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

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Date:	Thu, 23 Jan 2014 17:27:47 +0800
From:	Liu Ying <Ying.Liu@...escale.com>
To:	Jingoo Han <jg1.han@...sung.com>
CC:	'Jani Nikula' <jani.nikula@...ux.intel.com>,
	<linux-fbdev@...r.kernel.org>, <tomi.valkeinen@...com>,
	<plagnioj@...osoft.com>, <linux-kernel@...r.kernel.org>,
	<dri-devel@...ts.freedesktop.org>
Subject: Re: [PATCH v2] backlight: turn backlight on/off when necessary

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@...escale.com> 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@...escale.com>
>>> ---
>>> v1 can be found at https://lkml.org/lkml/2013/5/30/139
>>>
>>> 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(https://lkml.org/lkml/2014/1/22/126) 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@...el.com>
>>
>> *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 2nd point.

Please advice.

Regards,
Liu Ying

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