[Buildroot] Regarding /dev management

[Thomas Petazzoni <thomas.petazzoni@free-electrons.com>]

Dear Ezequiel Garcia,

On Sat, 9 Mar 2013 09:26:36 -0300, Ezequiel Garcia wrote:

> Yesterday I spent a couple hours trying to build my first buildroot fs.
> In particular, I wanted to use devtmpfs
> (BR2_ROOTFS_DEVICE_CREATION_DYNAMIC_DEVTMPFS)
> but found that /dev is not mounted by default in skeleton fs scripts,
> it should be mounted in inittab before some other things.
> 
> After some hacking in skeleton files, I managed to get the thing working.
> 
> Did I miss something? Is this the expected behavior?
> 
> Notice that I'm using the cpio image output and using the rootfs as in
> initramfs.
> In this case, CONFIG_DEVTMPFS=y and CONFIG_DEVTMPFS_MOUNT=y
> kernel options do not matter, because... (quoting Kconfig)
> 
> ""This will instruct the kernel to automatically mount the
> devtmpfs filesystem at /dev, directly after the kernel has
> mounted the root filesystem. The behavior can be overridden
> with the commandline parameter: devtmpfs.mount=0|1.
> 
> This option does not affect initramfs based booting, here
> the devtmpfs filesystem always needs to be mounted manually
> after the roots is mounted.""
> 
> Perhaps we can add some hack to add:
> 
> ::sysinit:/bin/mount -t devtmpfs devtmpfs /dev
> 
> to skeleton when BR2_ROOTFS_DEVICE_CREATION_DYNAMIC_DEVTMPFS=y.
> 
> However, being new to buildroot, perhaps I'm speaking non-sense and the
> developer is expected to use its own custom skeleton in this case.

This problem of devtmpfs not being mounted by the kernel when booting
inside an initramfs is handled by a special init script, fs/cpio/init,
which gets automatically installed as /init in your root filesystem is
you use BR2_ROOTFS_DEVICE_CREATION_DYNAMIC_DEVTMPFS. See the code in
fs/cpio/cpio.mk and fs/cpio/init.

Best regards,

Thomas
-- 
Thomas Petazzoni, Free Electrons
Kernel, drivers, real-time and embedded Linux
development, consulting, training and support.
http://free-electrons.com