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Re: JFFS2 and XIP



On Mon, 12 Nov 2001, A. Craig West wrote:
> of the jffs2 filesystem that could help me? Ideally, I want to be able to
> have XIP support in a writable file-system. Reading back over the the old
> thread, I can see that there are a lot of issues with this, but it still
> looks do-able. The Redmond guys have XIP, so it can't be THAT hard :-)
> It appears certain that XIP files would have to be un-compressed, and pages
> containing active XIP sessions would not be movable.

It's probably doable if you hack the page-cache code to not allocate a
memory page to hold the contents its trying to read but simply map the
relevant page in the flash.

But then you'd have to align all files in the flash filesystem to page
boundaries which could have disastrous consequences for image size, and
you'd have to remap the pages when the GC moves around the nodes.

I don't think XIP is worth much, because you loose the compression ability
and you don't really gain much memory. Even without XIP there is nothing
that says the memory allocated by flash pages cannot be flushed if it's
needed for something else (and indeed it is flushed). You'll unpack it
again next time it's needed so in the worst case you waste a bit of CPU
but you also waste a big potential headache... it's your call on what to
trade off.

8 MB ram is plenty for many applications without XIP. 

/BW


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