linux-kernel - Re: How to set fops in "struct platform_pwm_backlight

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Message-ID: <20131023105456.GB15082@ulmo.nvidia.com>
Date:	Wed, 23 Oct 2013 12:54:57 +0200
From:	Thierry Reding <thierry.reding@...il.com>
To:	Mark Zhang <nvmarkzhang@...il.com>
Cc:	rpurdie@...ys.net, jg1.han@...sung.com,
	Jean-Christophe PLAGNIOL-VILLARD <plagnioj@...osoft.com>,
	tomi.valkeinen@...com, linux-pwm@...r.kernel.org,
	"linux-fbdev@...r.kernel.org" <linux-fbdev@...r.kernel.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: Re: How to set fops in "struct platform_pwm_backlight_data"?

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.

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

Thierry

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