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

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Message-ID: <dea10517-11f7-0cca-fa2d-f7bc18501384@linaro.org>
Date:   Wed, 5 Jul 2017 11:24:41 +0100
From:   Daniel Thompson <daniel.thompson@...aro.org>
To:     Paul Kocialkowski <contact@...lk.fr>, linux-pwm@...r.kernel.org,
        linux-kernel@...r.kernel.org
Cc:     Thierry Reding <thierry.reding@...il.com>,
        Lee Jones <lee.jones@...aro.org>,
        Jingoo Han <jingoohan1@...il.com>,
        Philipp Zabel <p.zabel@...gutronix.de>,
        Linus Torvalds <torvalds@...ux-foundation.org>
Subject: Re: PWM backlight initial state assumptions, or how pwm_bl killed my
 (nyan) cat^W backlight support

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!


Daniel.


> 
> 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!
>