On Mon, Jun 24, 2019 at 04:31:57PM +0200, Paul Cercueil wrote: > > > Le lun. 24 juin 2019 à 13:28, Daniel Thompson <daniel.thompson@xxxxxxxxxx> 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. Good point. Thanks. > > > > > 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. That's the current behaviour. What I original asked about is whether a panel that was dark before the driver probes could end up flashing after the patch because it is activated pre-probe and only goes to sleep afterwards. Anyhow I got an answer; if it flashes after the patch then the problem does not originate in pwm_bl.c and is likely a problem with the handling of the pinctrl idel state (i.e. probably DT misconfiguration) So I think that just leaves my comment about the spurious sleep in the probe function. Daniel. _______________________________________________ dri-devel mailing list dri-devel@xxxxxxxxxxxxxxxxxxxxx https://lists.freedesktop.org/mailman/listinfo/dri-devel
