linux-kernel - Re: [PATCH 2.6.21] cramfs: add cramfs Linear XIP

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Date:	Thu, 07 Jun 2007 19:07:54 +0200
From:	Carsten Otte <cotte@...ibm.com>
To:	Christoph Hellwig <hch@...radead.org>, carsteno@...ibm.com,
	Jared Hulbert <jaredeh@...il.com>,
	Nick Piggin <nickpiggin@...oo.com.au>,
	Andrew Morton <akpm@...ux-foundation.org>,
	richard.griffiths@...driver.com,
	Richard Griffiths <res07ml0@...izon.net>,
	Linux-kernel@...r.kernel.org
Subject: Re: [PATCH 2.6.21] cramfs: add cramfs Linear XIP

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.
-
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