Re: How to set fops in "struct platform_pwm_backlight_data"?

[Mark Zhang]

On 10/23/2013 06:54 PM, Thierry Reding wrote:
> On Wed, Oct 23, 2013 at 06:31:30PM +0800, Mark Zhang wrote:
>> On 10/23/2013 05:09 PM, Thierry Reding wrote:
>>> On Wed, Oct 23, 2013 at 04:49:29PM +0800, Mark Zhang wrote:
>>>> On 10/23/2013 04:00 PM, Thierry Reding wrote:
>>>>> On Wed, Oct 23, 2013 at 10:16:24AM +0800, Mark Zhang wrote:
>>>>>> On 10/22/2013 08:49 PM, Thierry Reding wrote:
>>>>>>> On Tue, Oct 22, 2013 at 04:55:09PM +0800, Mark Zhang wrote:
>>>>>>>> On 10/22/2013 03:24 PM, Thierry Reding wrote:
>>>>>>>>> On Fri, Oct 18, 2013 at 12:48:12PM +0800, Mark Zhang wrote:
>>>>>>>> [...]
>>>>>>>>>>>
>>>>>>>>>>
>>>>>>>>>> Okay, I just want to set the "notify" function pointer in "struct
>>>>>>>>>> platform_pwm_backlight_data", because I want to tune the brightness
>>>>>>>>>> value before the pwm-bl sets the brightness to hardware. I don't know
>>>>>>>>>> how to do that, unless we define the platform data explicitly.
>>>>>>>>>
>>>>>>>>> Okay, my question should have been what you need the functions for and
>>>>>>>>> why you think you need them.
>>>>>>>>>
>>>>>>>>
>>>>>>>> If I understanding you correctly, I suppose I've said that: "because I
>>>>>>>> want to tune the brightness value before the pwm-bl sets the brightness
>>>>>>>> to hardware".
>>>>>>>
>>>>>>> Why do you want to tune the brightness value? What are you trying to
>>>>>>> achieve?
>>>>>>>
>>>>>>
>>>>>> Oh, Tegra has a feature named PRISM(aka SmartDimmer). It changes the
>>>>>> color value to make the display looks bright so that we can reduce the
>>>>>> backlight brightness to save power. So everytime PRISM is triggered, we
>>>>>> call "backlight_update_status", then in the "notify" callback, we change
>>>>>> the brightness value which pwm-bl provides by considering the PRISM
>>>>>> compensations.
>>>>>
>>>>> If you automatically call backlight_update_status() everytime PRISM
>>>>> gives you new data, can't you just pass the correct value in in the
>>>>> first place so that you don't have to tweak it in the .notify()
>>>>> callback?
>>>>
>>>> OK, how to do that? -- "pass the correct value in in the first place"?
>>>
>>> Well, if you call backlight_update_status(), then you can pass in a
>>> brightness value, right? You usually do that by setting the backlight's
>>> props.brightness field.
>>>
>>> So when PRISM gives you new data, you could just read out the current
>>> brightness, compute the new one based on the current one and the PRISM
>>> data, set the props.brightness field to that value and then call
>>> backlight_update_status().
>>>
>>
>> The param of "backlight_update_status" is "struct backlight_device *".
>> So you mean after I get a pointer of correct backlight device, just set
>> the brightness value I want to it's "props.brightness" directly?
> 
> Yes.
> 
>> I think the "struct backlight_device" should be opaque(although it's
>> definition is in include/linux/backlight.h, I know that), so it's better
>> not to touch it's member directly, that's why I wanna use that "notify"
>> callback.
> 
> Well, backlight_update_status() is the only way to change the brightness
> of a backlight. If nobody ever calls backlight_update_status() then the
> .notify() callback will never be called either.
> 

Yes, so this is exactly my understanding before I posted this thread. I
think "struct backlight_device" is a handle, we get it and call
"backlight_update_status", then if we wanna change it's default behavior
or brightness value, we overwrite the fops it provides.

> By the way, what method do you use to control the backlight brightness?
> Do you use the sysfs interface from userspace?
> 

Yes.

Mark
> Thierry
> 
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