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SubjectRe: What File System supports Application XIP
Tim Bird wrote:
> The patches I've seen require setting the CRAMFS_LINEAR option, to turn on
> linear addressing for cramfs, and CRAMFS_LINEAR_XIP. The result of these
> is to dispense with compression.

Compression is skipped for the XIP files, which are typically marked via
the sticky bit. You'll also need a version of mkcramfs that creates the
image without compressing those files.

> FYI - Here are some rough numbers:
> Time to run shell script which starts TinyX X server and "xsetroot -solid red",
> then shuts down:
> First invocation: Non-XIP 3.195 seconds, XIP 2.035 seconds
> Second invocation: Non-XIP 1.744 seconds, XIP 1.765 seconds
> I think this was on a 133 MHz PPC, but I'm not positive. In both cases
> the filesystem was in flash.

It was measured on a 168MHz ARM 925T TI OMAP 1510.

Others' advice that "you probably don't want XIP" is true in most cases.
But in producing a battery-operated product with certain requirements
for performance, power savings (due to reduced RAM requirements),
startup time (depending on the platform and software stack the
difference can be significant), etc. XIP is an option chosen by some CE
designers, who are willing to accept the performance penalty on a
product that will still run adequately for its intended uses. It would
be interesting to see an in-depth analysis of these topics on an actual
Linux-based product such as a cell phone. There are, of course, a
number of ways to address all these issues, not just XIP...

Todd Poynor
MontaVista Software

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