Messages in this thread | | | From | Rob Landley <> | Subject | Re: [PATCH] mtd: allow mtd and jffs2 when ARCH=um | Date | Wed, 15 Dec 2010 00:31:18 -0600 |
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On Tuesday 14 December 2010 19:19:05 Jason Lunz wrote: > On Tue, Dec 14, 2010 at 06:49:02PM -0600, Rob Landley wrote: > > The problem is that jffs2 is a filesystem, and thus something people > > would really like to be able to loopback mount, but it's hardwired to > > assume it's only ever stored on a certain type of hardware, and thus > > requies incestuous knowledge of the erase granularity of the flash layer > > in order to function. > > I assume you can turn your jffs2 image file into a block dev using > losetup, then turn the corresponding loop device into an mtd device > using block2mtd, at which point you ought to be able to mount it with > jffs2. I've never tried it.
That is awesome and I'm not finding any documentation on it... Ah:
http://wiki.maemo.org/Modifying_the_root_image#Block_device_emulating_an_MTD_device
Wow that's awkward. Let's see, that says...
mknod /tmp/mtdblock0 b 31 0 modprobe loop losetup /dev/loop0 rootfs.jffs2 modprobe mtdblock modprobe block2mtd # Note the ,128KiB is needed (on 2.6.26 at least) to set the # eraseblock size. echo "/dev/loop0,128KiB" > /sys/module/block2mtd/parameters/block2mtd modprobe jffs2 mount -t jffs2 /tmp/mtdblock0 /media/jffs2
So the system isn't automatically loading mtdblock, udev isn't creating any /dev nodes for it even thought it is for loop, the associations are created by writing strings into a filesystem with a _notoriously_ unstable API (and it expects a three letter mixed case suffix to specify units), and despite "mtdblock0" I have no idea how that echo syntax would specify more than one association at a time.
I'm trying to figure out whether creating a shell script for this or trying to modify the busybox mount command would be a better approach to beating some sort of usability out of doing this.
Hmmm, mkfs.jffs2 is named as a mkfs but acts a a generator ala genext2fs. How about...
mkdir empty mkfs.jffs2 -r empty -o rootfs.jffs2 -e 128 -l -n
And it created an empty (zero byte) file. That's nice. If I touch a file in "empty" now 116 bytes. Adding -p makes it one erase block, but adding "-p 2048" doesn't specify a multiple of the erase block size, it instead rounded rounded it _down_ to that many bytes. But --pad=$((2048*1024*1024)) was apparently ignored...? How do I specify a _size_ for this thing so it has empty space I can write into after the fact? (The man page says pad with 0xFF. Am I going to have to do this by hand?)
mkdir empty touch empty/hello mkfs.jffs2 -r empty -o temp.jffs2 -e 128 -l -n -p python -c "import sys; sys.stdout.write(131072*63*chr(0xff))" >> temp.jffs2 losetup /dev/loop0 temp.jffs2 echo "/dev/loop0,128KiB" > /sys/module/block2mtd/parameters/block2mtd mount -t jffs2 mtdblock0 empty
Yay! I have a filesystem! And df claims it's 8 megabytes.
Woot.
> > What any of this has to do with UML is an open question. I don't want to > > require UML to loopback mount a jffs2 image, I want to be able to do it > > from my host. From my perspective, you're solving the wrong problem. > > There's more than just loopback-mounting jffs2 images. I use this to > make entire uml+jffs2 virtual machines for testing purposes.
As I said, I'm almost exclusively using qemu/kvm these days, but this is fun...
> Jason
Rob -- GPLv3: as worthy a successor as The Phantom Menace, as timely as Duke Nukem Forever, and as welcome as New Coke.
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