Re: MTD, generic physmap memory, MTD_BLOCK

[Shane Nay <shane@agendacomputing.com>]

> I did some work in the direction suggested by (thanks!) Nicolas--modified
> mtdram.c to act as a conduit device for plain memory.  I did get a
> successful boot from a cramfs rootdisk.
>
> When I said XIP, I didn't realize that this crowd had a very "serious"
> definition of it.  I simply meant I was trying to get around the initrd
> buffering issue:  As far as I can tell, up until now the only mechanism
> that a CPU (with only RAM and ROM visibility) has at its disposal to boot
> from is initrd, that would copy all the ROM image into RAM buffer cache..
> and only understand three file systems (cramfs is not one of them).  I
> don't mind cramfs using a buffer page (4k, LinuxPPC) or two to do its
> work--I was trying to slay the complete-image-copy daemon.  (My embedded
> system is 6 meg RAM, 8 meg ROM.. and I want shared libraries, etc)

It's 32k BTW.  Boot from initrd?..., initial romdisk?..., nope.  Try this 
instead..., /dev/rom.  Patches are on ftp.agendacomputing.com site.  Make a 
cramfs partition with the headers necessary to tag it as a cramfs partition 
(in the scripts/ directory on linux-vr.org ..., mkromfs I think applied to a 
cramfs filesystem)  Then enable /dev/rom, and set it's address at the start 
of the rom.  Then root=/dev/rom as the boot options.

> It seems to me that for the time being and the immediate future, XIP as
> we're talking about it would be overkill.  The streamlining and elegance
> gained in the operation of XIP is probably offset by the ugliness of
> implementation and the potential loss of compression that seems so crucial
> in embedded devices, the primary target (right?) of MTD.  I like XIP but I
> don't think I'd ever use it.

With only 6MB of ram..., it might be a good thing for your device, but I 
don't know what you're planning on running on it.

> One very relevant topic that I'd like to discuss is where a generic memory
> conduit device belongs.  I would suspect it belongs as a fallback default
> if CFI devices can't be probed (I forget the file), but it also might make
> sense to give it a separate file for "neatness" sake.
Thanks,
Shane.


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