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

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Date:	Wed, 6 Jun 2007 17:23:05 +0100
From:	Christoph Hellwig <hch@...radead.org>
To:	carsteno@...ibm.com
Cc:	Christoph Hellwig <hch@...radead.org>,
	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

On Wed, Jun 06, 2007 at 06:15:04PM +0200, Carsten Otte wrote:
> The clear advantage of using cramfs on embedded platforms over using the
> ext2 stuff is, that one can choose per-file whether it should be
> compressed or xip.
> The real key is, to put both our ext2 stuff and the cramfs xip on a 
> common infrastructure. They should use the same file operations and 
> adress space operations for xip files rather then replicating each 
> others bugs.
> If cramfs shall be kept simple, it might be time to fork that file 
> system. I don't see that need arise from the proposed solution. It can 
> become clean and sane with a little work on it. Look at the xip 
> extensions for ext2 for example, they don't bloat the filesystem too much.

ext2 is a multi-purpose block based filesystem and you add support for
another bit of storage.  cramfs is very specialized to storing read-only
compressed data on block devices.  The embedded people already use them
on flash which is a little dumb, but now we add even more cludge for
a non-block based access.

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.
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