Re: [PATCH v2] backlight: turn backlight on/off when necessary

From: Liu Ying
Date: Thu Jan 23 2014 - 04:53:55 EST

On 01/23/2014 05:27 PM, Liu Ying wrote:
> On 01/23/2014 01:44 PM, Jingoo Han wrote:
>> On Wednesday, January 22, 2014 6:36 PM, Jani Nikula wrote:
>>> On Mon, 20 Jan 2014, Liu Ying <Ying.Liu@xxxxxxxxxxxxx> wrote:
>>>> We don't have to turn backlight on/off everytime a blanking
>>>> or unblanking event comes because the backlight status may
>>>> have already been what we want. Another thought is that one
>>>> backlight device may be shared by multiple framebuffers. We
>>>> don't hope blanking one of the framebuffers may turn the
>>>> backlight off for all the other framebuffers, since they are
>>>> likely being active to display something. This patch adds
>>>> some logics to record each framebuffer's backlight usage to
>>>> determine the backlight device use count and whether the
>>>> backlight should be turned on or off. To be more specific,
>>>> only one unblank operation on a certain blanked framebuffer
>>>> may increase the backlight device's use count by one, while
>>>> one blank operation on a certain unblanked framebuffer may
>>>> decrease the use count by one, because the userspace is
>>>> likely to unblank a unblanked framebuffer or blank a blanked
>>>> framebuffer.
>>>> Signed-off-by: Liu Ying <Ying.Liu@xxxxxxxxxxxxx>
>>>> ---
>>>> v1 can be found at
>>>> v1->v2:
>>>> * Make the commit message be more specific about the condition
>>>> in which backlight device use count can be increased/decreased.
>>>> * Correct the setting for bd->props.fb_blank.
>>>> drivers/video/backlight/backlight.c | 28 +++++++++++++++++++++-------
>>>> include/linux/backlight.h | 6 ++++++
>>>> 2 files changed, 27 insertions(+), 7 deletions(-)
>> [.....]
>>> Anything backlight worries me a little, and there are actually three
>>> changes bundled into one patch here:
>>> 1. Changing bd->props.state and bd->props.fb_blank only when use_count
>>> changes from 0->1 or 1->0.
>>> 2. Calling backlight_update_status() only with the above change, and not
>>> on all notifier callbacks.
>>> 3. Setting bd->props.fb_blank always to either FB_BLANK_UNBLANK or
>>> FB_BLANK_POWERDOWN instead of *(int *)evdata->data.
> Since I have already post v3( to change the setting for bd->props.fb_blank, the idea of the 3rd point is not very appropriate any more.
>>> The rationale in the commit message seems plausible, and AFAICT the code
>>> does what it says on the box, so for that (and for that alone) you can
>>> have my
>>> Reviewed-by: Jani Nikula <jani.nikula@xxxxxxxxx>
>>> *BUT* it would be laborous to figure out whether this change in
>>> behaviour might regress some drivers. I'm just punting on that. And that
>>> brings us back to the three changes above - in a bisect POV it might be
>>> helpful to split the patch up. Up to the maintainers.
>> I agree with Jani Nikula's opinion.
>> Please split this patch into three patches as above mentioned.
> I am open to split the patch up.
> However, IMHO, this patch is somewhat self-contained.
> For example, if we try to create 2 patches for the 1st point and the 2nd point Jani mentioned, one patch would invent the use_count and call backlight_update_status() on all notifier callbacks(just
> ignore the use_count).
> Do you think this is a good patch?
> It also doesn't look straightforward for me to create 2 patches for the 1st point and the 3rd point.
^ Sorry, fix typo(2nd -> 3rd).

> Please advice.

To unsubscribe from this list: send the line "unsubscribe linux-kernel" in
the body of a message to majordomo@xxxxxxxxxxxxxxx
More majordomo info at
Please read the FAQ at