[yocto] busybox and mdev, how does it really work?

[Hans Beckérus]

On Tue, Jan 21, 2014 at 12:10 PM, Hans Beckérus <hans.beckerus at gmail.com> wrote:
> On Tue, Jan 21, 2014 at 11:23 AM, Anders Darander <anders at chargestorm.se> wrote:
>> * Hans Beckérus <hans.beckerus at gmail.com> [140121 11:05]:
>>> Things looked ok for a while, /dev was populated properly after boot
>>> and all necessary file systems / mount points were created.
>>> We were using an ext2 fs in RAM for mounting /.
>>> Now, we made a change to instead use a CPIO image compressed using xz
>>> and enabled the support in kernel to handle this.
>>> This is when our problems started :( Suddenly our system booted with
>>> just a very minimalistic /dev folder, containing basically only a few
>>> of the devices probed at boot time.
>>
>>> So, the questions now are:
>>
>>> - how was /dev populated before when there was no /etc/init.d/mdev?
>>
>> I haven't checked poky-tiny, or more specifically the kernel
>> configurations for it. But based on the description that I cited above,
>> I'd guess that poky-tiny has
>>         CONFIG_DEVTMPFS=y
>>         CONFIG_DEVTMPFS_MOUNT=y
>> enabled.
>>
> Indeed  (from our .config):
>
> CONFIG_DEVTMPFS=y
> CONFIG_DEVTMPFS_MOUNT=y
>
>> That would mean the the kernel itself were managing /dev for you when
>> you were using ext2. Unfortunately, CONFIG_DEVTMPFS_MOUNT do only
>> automount devmptfs on /dev if you're not in an initramfs (or initrd).
>>
> Sounds reasonable. But, when moving from ext2.gz to cpio.xz, we still
> did not add any init script for mounting /dev, yet it is still created
> and populated? But only with a very limited set of devices.
> I guess there is a difference when the kernel mounts the cpio.xz image
> compared to expanding the ext2.gz in RAM with respect to how /dev is
> handled. But I am only guessing here.
>
Another thing, You say CONFIG_DEVTMPFS_MOUNT does *not* automount /dev
if using the initrd scheme?
But I think it did when using the compressed ext2 file system? How
come /dev was mounted and populated automatically in the ext2 case but
when we changed to cpio.xz things seem to break?


>> Thus, you should add a simple initscript which mounts devtmpfs on /dev.
>> Thereafter, you should be fine.
>>
> Well, using the one that came from busybox-mdev rendered our /dev
> completely useless :(
> As I said, if I reduce the script to perform only 'mdev -s', then it
> works a lot better. So mounting /dev again seems like a bad idea.
>
>> Regarding your other questions on mdev, it's been a little while since I
>> used mdev. Let us know if the ideas above is enough, or if you want to
>> dig more into mdev.
>>
> mdev as a device manager I think we have under control, the confusion
> is more in our current system behavior at boot.
>
>> Cheers,
>> Anders
>>
>> --
>> Anders Darander
>> ChargeStorm AB / eStorm AB