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

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Date:	Fri, 08 Jun 2007 09:17:18 +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:
> On Thu, Jun 07, 2007 at 07:07:54PM +0200, Carsten Otte wrote:
>> 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)
> 
> And we'll need that even when using cramfs.  There's not way we'd
> merge a hack where the user has to specify a physical address on
> the mount command line.
Nay, the proposed solution is read only. From what I understood of the 
discussion, it does'nt need that. The entire flash memory can be read 
at the same time. Things start to get interresting when you want to 
write, then you need to care about erase blocks and such.

>> 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.
> 
> You'd of course want all that for a full flash filesystem aswell.
True.

so long,
Carsten
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