644 >> > > --- a/drivers/video/backlight/pwm_bl.c >> > > +++ b/drivers/video/backlight/pwm_bl.c >> > > @@ -16,6 +16,7 @@ >> > > #include <linux/module.h> >> > > #include <linux/kernel.h> >> > > #include <linux/init.h> >> > > +#include <linux/pinctrl/consumer.h> >> > > #include <linux/platform_device.h> >> > > #include <linux/fb.h> >> > > #include <linux/backlight.h> >> > > @@ -50,6 +51,8 @@ static void pwm_backlight_power_on(struct >> pwm_bl_data *pb) >> > > struct pwm_state state; >> > > int err; >> > > + pinctrl_pm_select_default_state(pb->dev); >> > > + >> > > pwm_get_state(pb->pwm, &state); >> > > if (pb->enabled) >> > > return; >> > > @@ -90,6 +93,8 @@ static void pwm_backlight_power_off(struct >> pwm_bl_data *pb) >> > > regulator_disable(pb->power_supply); >> > > pb->enabled = false; >> > > + >> > > + pinctrl_pm_select_sleep_state(pb->dev); >> > > } >> > > static int compute_duty_cycle(struct pwm_bl_data *pb, int >> brightness) >> > > @@ -626,6 +631,10 @@ static int pwm_backlight_probe(struct >> platform_device *pdev) >> > > backlight_update_status(bl); >> > > platform_set_drvdata(pdev, bl); >> > > + >> > > + if (bl->props.power == FB_BLANK_POWERDOWN) >> > > + pinctrl_pm_select_sleep_state(&pdev->dev); >> > >> > Didn't backlight_update_status(bl) already do this? >> > >> > >> > Daniel. >> > >> > >> > > + >> > > return 0; >> > > err_alloc: >> > > >> > > >
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