[Buildroot] Question about using mdev for /dev management

[Michael S. Zick <minimod@morethan.org>]

On Tue December 13 2011, Arnout Vandecappelle wrote:
> On Monday 05 December 2011 19:45:29 Will Wagner wrote:
> > Firstly it replaces $(TARGET_DIR)/init with fs/cpio/init which attempts 
> > to mount devtmpfs. My kernel doesn't support this (it's too old) and I'd 
> > rather not get this trace in the boot (it worries other developers).
> 
>  I didn't realize that mdev worked without devtmpfs in the kernel...
>

It does, and it has for nearly a decade.  ;-)

With tmpfs sysfs available in the kernel and a tmpfs mounted on /dev -
an mdev -s will populate it for you.

It was only recent changes to the BR configuration dependences that make
(when looking at the BR menu) that mdev/udev requires devtmpfs.

That was just a decision by the project to move forward with the new
and drop the old but it isn't dropped in the kernel. ;-)

Mike
> If devtmpfs exists (and it does for all currently supported kernels), then
> it should be mounted as soon as possible.  It is normally mounted
> automatically by the kernel, except when it runs /init for an initramfs.
> That's why we have this extra /init script for the cpio rootfs.
> 
>  You could add a check to the init script that mounts a normal tmpfs on
> /dev if the mount fails (and redirect its output to /dev/null).
> 
> > I assume that for mdev we could not replace init but instead make sure 
> > that /dev/console (and possibly /dev/null?) exist?
> 
>  Yeah, actually, I think /dev/console and /dev/null should be created
> on the rootfs even if devtmpfs is used.  Which brings us to:
> 
> 
> > The other small issue I have is that mdev fails to spot one of the 
> > kernel devices (as the driver doesn't have a sysfs entry) so it needs 
> > adding manually. I do that by adding an entry to BR2_ROOTFS_DEVICE_TABLE 
> > which works fine, but doesn't seem ideal as I thought device entries 
> > were meant to be set in BR2_ROOTFS_STATIC_DEVICE_TABLE, but that is not 
> > offered unless static devices used. Is there a better way to do this or 
> > should I just leave it as is?
> 
>  If you ask me, it's OK to add /dev entries in the BR2_ROOTFS_DEVICE_TABLE.
> In fact, I think /dev/console and /dev/null should be put in there.  But
> I've never gotten around to roll a patch for it.
> 
>  Regards,
>  Arnout
>