On Thu, 31 Jul 2014, Thierry Reding wrote:

From: Thierry Reding <redacted>

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 <redacted>
---
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.

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.

 .../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(+)

Some people on the list are talking about this again.

What was the verdict?

-- 
Lee Jones
Linaro STMicroelectronics Landing Team Lead
Linaro.org ? Open source software for ARM SoCs
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