linux-kernel - Re: [PATCH 1/5] Backlight: Add backlight type

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Message-ID: <20101119202558.GA10898@srcf.ucam.org>
Date:	Fri, 19 Nov 2010 20:25:59 +0000
From:	Matthew Garrett <mjg59@...f.ucam.org>
To:	Andrew Morton <akpm@...ux-foundation.org>
Cc:	linux-kernel@...r.kernel.org, rpurdie@...ys.net,
	nouveau@...ts.freedesktop.org, dri-devel@...ts.freedesktop.org,
	intel-gfx@...ts.freedesktop.org, linux-acpi@...r.kernel.org,
	lenb@...nel.org
Subject: Re: [PATCH 1/5] Backlight: Add backlight type

On Fri, Nov 19, 2010 at 12:05:23PM -0800, Andrew Morton wrote:
> On Fri, 19 Nov 2010 10:53:52 -0500
> Matthew Garrett <mjg@...hat.com> wrote:
> 
> > There may be multiple ways of controlling the backlight on a given machine.
> > Allow drivers to expose the type of interface they are providing, making
> > it possible for userspace to make appropriate policy decisions.
> > 
> > ...
> >
> >  60 files changed, 102 insertions(+), 0 deletions(-)
> 
> This patch has a pretty short half-life.

Well, ideally it would have landed in the backlight tree when I sent it 
months ago. Then we'd have the opportunity to ensure that everything was 
fixed up before it went in in the merge window.

> > @@ -62,6 +68,8 @@ struct backlight_properties {
> >  	/* FB Blanking active? (values as for power) */
> >  	/* Due to be removed, please use (state & BL_CORE_FBBLANK) */
> >  	int fb_blank;
> > +	/* Backlight type */
> > +	enum backlight_type type;
> >  	/* Flags used to signal drivers of state changes */
> >  	/* Upper 4 bits are reserved for driver internal use */
> >  	unsigned int state;
> 
> And if/when the half-life expires, we'll have drivers in-tree which
> forget to set backlight_properties.type.  I haven't checked, but if
> we're lucky they will default to "0".

Depends entirely on whether they kzalloc the structure or not before
calling backlight_device_register(). 

> What will be the runtime effects upon such unconverted drivers? 
> Ideally we'd like them to continue to work OK, and to emit a runtime
> warning.  In which case you'll need BACKLIGHT_RAW=1 so the unconverted
> driver can be detected, warned about and fixed up by the core code.

The worst case I can think of is that we walk off the array - I guess 
there's an argument for sanity checking that in backlight_show_type().

-- 
Matthew Garrett | mjg59@...f.ucam.org
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