MTD Partition problems

[Jörn Engel]

On Sat, 3 August 2002 14:24:32 -0500, Vipin Malik wrote:
> >Fact is that it is useless, the superuser can create a read only
> >device simply by 'chmod -w /dev/mtd0', if she wishes. That is all
> >those extra devices do, except for causing confusion and eating up
> >minor numbers.
> 
> So if I read you correctly, you are saying that the partitioning code 
> really works but in a quirky way: the presence of duplicate read only 
> paired devices, which really provide no useful functionality.

Yes.

> However, if one ignores the duplicated devices, one can still use 
> partitioning and mount a JFFS2 fs on one of the partitions while having 
> something completely different on another and device access locking will be 
> taken care of by the mtd driver (which is essentially what partitioning 
> provides vs. just calling add_mtd() NUM_PARTITION times in your map file on 
> the same flash chip).

Yes. The partitioning code basically checks the limits and hands the
work to the base device with adjusted offset.

> Additionally, (say) /dev/mtdblock2 will really provide a handle to 
> /dev/mtd4 the writable third partition on the partitioned flash chip, thus 
> mounting the JFFS2 fs on the partition originally intended (the third 
> partition).

Almost. /dev/mtd4 should be minor number 8. If you check
/usr/src/linux/Documentation/devices.txt, you will read the following:
                  0 = /dev/mtd0         First MTD (rw)
                  1 = /dev/mtdr0        First MTD (ro)
Thus mtd0 has minor 0, mtd1 has minor 2, ...

Also, the third partition is /dev/mtd3, as /dev/mtd0 is the real
device. Look at the minor numbers for /dev/hda*, just the same.

But what you *meant* is correct.

> Unfortunately, my tests indicate that this is not happening. I know that 
> /dev/mtdblock2 (the intended third partition) is formatted. However JFFS2 
> finds data here and refuses to mount on it. I think it is really being 
> passes a handle to and accesses /dev/mtd2, which is the wrong partition.

See above.

> /dev/mtdblock[4,5] do not exist, even though /dev/mtd[4,5] do and /dev/mtd4 
> is the correct partition that I want!

See above.

> Argh! I'll try your patches if you like.

I would be happy if you did. But you don't need my patches, unless you
have more than one device that you want to partition. With the two
thinkos out of the way, you should be doing fine. :)

Jörn

-- 
Mundie uses a textbook tactic of manipulation: start with some
reasonable talk, and lead the audience to an unreasonable conclusion.
-- Bruce Perens