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

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Message-ID: <20070607211519.GA22348@infradead.org>
Date:	Thu, 7 Jun 2007 22:15:19 +0100
From:	Christoph Hellwig <hch@...radead.org>
To:	Jared Hulbert <jaredeh@...il.com>
Cc:	Christoph Hellwig <hch@...radead.org>, carsteno@...ibm.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 Thu, Jun 07, 2007 at 02:11:44PM -0700, Jared Hulbert wrote:
> >that even more important doesn't require pulling in
> >the whole block layer which is especially important for embedded
> >devices at the lower end of the scala.
> 
> Good point.  That is a big oversight.  Though I would prefer to handle
> that in the same fs rather than fork.

If if were actually talking about complex filesystem I'd agree.  But
the cramfs xip patch posted here touches about 2/3 of the number of
lines that cramfs has in total.  And cramfs is not exactly the best
base to start with..

> >I still think it'd be even better to just
> >hook xip support into jffs or logfs because they give you a full
> >featured flash filesystem for all needs without the complexity
> >of strictly partitioning between xip-capable and write parts
> >of your storage.
> 
> This is nirvana.   But it is not the goal of the patches in question.
> In fact there are several use cases that don't need and don't value
> the writeability and don't need therefore the overhead.  It is a long
> term goal never the less.

With the filemap_xip.c helpers adding xip support to any filesystem
is pretty trivial for the highlevel filesystem operations.  The only
interesting bit is the lowlevel code (the get_xip_page method and
the others Carsten mentioned), but we need to do these lowlevel
code in a generic and proper way anyway.

I'll try to hack up an xip prototype for jffs2 next week.
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