Hi Thierry, On Friday 01 November 2013 11:13:47 Thierry Reding wrote: > On Fri, Nov 01, 2013 at 08:37:23AM +0900, Jingoo Han wrote: > > On Wednesday, October 23, 2013 5:02 AM, Thierry Reding wrote: > > > On Tue, Oct 22, 2013 at 05:34:45PM +0200, Jean-Christophe PLAGNIOL- VILLARD wrote: > > > > On 09:23 Tue 22 Oct , Thierry Reding wrote: > > > > > On Tue, Oct 22, 2013 at 06:58:33AM +0200, Jean-Christophe PLAGNIOL- VILLARD wrote: > > > > > > On 11:13 Mon 21 Oct , Denis Carikli wrote: > > > > > > > Cc: Richard Purdie <rpurdie@xxxxxxxxx> > > > > > > > Cc: Jingoo Han <jg1.han@xxxxxxxxxxx> > > > > > > > Cc: Laurent Pinchart <laurent.pinchart@xxxxxxxxxxxxxxxx> > > > > > > > Cc: Rob Herring <rob.herring@xxxxxxxxxxx> > > > > > > > Cc: Pawel Moll <pawel.moll@xxxxxxx> > > > > > > > Cc: Mark Rutland <mark.rutland@xxxxxxx> > > > > > > > Cc: Stephen Warren <swarren@xxxxxxxxxxxxx> > > > > > > > Cc: Ian Campbell <ijc+devicetree@xxxxxxxxxxxxxx> > > > > > > > Cc: devicetree@xxxxxxxxxxxxxxx > > > > > > > Cc: Sascha Hauer <kernel@xxxxxxxxxxxxxx> > > > > > > > Cc: linux-arm-kernel@xxxxxxxxxxxxxxxxxxx > > > > > > > Cc: Lothar Waßmann <LW@xxxxxxxxxxxxxxxxxxx> > > > > > > > Cc: Jean-Christophe Plagniol-Villard <plagnioj@xxxxxxxxxxxx> > > > > > > > Cc: Eric Bénard <eric@xxxxxxxxxx> > > > > > > > Signed-off-by: Denis Carikli <denis@xxxxxxxxxx> > > > > > > > --- > > > > > > > ChangeLog v3->v4: > > > > > > > - The default-brightness property is now optional, it defaults > > > > > > > to 1 if not set.> > > > > > > > > > > by default we set OFF not ON > > > > > > > > > > > > do not actiate driver or properti by default you can not known to > > > > > > consequence on the hw > > > > > > > > > > Turning on a backlight by default is what pretty much every > > > > > backlight driver does. I personally think that's the wrong default, > > > > > I even tried to get some discussion started recently about how we > > > > > could change this. However, given that this has been the case for > > > > > possibly as long as the subsystem has existed, suddenly changing it > > > > > might cause quite a few of our users to boot the new kernel and not > > > > > see their display come up. As with any other ABI, this isn't > > > > > something we can just change without a very good migration path. > > > > > > > > I'm sorry but the blacklight descibe in DT have nothing to do with the > > > > common pratice that the current driver have today > > > > > > That's not at all what I said. What I said was that the majority of > > > backlight drivers currently default to turning the backlight on when > > > probed. Therefore I think it would be consistent if this driver did the > > > same. > > > > > > I also said that I don't think it's a very good default, but at the same > > > time we can't just go and change the default behaviour at will because > > > people may rely on it. > > > > I agree with your opinion. > > But, I can't decide how to change it. > > One solution that Stephen proposed was to make all DT-based setups use a > new default (off). An advantage of that is that such setups are still > fairly actively being worked on, so we can probably get early adopters > to cope with the new default. > > Looking through some DTS files it seems that many use the pwm-backlight > driver. So if we can get some concensus from all the users that changing > the default would be okay, then I think we could reasonably change it. > > Another solution perhaps would be to add a property to DT which encodes > the default state of the backlight on boot. This used to be impossible > because the general concensus was that DT should describe hardware only > and not software policy. During the kernel summit this requirement was > somewhat relaxed and it should now be okay to describe system > configuration data, and I think this would be a good match. If the > system is designed to keep the backlight off by default because some > other component (display panel driver) is meant to turn it on later, > that's system configuration data, right? > > Perhaps a boolean property could be used: > > backlight { > compatible = "pwm-backlight"; > > ... > > backlight-default-off; > }; > > > > > put on by default if wrong specially without the property define. Even > > > > put it on by default it wrong as the bootloader may have set it > > > > already for splash screen and to avoid glitch the drivers need to > > > > detect this. > > > > > > I agree that would be preferable, but I don't know of any way to detect > > > what value the bootloader set a GPIO to. The GPIO API requires that you > > > call gpio_direction_output(), and that requires a value parameter which > > > will be used as the output level of the GPIO. > > > > Jean-Christophe's point is right. > > > > We may need to discuss 'the way to detect what value the bootloader > > set a GPIO to'. > > Since some GPIO hardware simply can't do it, I guess we could resolve to > passing such information via DT. That obviously won't work for non-DT > setups, but I guess that's something we could live with. For what it's worth, the pcf857x GPIO controllers suffer from this problem, and the solution was to use a DT property to specify the initial GPIOs state. The driver can thus implement gpio_get_value() properly and the hardware limitation is hidden from the clients. https://lkml.org/lkml/2013/9/21/105 > One possibility would be to supplement the backlight-default-off property > with another property (backlight-boot-on) that can be passed on to the > kernel from the bootloader to signal that it has turned the backlight on. -- Regards, Laurent Pinchart
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