Re: 2.6.21-rc1 dims my LCD

Re: 2.6.21-rc1 dims my LCD

[Richard Purdie]

On Mon, 2007-02-26 at 16:24 +0100, Jiri Kosina wrote:
> On Mon, 26 Feb 2007, Richard Purdie wrote:
> 
> > Jiri: I've appended a patch that should already be queued, could you
> > test and see if it solves the problem.
> 
> Thanks. In the meantime I have gone through the code and I can confirm 
> that this is the root cause of what I am observing.
> 
> Now regarding the patch - at the time when the dim happened previously, 
> currently there is a observable blink (after which the brightness is 
> correct). 

The reason for the behaviour is that its turning the backlight down/off
to save power as it thinks the screen is blank. The blink therefore
makes sense as far as the backlight class is concerned and its working
as intended now.

> I have put some debugging printk() into fb_notifier_callback(), 
> and it turns out that on FB_EVENT_CONBLANK, there are two successive calls 
> to backlight_update_status(), second immediately following the first one:
> 
> Feb 26 15:11:14 thunder kernel: calling backlight_update_status() with bd->props.fb_blank == 1, bd->props.brightness == 0
> Feb 26 15:11:14 thunder kernel: calling backlight_update_status() with bd->props.fb_blank == 0, bd->props.brightness == 0
> 
> Is this really a right thing to do?

This should come from two calls to fbcon_generic_blank() in
drivers/video/console/fbcon.c with a different blank parameter and is
therefore in a way outside the scope of the backlight class.

It would be interesting to know why it decides to blank then immediately
unblank the display though. I've cc'd the fbdev-devel list.

Richard



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Re: 2.6.21-rc1 dims my LCD

[Antonino A. Daplas]

On Mon, 2007-02-26 at 15:43 +0000, Richard Purdie wrote:
> On Mon, 2007-02-26 at 16:24 +0100, Jiri Kosina wrote:
> > On Mon, 26 Feb 2007, Richard Purdie wrote:
> > 
> > > Jiri: I've appended a patch that should already be queued, could you
> > > test and see if it solves the problem.
> > 
> > Thanks. In the meantime I have gone through the code and I can confirm 
> > that this is the root cause of what I am observing.
> > 
> > Now regarding the patch - at the time when the dim happened previously, 
> > currently there is a observable blink (after which the brightness is 
> > correct). 
> 
> The reason for the behaviour is that its turning the backlight down/off
> to save power as it thinks the screen is blank. The blink therefore
> makes sense as far as the backlight class is concerned and its working
> as intended now.
> 
> > I have put some debugging printk() into fb_notifier_callback(), 
> > and it turns out that on FB_EVENT_CONBLANK, there are two successive calls 
> > to backlight_update_status(), second immediately following the first one:
> > 
> > Feb 26 15:11:14 thunder kernel: calling backlight_update_status() with bd->props.fb_blank == 1, bd->props.brightness == 0
> > Feb 26 15:11:14 thunder kernel: calling backlight_update_status() with bd->props.fb_blank == 0, bd->props.brightness == 0
> > 
> > Is this really a right thing to do?

Well, no, the console should not blank and immediately unblank. Of
course, the console can blank after some time and can be induced to
unblank after a keyboard event, for example.

Besides the debugging printk's, can you also add something like
WARN_ON(1); so you can get a tracing too.

Tony



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