linux-kernel - Re: Git backlight subsystem tree

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Message-Id: <1170978623.5849.63.camel@localhost.localdomain>
Date:	Thu, 08 Feb 2007 23:50:23 +0000
From:	Richard Purdie <rpurdie@...ys.net>
To:	Greg KH <greg@...ah.com>
Cc:	James Simmons <jsimmons@...radead.org>,
	LKML <linux-kernel@...r.kernel.org>,
	akpm <akpm@...ux-foundation.org>,
	Marcin Juszkiewicz <openembedded@....one.pl>
Subject: Re: Git backlight subsystem tree

Hi Greg,

On Thu, 2007-02-08 at 13:23 -0800, Greg KH wrote:
> On Thu, Feb 08, 2007 at 06:32:02PM +0000, James Simmons wrote:
> > I CC Greg to explain. The backlight class didn't go away. The way it is 
> > handled is different.
> 
> Have a pointer to the patch so I can help explain better?

You've been cc'd on the proposed patch a couple of times in this thread
so it should be in your mailbox now.

> As a short summary, 'struct class_device' is going away.  Using a
> 'struct device' in its place is what the conversion should have just
> done, no functionality change otherwise.

The backlight class is drivers/video/backlight/backlight.c and if I
understand things correctly what shows up in sysfs changes.

At the moment (as I understand the code which could be wrong), the
backlight functionality is tagged onto an existing device. Taking
drivers/video/backlight/corgi_bl.c as an example, the corgi_bl device
exists under /sys/devices/platform/corgi_bl with an associated struct
device. The backlight code then appends some magic to this to link in
some class attributes that show up under /sys/class/backlight. There is
only ever one device.

By replacing class_device_register() with device_create(), the proposed
patch appears to generate a second struct device with the original as
its parent. I'm not sure where it appears in /sys/devices? Having
another full blown struct device around makes me uneasy as wherever it
appears, we only have one device, not two.

If I had some kind of platform device which had an LED, backlight and
say battery component and registered with the three appropriate classes
would that mean four struct devices? Where under /sys/devices do they
each appear?

Also, it was mentioned that a parent struct device is a requirement and
this isn't the case for all backlight users. I think this constraint on
device_create has been removed in more recent code though? 

Richard

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