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). > > > 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" ;-). 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@xxxxxxxxxxxxxxx> > --- > > > 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: > > > > > _______________________________________________ dri-devel mailing list dri-devel@xxxxxxxxxxxxxxxxxxxxx https://lists.freedesktop.org/mailman/listinfo/dri-devel
