Date:	Tue, 29 Jan 2013 13:30:25 +0100
From:	Thierry Reding <thierry.reding@...onic-design.de>
To:	Peter Ujfalusi <peter.ujfalusi@...com>
Cc:	Richard Purdie <rpurdie@...ys.net>,
	Grant Likely <grant.likely@...retlab.ca>,
	Rob Landley <rob@...dley.net>,
	Florian Tobias Schandinat <FlorianSchandinat@....de>,
	Andrew Morton <akpm@...ux-foundation.org>,
	devicetree-discuss@...ts.ozlabs.org, linux-doc@...r.kernel.org,
	linux-kernel@...r.kernel.org, linux-fbdev@...r.kernel.org
Subject: Re: [PATCH 1/4] pwm_backlight: Fix PWM levels support in non DT case

On Tue, Jan 29, 2013 at 01:10:19PM +0100, Peter Ujfalusi wrote:
> On 01/29/2013 11:17 AM, Thierry Reding wrote:
> > On Tue, Jan 29, 2013 at 09:17:04AM +0100, Peter Ujfalusi wrote:
> >> On 01/28/2013 10:01 PM, Thierry Reding wrote:
> >>> On Tue, Jan 22, 2013 at 02:39:53PM +0100, Peter Ujfalusi wrote:
> >>>> It is expected that board files would have:
> >>>> static unsigned int bl_levels[] = { 0, 50, 100, 150, 200, 250, };
> >>>>
> >>>> static struct platform_pwm_backlight_data bl_data = {
> >>>> 	.levels = bl_levels,
> >>>> 	.max_brightness = ARRAY_SIZE(bl_levels),
> >>>> 	.dft_brightness = 4,
> >>>> 	.pwm_period_ns = 7812500,
> >>>> };
> >>>>
> >>>> In this case the max_brightness would be out of range in the levels array.
> >>>> Decrement the received max_brightness in every case (DT or non DT) when the
> >>>> levels has been provided.
> >>>
> >>> What's wrong with specifying .max_brightness = ARRAY_SIZE(bl_levels) - 1
> >>> instead?
> >>
> >> There is nothing wrong with that either but IMHO it is more natural for board
> >> files to use just ARRAY_SIZE(bl_levels). In this way the handling of
> >> data->max_brightness among non DT and DT booted kernel is more uniform in the
> >> driver itself.
> > 
> > The .max_brightness field is defined to be the maximum value that you
> > can set the brightness to. If you use .levels and set .max_brightness to
> > ARRAY_SIZE() then that's no longer true because the maximum value for
> > the brightness will actually be ARRAY_SIZE() - 1.
> 
> Yes, in conjunction with .levels it would be better to have .nr_levels instead
> reusing the max_brightness.
> 
> > The reason why in the DT case we decrement .max_brightness is that it is
> > used as a temporary variable to keep the number of levels, which
> > corresponds to ARRAY_SIZE() in your example, and adjust it later on to
> > match the definition. That's an implementation detail.
> > 
> > Platform data content should be encoded properly without knowledge of
> > the internal implementation.
> > 
> >> Right now all board files are using only the .max_brightness to specify the
> >> maximum value, I could not find any users of .levels in the kernel.
> > 
> > Yes. But this is the legacy mode which should be considered deprecated.
> > The reason why the concept of brightness-levels was introduced back when
> > the DT bindings were created was that pretty much no hardware in
> > existence actually works that way. This was a topic of discussion at the
> > time when I first proposed these changes, see for example:
> > 
> > 	http://www.mail-archive.com/devicetree-discuss@lists.ozlabs.org/msg09472.html
> 
> Right. Now I know the background.
> However I do not agree with the conclusion. Probably it is fine in some cases
> to limit the number of configurable duty cycles to have only distinct steps.
> To not go too far, on my laptop I have:
> # cat /sys/class/backlight/acpi_video0/max_brightness
> 15
> # cat /sys/class/backlight/intel_backlight/max_brightness
> 93

FWIW, I have hardware with an Intel chipset that has max_brightness
13666. That doesn't mean all of these are necessarily valid or useful.

> In this case I would be more happier if the user space would use the
> intel_backlight than the acpi_video0. I'm fine if user space decides to allow
> me only 15 distinct steps on the intel_backlight but I would expect it to do
> so in a way when I change the brightness in the UI it would step down or up to
> the next user configurable level. Right now it uses the acpi_video0 to control
> the levels which means that I have (ugly) jumps in the backlight every time I
> adjust it.
> 
> In my phone and tablet all transitions between backlight levels are smooth.
> This is not a case in my laptop (with exception when the backlight is set to
> auto, FW controlled).
> 
> The perceived brightness change depends on the surrounding environment, you
> can not say that in high level you would need less steps or you need to have
> less steps in low brightness. If you in a dark room the low changes can be
> observed, while the same change when you are outside in a sunny day would not
> reflect the same.
> 
> Sure we could do the ramps in driver, but what are the parameters? how many
> step between the two level? What is the time between the steps?
> 
> If you are about to make a product you will end up specifying all the possible
> steps to provide the best user experience. So if the PWM have 127 duty cycle
> you will have levels from 0, 1, 2, 3, ...., 125, 126, 127.

That's not true. The duty-cycle merely defines a percentage of how long
the PWM signal will be high. You can choose an arbitrary number of
subdivisions.

Since the brightness of a display isn't linearly proportional to the
duty cycle of the PWM driving it, representing that brightness by a
linear range is misleading. Furthermore some panels have regions where
the backlight isn't lit at all or where changes in the PWM duty-cycle
don't make any difference.

Thierry

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