Re: [PATCH 2.6.21] cramfs: add cramfs Linear XIP

[Carsten Otte <cotte@de.ibm.com>]

Christoph Hellwig wrote:
> The right way to architect xip for flash-based devices is to implement
> a generic get_xip_page for mtd-based devices and integrate that into
> an existing flash filesystem or write a simple new flash filesystem
> tailored to that use case.
I've had a few beer long discussion with Joern Engel and David 
Woodhouse on this one. To cut a long discussion short: the current XIP 
infrastructure is not sufficient to be used on top of mtd. We'd need 
some extenstions:
- on get_xip_page() we'd need to state if we want the reference 
read-only or read+write
- we need a put_xip_page() to return references
- and finally we need a callback for the referece, so that the mtd 
driver can ask to get its reference back (in order to unmap from 
userland when erasing a block)

While I fully agree, that a flash filesystem using xip would be very 
desireable, the proposed cramfs extension is a totaly different beast 
that has its own value to me: the ability to select per file whether 
xip or compression is more efficient.