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

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Date:	Wed, 23 Oct 2013 12:58:22 +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 06:46:17PM +0800, Mark Zhang wrote:
> On 10/23/2013 06:36 PM, 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().
> >>
> > 
> > Okay, anyway, I got the idea. Actually it's simpler. :)
> > I'm just curious that if we can do that in this way, why we need
> > "notify" anymore?
> 
> Oh, by the way, how about "check_fb" fops? Is there some kind of
> substitution as well? I mean, if I wanna set "check_fb" and also want to
> define the backlight device via DT, is there a way to do that?

No, there's no substitution for that. .check_fb() is used to verify that
a backlight device is associated with a framebuffer device. There are
better ways to check for that with DT, although nothing has been merged
into mainline yet.

Thierry

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