MTD and EEPROM

MTD and EEPROM

[Ben Hutchings]

There is currently no MTD type defined for EEPROM - only RAM, ROM and
various types of flash.

Is there any particular reason for this?  Is it simply that EEPROMs tend
to be rather small and not worth putting filesystems on?

Ben.

-- 
Ben Hutchings, Staff Engineer, Solarflare
Not speaking for my employer; that's the marketing department's job.
They asked us to note that Solarflare product names are trademarked.

Re: MTD and EEPROM

[Artem Bityutskiy]

On Mon, 2013-04-15 at 15:48 +0100, Ben Hutchings wrote:
> There is currently no MTD type defined for EEPROM - only RAM, ROM and
> various types of flash.

I do not really know why, may be no one needed it?

> Is there any particular reason for this?  Is it simply that EEPROMs tend
> to be rather small and not worth putting filesystems on?

One may want to have access to raw EEPROM without having any
file-system. So bare /dev/mtdX could be useful.

-- 
Best Regards,
Artem Bityutskiy

Re: MTD and EEPROM

[Ben Hutchings]

On Mon, 2013-05-13 at 09:37 +0300, Artem Bityutskiy wrote:
> On Mon, 2013-04-15 at 15:48 +0100, Ben Hutchings wrote:
> > There is currently no MTD type defined for EEPROM - only RAM, ROM and
> > various types of flash.
> 
> I do not really know why, may be no one needed it?
> 
> > Is there any particular reason for this?  Is it simply that EEPROMs tend
> > to be rather small and not worth putting filesystems on?
> 
> One may want to have access to raw EEPROM without having any
> file-system. So bare /dev/mtdX could be useful.

Indeed, and the EEPROM on SFC4000 boards is already exposed as MTD by
the sfc driver.  It's labelled as type MTD_RAM.

The reason I asked was that I had became aware of this change:

commit dd02b67d5e9e7896891fa27eb5db65f55a290998
Author: Anatolij Gustschin <agust@denx.de>
Date:   Tue Jun 15 09:30:15 2010 +0200

    mtd: mtdchar: fix mmap for MTD RAM/ROM char devices
    
mmap() of an EEPROM would then do something very nasty, as the code in
mtdchar assumes that MTD_RAM indicates a device created by the mtdram
driver and uses its 'private' field accordingly.  I thought that it must
therefore be a bug for the sfc driver to use MTD_RAM.  (But I didn't
want to say too much immediately as this could be a security issue.)

I later realised that this code had more recently been disabled, so this
was not a bug in sfc:

commit f5cf8f07423b2677cebebcebc863af77223a4972
Author: David Woodhouse <David.Woodhouse@intel.com>
Date:   Tue Oct 9 15:08:10 2012 +0100

    mtd: Disable mtdchar mmap on MMU systems

I requested this be applied to the earlier stable branches, which was
accepted, so this should all be cleared up now.

Ben.

-- 
Ben Hutchings, Staff Engineer, Solarflare
Not speaking for my employer; that's the marketing department's job.
They asked us to note that Solarflare product names are trademarked.