[Buildroot] Freescale i.mx Arm serial port cramfs and mdev

[Buildroot] Freescale i.mx Arm serial port cramfs and mdev

[Allen Shockley]

To the list,

Today is my 34th birthday and I what I want more than anything is some help
with this problem, so I can move on to the hard stuff.

A brief project history.

When using an i.mxl Arm processor and 2.4 Kernel with cramfs root disk, all
was calm and easily configured.

Necessary changes arose after a year and a half, (Specifically a change from
Master I2C to Slave), which I thought would be better undertaken on the
latest branch of the Kernel (and a much better mainline drivers).
I did not like my previous build system. If you call building each piece by
hand on the command line and creating a filesystem, a system.
I looked around and found a couple promising candidates. I settled on
BuildRoot for its simplicity of design and extensibility. (Kudos all around
fellas)
Penguintronix has a similar system ptxdist but its a little to hectic to
parse easily and I'm on a schedule.
After a few initial bad builds i was successful. (Damn that soft-float)

My problems started cropping up pretty early on. Mostly due to the fact that
the i.mx serial port has been redesignated ttySMX0 in the new kernel tree.
Bummer can't open tty1
Bummer can't open tty2
Bummer can't open tty3

I decide to go with the flow of things and work through it step by step.
which has led me through alternating between the above output repaeting
nonstop or
Can't open intitial console.

I hated the enormity of the /dev directory. Slowly I was able to piece
enough information together to decide that udev is not very apealing, but
mdev sounds nice. So for around about 10 days I have been playing chicken
and the egg on a ro filesystem trying to uncover the secret on how to
populate the /dev entry for the initial console and still mount it in ram
(tmpfs) to use mdev -s to fill it with devices found in /sys.

If I comment out the device table entries pointing to /dev, I have no
console. If I populate it with the barest essentials needed for console, I
have no way to mount it as tmpfs.
The obvious solution is to mount it then add bare essentails and then use
mdev. I believe this would work if I was using a standard serial port but
right now I'm stuck trying to get it to see ttySMX0 again.

I am absolutely sure I am missing a very small detail somewhere but cannot
find it.

Any thoughts, or direction where I can get more information are welcomed.
Even suggestions that I'm being foolhardy for implementing mdev in a fixed
embedded system that never changes. I considered this, but there is an
attraction to the idea of the kernel filing the /dev entry over hard coding
the root disk device entries. I'd hate for it to lead to this problem on the
next kernel change.

I can post configs and copies of the device table and inittab and others if
you need it. (Unfortunately, I am building inside a VM and I'm away from it
now.)

Allen Shockley
ashockley at consumerengineering.com
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[Buildroot] Freescale i.mx Arm serial port cramfs and mdev

[Rob Landley]

On Tuesday 10 October 2006 11:33 am, Allen Shockley wrote:
> I hated the enormity of the /dev directory. Slowly I was able to piece
> enough information together to decide that udev is not very apealing, but
> mdev sounds nice. So for around about 10 days I have been playing chicken
> and the egg on a ro filesystem trying to uncover the secret on how to
> populate the /dev entry for the initial console and still mount it in ram
> (tmpfs) to use mdev -s to fill it with devices found in /sys.

Well, as the author of mdev I find it fairly straightforward to use, but I 
would, wouldn't I? :)

Just add a /dev/console to initramfs, then overmount /dev with a tmpfs 
(mount -t tmpfs /dev /dev), populate it with mdev (mount -t sysfs /sys /sys; 
mdev -s), and then if you're going to switch_root to another filesystem do a 
mount --move /dev new/dev (and either umount /sys or --move it too).

Your initramfs needs a /dev/console for the kernel to open stdin/stdout/stderr 
for init before it can exec pid1, but you don't need that after init is 
running.  I wouldn't bother to delete it either, switch_root should do that 
for you if you remove the overmount.  (Unless you're not going to call 
switch_root, in which case an rm statement in your init script is probably 
slightly bigger than the amount of memory you'll save by doing so. :P )

> If I comment out the device table entries pointing to /dev, I have no
> console. If I populate it with the barest essentials needed for console, I
> have no way to mount it as tmpfs.

A) You only need the one entry, which is two initramfs entries (one to 
create /dev and one to create /dev/console).

B) Just overmount it.

Or if you're staying with initramfs just call mdev -s, which doesn't mind a 
node it's trying to create already exists.

> The obvious solution is to mount it then add bare essentails and then use
> mdev. I believe this would work if I was using a standard serial port but
> right now I'm stuck trying to get it to see ttySMX0 again.

I don't know if the "console=" command line parsing actually needs a /dev 
entry in initramfs for the device you point it at.  I don't think it does, 
that would be in the linux kernel source's init directory, I think.

> I am absolutely sure I am missing a very small detail somewhere but cannot
> find it.

What exactly are you currently trying to do?

> Any thoughts, or direction where I can get more information are welcomed.

Ask me. :)

> Even suggestions that I'm being foolhardy for implementing mdev in a fixed
> embedded system that never changes.

Nah, that's part of what it's for.  It's small and simple.

> I considered this, but there is an 
> attraction to the idea of the kernel filing the /dev entry over hard coding
> the root disk device entries. I'd hate for it to lead to this problem on the
> next kernel change.

Yahtzee.  Bingo.  That sort of thing.

> I can post configs and copies of the device table and inittab and others if
> you need it. (Unfortunately, I am building inside a VM and I'm away from it
> now.)

Feel free.  In general mdev just looks at /sys and does a lot of mknods 
in /dev.  Setting up partitions and stuff is the caller's problem...

> Allen Shockley
> ashockley at consumerengineering.com

Rob
-- 
"Perfection is reached, not when there is no longer anything to add, but when 
there is no longer anything to take away." - Antoine de Saint-Exupery