u-boot and root filesystems

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Hi,

I have some questions about u-boot for a project I'm working on. The uP
is an ARM9.

I am wondering if it's possible to use u-boot to two things in one
boot-up:
- to create a root filesystem on an SD card (I'm not interested in the
secure digital functionality)
- to boot Linux using a Linux kernel image from inside this newly
created root filesystem

Another question is how small can a functional rootfs be? I am hoping I
can get it under 20 MB.

And a final question is: Is it possible to have a read-only filesystem?
Or do some things (ie. /dev devices) have to be read/write?

Thanks, Pete


Re: u-boot and root filesystems

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You can have a bare bone system in 8M or so.  But there is not much you
can do other than starting a custom app.

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No, /dev can be read-only, but /tmp amd /var must be writable.

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Re: u-boot and root filesystems
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You can use U-boot to store a file system, not to create it.
Whether you can use your specific ARM9 or not is depending on
if u-boot supports the SD-interface on your chip.
If it doesn't then you have to write a driver.

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May beed modifications to U-boot, but I know of people that have done such
modifications.


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You can have a complete system with Linux kernel and small file system in 4
MB
but you would have very little room for your own application

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Yes, in a way,
if you use a RAM disk, then the underlying flash image is not writable.

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--
Best Regards,
Ulf Samuelsson
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Re: u-boot and root filesystems


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In answer to most of your questions, please refer to
<http://www-128.ibm.com/developerworks/library/pa-migrate2/ which is
an article I wrote last year on related topics.

It is irksome to run Linux off a write-protected root filesystem.
Possible, yes - with tweaking  - but it's much easier just to load the
root filesystem into a RAMdisk and run entirely out of RAM. Faster,
too.

As far as memory footprint goes, are you asking about flash storage
requirements or RAM requirements? You can fit a fully functional Linux
distribution and applications into 8Mb without difficulty. (The root
filesystem can be compressed, remember. I've implemented turnkey
x86-based Linux devices on a 4Mb CompactFlash). I would suggest 64Mb
RAM, more if you intend to use X. It can be squeezed into less, but
again it's irritating.


Re: u-boot and root filesystems

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There are no set rules for flash vs. ram.  Very often, it's a
trade off between costs and limitations.  Nowadays, flash
is sometimes cheaper than ram.  There are still many systems
with limited ram space, but flash is virtually unlimited.

You can compress everything on flash, but need more ram to
decompress and run them anyway.


Re: u-boot and root filesystems

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I worked on linux project last year, which may give you some
guidelines...  We had a very functional rootfs that fit snuggly into
4MB (with JFFS2).  We kept our app and additional packages (the most
notable large ones were Python and Qt) in a separate partition in flash
- that took up a little over 20MB (also JFFS2, and we got better than
2:1 compression).

Of course, it all depends on what you want...

I'm now again working on a linux project, and I've just the other day
used Busybox to get a minimal rootfs (NOTHING, except init and sh).
It's only 75K.  Of course, the caveat is you need glibc, too, and
that's 2MB.

-bri


Re: u-boot and root filesystems
Thanks everyone for the answers. Very useful info.

-- Pete

Brian Silverman wrote:
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