linux-kernel - Re: [PATCH/RFC] platform: Add support for automatic device IDs

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Date:	Fri, 27 Jul 2012 21:37:23 +0200
From:	Jean Delvare <khali@...ux-fr.org>
To:	Greg Kroah-Hartman <gregkh@...uxfoundation.org>
Cc:	LKML <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH/RFC] platform: Add support for automatic device IDs

Hi Greg,

On Fri, 27 Jul 2012 11:21:55 -0700, Greg Kroah-Hartman wrote:
> On Fri, Jul 27, 2012 at 01:46:25PM +0200, Jean Delvare wrote:
> > Right now we have support for explicit platform device IDs, as well as
> > ID-less platform devices when a given device type can only have one
> > instance. However there are cases where multiple instances of a device
> > type can exist, and their IDs aren't (and can't be) known in advance
> > and do not matter. In that case we need automatic device IDs to avoid
> > device name collisions.
> > 
> > I am using magic ID value -4 for this (I left -2 and -3 free in case
> > we ever need a couple of other magic values.) The automatically
> > allocated device IDs are global (to avoid an additional per-driver
> > cost) and are stored internally as negative numbers, starting with -4.
> > This is required so that the IDs can be freed later. Externally the
> > positive value is used.
> > 
> > Signed-off-by: Jean Delvare <khali@...ux-fr.org>
> > Cc: Greg Kroah-Hartman <gregkh@...uxfoundation.org>
> > ---
> > If anyone has a problem with the -4 or using negative device IDs
> > internally, it would be possible to avoid that by adding a boolean
> > attribute to every platform device to record whether the ID needs to
> > be freed. This would cost some memory.
> 
> A boolean is "free" don't worry about that.  Don't overload the device
> id with "magic" values like this, I don't like that at all.

I suspected so...

> I'd prefer
> to see another field added if this is really going to be needed.

OK, I'll send a different implementation in a moment. Note though that
PLATFORM_DEVID_AUTO will have to stay, as we still need a way to tell
the platform subsystem that we want an automatic ID.

Thanks,
-- 
Jean Delvare
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