XIP on mtdram

XIP on mtdram

[Rogelio M. Serrano Jr.]

How can we do XIP on mtdram? Please enlighten me on this subject. I have
not heard of the subject before. And I have no idea how it is done. I
need to eliminate the use of ramdisks due to severe hardware
requirements and very difficult specs. The mtdram device is the best
answer to my problem. This must be put on production next week.



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Re: XIP on mtdram

[David Woodhouse]

Why can you use mtdram but not a normal ramdisk? The main advantage in that 
would be if you could eliminate the block device code and save a few pages 
of kernel code/data - but we haven't actually extracted it from the VM code 
yet, so we can't do that yet.

We don't currently have XIP in place for JFFS. You'd have to make sure the 
actual data in the nodes were page-aligned in the host memory, and remained 
that way on GC. It's possible, but we've not looked at it yet. 

Also, as flash is more expensive than RAM, it's cheaper to do compression, 
which means you obviously can't do XIP.

If you really have to have XIP from a filesystem stored in RAM, look at 
ramfs.


--
dwmw2




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Re: XIP on mtdram

[David Woodhouse]

rogelio@evoserve.com said:
> I really have to because i can have no more than 32 M ram and 4 M
> norflash. Can ramfs be mounted as root partition? I can almost boot
> linux straight off the bios chip now. I need to read a filesystem
> image off the flash, mount it as root and mount the flash filesystem
> array.

At the moment, you _have_ to execute your binaries from RAM. There's no 
alternative. Using a ramdisk, ramfs, or mtdram will all use up pages of 
that RAM, so there's no advantage in doing that over just using JFFS on the 
flash and having the page cache store it for you.

In fact, by using JFFS on the flash, at least the kernel can discard the
pages from RAM when they're not in use, so in fact you're better off that
way.

With some hacking, it's theoretically possible to get XIP working on JFFS. 
You're unlikely to get that running in a week, though. My advice is to go 
with JFFS directly on the flash.

--
dwmw2




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