Re: PWM backlight initial state assumptions, or how pwm_bl killed my (nyan) cat^W backlight support

[Paul Kocialkowski]

Hi,

On Wed, 2017-07-05 at 13:07 +0200, Philipp Zabel wrote:
> Hi Paul,
> 
> On Wed, 2017-07-05 at 13:41 +0300, Paul Kocialkowski wrote:
> > Hey,
> > 
> > On Wed, 2017-07-05 at 11:24 +0100, Daniel Thompson wrote:
> > > On 04/07/17 21:13, Paul Kocialkowski wrote:
> > > > As I try to maintain support for ARM CrOS (read, ChromeOS/ChromiumOS)
> > > > devices in
> > > > upstream Linux on my spare time, I try to test out rc and stable
> > > > versions as
> > > > often as time allows. I have been rolling out 4.12 since Monday and
> > > > noticed
> > > > that
> > > > the backlight on my tegra124 nyan big stayed dark for this release.
> > > > 
> > > > Not very cool, although I'm not blaming anyone else than myself on this,
> > > > I should have just tested it and brought the issue up during the rc
> > > > cycle.
> > > > Still, let's try to move forward.
> > > 
> > > Personally I might be inclined to spread the blame a bit wider ;-).
> > > 
> > > Did you bisect it down to a specific patch? An SHA-1 would be something 
> > > of a time saver here!
> > 
> > The offending commit here is d1b81294575098d989be1f2f6bb628091ceaa87b, that
> > added a check on the intial PWM state.
> > 
> > The policy I'm describing was introduced with
> > 3698d7e7d221a5c90d4b55e96d0c8f98a8b4d7df which did not break my use case at
> > this
> > point. It will still disable the driver if the regulator was not enabled
> > already, which I think is not desirable. The policy on the initial GPIO
> > state is
> > also quite disputable.
> 
> Some panels have a documented powerup sequence, which usually ends with
> the backlight being enabled to avoid showing garbage before the panel is
> initialized completely.
> The reason for 3698d7e7d221 was a device with the display disabled out
> of the bootloader, where the backlight is controlled by the simple-panel
> driver. Enabling the backlight from the backlight driver before the
> panel driver requests the backlight to be enabled (before the panel is
> powered) would result in a white flash during boot.
> 
> I tried to be careful to only let the backlight driver set the initial
> state to disabled if a few conditions are met: the GPIO is already
> configured as output and disabled, and the backlight device tree node
> has a phandle pointing to it, so we can expect there to be some
> controlling instance that will enable it when appropriate.
>
> I wonder why in your case there is a phandle link to the backlight node
> but nothing actually enables the backlight during boot. I would expect
> that to be handled by the panel driver.

I had completely missed the fact that the panel driver is supposed to request
backlight enable! With that in mind, this policy no longer concerns "the whole
lifetime of the driver" but only the timeframe between backlight probe and panel
probe. Thanks for clarifying this. I suppose I agree with this policy now.

So remains the question: why does the panel not enable backlight on my device?

I will investigate this later today. Hopefully, the probe defer logic should
prevent a case where the panel is probed (to the end) before the backlight
driver.

> > > > After investigating, it appears that the pwm_bl driver is enforcing a
> > > > policy
> > > > on
> > > > heavily relying on the backlight initial state
> > > > (pwm_backlight_initial_power_state). To make it short, if backlight
> > > > wasn't
> > > > detected as already enabled by the bootloader, it's going to refuse to
> > > > enable it
> > > > during the whole lifetime of the driver.
> > > > 
> > > > This policy isn't exactly new (so I do realize that I'm a bit late to
> > > > the
> > > > party), but it went one step further this cycle by adding a check on the
> > > > PWM
> > > > state. This broke support for my nyan big, as the pwm driver does not
> > > > check
> > > > for
> > > > the previous state at probe time and reports it as disabled initially.
> > > > 
> > > > One could say that the driver has to be fixed to report that state (and
> > > > I
> > > > agree
> > > > it is a desirable thing to do), but I think it is a symptom of a broader
> > > > issue.
> > > > 
> > > > Basically, do we really want pwm_bl to behave this way? What is the
> > > > rationale
> > > > behind this decision, other than "because we can"? A strong argument
> > > > against
> > > > it
> > > > is that not all bootloaders have support for turning the backlight on
> > > > (that
> > > > is
> > > > definitely not the case on the omap3 sniper and omap4 kc1 boards with
> > > > upstream
> > > > U-Boot, that I introduced to mainline Linux).
> > > > 
> > > > Also, we can still expect the gpio/regulator/pwm drivers to be reset at
> > > > probe
> > > > time (and I also agree it's not necessarily a good thing, especially as
> > > > far
> > > > as
> > > > backlight is concerned, but that's the reality and dropping backlight
> > > > support in
> > > > those cases doesn't seem like an appropriate course of action). This
> > > > will
> > > > result
> > > > in pwm_bl assuming that backlight was not enabled by the bootloader and
> > > > thus
> > > > refuse to enable it at all times.
> > > > 
> > > > Comments and reactions are welcome, as I'd really like to find a sane
> > > > way to
> > > > resolve this problem.
> > > > 
> > > > Cheers!
> 
> regards
> Philipp
> 
-- 
Paul Kocialkowski, developer of free digital technology and hardware support

Website: https://www.paulk.fr/
Coding blog: https://code.paulk.fr/
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