linux-kernel - Re: [PATCH] driver-core: devtmpfs - driver core maintained /dev tmpfs

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Message-Id: <20090501000353.b89caa8c.akpm@linux-foundation.org>
Date:	Fri, 1 May 2009 00:03:53 -0700
From:	Andrew Morton <akpm@...ux-foundation.org>
To:	Greg KH <greg@...ah.com>
Cc:	Kay Sievers <kay.sievers@...y.org>,
	linux-kernel <linux-kernel@...r.kernel.org>,
	Jan Blunck <jblunck@...e.de>
Subject: Re: [PATCH] driver-core: devtmpfs - driver core maintained /dev
 tmpfs

On Thu, 30 Apr 2009 23:55:27 -0700 Greg KH <greg@...ah.com> wrote:

> On Thu, Apr 30, 2009 at 11:43:12PM -0700, Andrew Morton wrote:
> > On Thu, 30 Apr 2009 23:17:01 -0700 Greg KH <greg@...ah.com> wrote:
> > 
> > > On Thu, Apr 30, 2009 at 10:29:00PM -0700, Andrew Morton wrote:
> > > > On Thu, 30 Apr 2009 15:23:42 +0200 Kay Sievers <kay.sievers@...y.org> wrote:
> > > > 
> > > > > From: Kay Sievers <kay.sievers@...y.org>
> > > > > Subject: driver-core: devtmpfs - driver core maintained /dev tmpfs
> > > > > 
> > > > > Devtmpfs lets the kernel create a tmpfs very early at kernel
> > > > > initialization, before any driver core device is registered. Every
> > > > > device with a major/minor will have a device node created in this
> > > > > tmpfs instance. After the rootfs is mounted by the kernel, the
> > > > > populated tmpfs is mounted at /dev. In initramfs, it can be moved
> > > > > to the manually mounted root filesystem before /sbin/init is
> > > > > executed.
> > > > 
> > > > Lol, devfs.
> > > 
> > > Well, devfs "done right" with hopefully none of the vfs problems the
> > > last devfs had. :)
> > 
> > I think Adam Richter's devfs rewrite (which, iirc, was tmpfs-based)
> > would have fixed up these things.  But it was never quite completed and
> > came when minds were already made up.
> > 
> > I don't understand why we need devfs2, really.  What problems are
> > people having with teh existing design?
> 
> Boot speed, boot speed, boot speed.
> 
> Oh, and reduction in complexity in init scripts, and saving embedded
> systems a lot of effort to implement a dynamic /dev properly (have you
> _seen_ what Android does to keep from having to ship udev?  It's
> horrible...)

Why can't they ship udev?

> > > > >  block/bsg.c                         |    6 
> > > > >  drivers/gpu/drm/drm_sysfs.c         |    7 
> > > > >  drivers/input/input.c               |    6 
> > > > >  drivers/media/dvb/dvb-core/dvbdev.c |   10 +
> > > > >  drivers/usb/core/usb.c              |   11 +
> > > > 
> > > > These five subsystems were updated, but there are so many others.  Why
> > > > these five in particular?
> > > 
> > > These are the ones that create a subdirectory in /dev/   None of the
> > > others do.
> > 
> > oic.
> > 
> > Where is it determined that these subsystems create /dev subdirectories? 
> > udev rules?  If so, do we need to henceforth keep devfs2 (sorry, I
> > can't resist) in sync with udev?
> 
> No, with this, udev rules can get simpler and remove these directory
> names, keeping them only in one place, preventing anything from getting
> out of sync.

This assumes that devtmpfs is enabled in config, yes?

Does that means that we need two versions of udev out there, or can one
version be feasibly extended to handle both flavours of kernel?

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