Support of removable MTD devices and other advanced features (follow-up from lkml)

[Jörn Engel]

On Sat, 24 May 2008 20:41:17 -0700, Alex Dubov wrote:
> --- Jörn Engel <joern at logfs.org> wrote:
> 
> > Writes happen in multiples of mtd->writesize.  Which for NAND is
> > pagesize.  There are also special cases with subpage writes.  AFAIK only
> > UBI exploits that feature.
> 
> Most xd cards can only be written a whole PEB in a time (can be handled with
> appropriate writesize, I suppose).

Up to 256M writing pages at a time worked for me.  1G didn't work
anymore.  Whether this was due to your requirement above or something
else I didn't work out.

> Memorystick cards can be written page at a time, but only in progressive
> fashion - only if all pages at lower offsets to the current page were written
> before. This can be made to work as a useful optimization.

The same limitation is true for some raw NAND chips as well.  Afaik all
of current MTD honors that limitation.

> Are there any special tricks with subpage writes or it all amounts to "read
> block" -> "merge changes" -> "write block"?

Subpage writes are special.  On some chips less than a page can be
written at once, e.g. 512 bytes for a 2k page chip.  But there are
additional limitations on the numbers of writes.  The 2k chip may limit
you to 3 writes.  So you have to do something like 512, 512, 1k.

Unless you really really need this, you'd better forget about it.

Jörn

-- 
I can say that I spend most of my time fixing bugs even if I have lots
of new features to implement in mind, but I give bugs more priority.
-- Andrea Arcangeli, 2000