Re: [RFC] pwm-backlight: Allow backlight to remain disabled on boot

[Ajay kumar]

Hi Thierry,

Thanks for adding me.

On Thu, Jul 31, 2014 at 5:26 PM, Thierry Reding
<thierry.reding@xxxxxxxxx> wrote:
> Cc'ing Ajay (who raised this on a different thread recently), therefore
> quoting all of the original email.
>
> Thierry
>
> On Thu, Jul 31, 2014 at 01:42:50PM +0200, Thierry Reding wrote:
>> From: Thierry Reding <treding@xxxxxxxxxx>
>>
>> The default for backlight devices is to be enabled immediately when
>> registering with the backlight core. This can be useful for setups that
>> use a simple framebuffer device and where the backlight cannot otherwise
>> be hooked up to the panel.
>>
>> However, when dealing with more complex setups, such as those of recent
>> ARM SoCs, this can be problematic. Since the backlight is usually setup
>> separately from the display controller, the probe order is not usually
>> deterministic. That can lead to situations where the backlight will be
>> powered up and the panel will show an uninitialized framebuffer.
>>
>> Furthermore, subsystems such as DRM have advanced functionality to set
>> the power mode of a panel. In order to allow such setups to power up the
>> panel at exactly the right moment, a way is needed to prevent the
>> backlight core from powering the backlight up automatically when it is
>> registered.
>>
>> This commit introduces a new boot_off field in the platform data (and
>> also implements getting the same information from device tree). When set
>> the initial backlight power mode will be set to "off".
>>
>> Signed-off-by: Thierry Reding <treding@xxxxxxxxxx>
>> ---
>> I've been meaning to send this for a while but was always holding back
>> because of the indoctrination that this type of configuration shouldn't
>> be part of device tree. However this issue was recently raised again in
>> the context of power up sequences for display panels. As described above
>> the issue is that panel datasheets recommend that the backlight attached
>> to a panel be turned on at the very last step to avoid visual glitches
>> during the panel's power up sequence. With the current implementation it
>> is typical for the backlight to be probed before the display panel. That
>> has, in many cases, the side-effect of enabling the backlight, therefore
>> making the screen content visible before it's actually initialized.
>>
>> Some panels come up with random garbage when uninitialized, others show
>> all white. With some luck the panel will be all black and users won't
>> really notice.
Right, most of the DSI panels are lucky. But the case is not the same
with few eDP/LVDS panels.

>> This patch is an attempt to enable boards to override the default of
>> turning on the backlight for the pwm-backlight driver. I'm not sure if
>> there was a specific reason to turn on the backlight by default when
>> this driver was initially written, but the fact is that since it has
>> pretty much always been like this we can't really go and change the
>> default, otherwise a lot of people may end up with no backlight and no
>> clue as to how to enable it. So the only reasonable thing we can do is
>> to keep the old behaviour and give new boards a way to override it if
>> they know that some other part of the stack will enable it at the right
>> moment.
Agreed! And, I have tested this patch.
This resolves the only remaining problem for display on peach_pi.
I cannot see the boot time glitch anymore :)

Ajay

>>  .../devicetree/bindings/video/backlight/pwm-backlight.txt         | 1 +
>>  drivers/video/backlight/pwm_bl.c                                  | 8 ++++++++
>>  include/linux/pwm_backlight.h                                     | 2 ++
>>  3 files changed, 11 insertions(+)
>>
>> diff --git a/Documentation/devicetree/bindings/video/backlight/pwm-backlight.txt b/Documentation/devicetree/bindings/video/backlight/pwm-backlight.txt
>> index 764db86d441a..65e001a1733d 100644
>> --- a/Documentation/devicetree/bindings/video/backlight/pwm-backlight.txt
>> +++ b/Documentation/devicetree/bindings/video/backlight/pwm-backlight.txt
>> @@ -17,6 +17,7 @@ Optional properties:
>>                 "pwms" property (see PWM binding[0])
>>    - enable-gpios: contains a single GPIO specifier for the GPIO which enables
>>                    and disables the backlight (see GPIO binding[1])
>> +  - backlight-boot-off: keep the backlight disabled on boot
>>
>>  [0]: Documentation/devicetree/bindings/pwm/pwm.txt
>>  [1]: Documentation/devicetree/bindings/gpio/gpio.txt
>> diff --git a/drivers/video/backlight/pwm_bl.c b/drivers/video/backlight/pwm_bl.c
>> index d7a3d13e72ec..62adfc9d37a7 100644
>> --- a/drivers/video/backlight/pwm_bl.c
>> +++ b/drivers/video/backlight/pwm_bl.c
>> @@ -173,6 +173,8 @@ static int pwm_backlight_parse_dt(struct device *dev,
>>               data->max_brightness--;
>>       }
>>
>> +     data->boot_off = of_property_read_bool(node, "backlight-boot-off");
>> +
>>       return 0;
>>  }
>>
>> @@ -317,6 +319,12 @@ static int pwm_backlight_probe(struct platform_device *pdev)
>>       }
>>
>>       bl->props.brightness = data->dft_brightness;
>> +
>> +     if (data->boot_off)
>> +             bl->props.power = FB_BLANK_POWERDOWN;
>> +     else
>> +             bl->props.power = FB_BLANK_UNBLANK;
>> +
>>       backlight_update_status(bl);
>>
>>       platform_set_drvdata(pdev, bl);
>> diff --git a/include/linux/pwm_backlight.h b/include/linux/pwm_backlight.h
>> index efdd9227a49c..1fc14989da4a 100644
>> --- a/include/linux/pwm_backlight.h
>> +++ b/include/linux/pwm_backlight.h
>> @@ -15,6 +15,8 @@ struct platform_pwm_backlight_data {
>>       unsigned int *levels;
>>       /* TODO remove once all users are switched to gpiod_* API */
>>       int enable_gpio;
>> +     bool boot_off;
>> +
>>       int (*init)(struct device *dev);
>>       int (*notify)(struct device *dev, int brightness);
>>       void (*notify_after)(struct device *dev, int brightness);
>> --
>> 2.0.3
>>
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