Date:   Wed, 05 Jul 2017 13:07:53 +0200
From:   Philipp Zabel <p.zabel@...gutronix.de>
To:     Paul Kocialkowski <contact@...lk.fr>
Cc:     Daniel Thompson <daniel.thompson@...aro.org>,
        linux-pwm@...r.kernel.org, linux-kernel@...r.kernel.org,
        Thierry Reding <thierry.reding@...il.com>,
        Lee Jones <lee.jones@...aro.org>,
        Jingoo Han <jingoohan1@...il.com>,
        Linus Torvalds <torvalds@...ux-foundation.org>
Subject: Re: PWM backlight initial state assumptions, or how pwm_bl killed
 my (nyan) cat^W backlight support

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.

> > > 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

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