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SubjectARM: add xz kernel compression support
Date
This patchset applies on Linus' tree

It adds xz support to the ARM tree. x86 already has it, so let's have it too!
My benchmarks below show that xz is better than lzma in terms of image size
and uncompression time.

My benchmarks below show that xz is better than lzma in terms of image size
and uncompression time.

lzma and xz remain pretty bad in terms of boot time, but they may be useful
in cases where RAM usage and kernel size are critical (booting on FPGAs in early
CPU design?).

However, on fast CPUs like OMAP4 with slow storage, xz is almost as good as
lzo, but uses much less space. Therefore, it can be a good compromise.

This patch and all other compression options were tested on two boards:
- OMAP4430 Panda board
- AT91SAM9623 USB-A9263 board from Calao Systems

Results on OMAP4 Panda
----------------------

Booting from MMC

Comp. Size Loading time Uncompressing time Total time
gzip 3355768 2.213376 0.501500 2.714876
lzma 2488144 1.647410 1.399552 3.046962
xz 2366192 1.566978 1.299516 2.866494
lzo 3697840 2.471497 0.160596 2.632093
None 6965644 4.626749 0 4.626749

Results on Calao Systems USB-A9263 (AT91)
-----------------------------------------

Booting from NAND

Compression Size Loading time Uncompressing time Total time
gzip 2386936 5.843289 0.935495 6.778784
lzma 1794344 4.465542 6.513644 10.979186
xz 1725360 4.308605 4.816191 9.124796
lzo 2608624 6.351539 0.447336 6.798875
None 4647908 11.080560 0 11.080560

Loading time: between "reading uImage" and "OK" (right before "Starting kernel")
Compression time measured by:
~/bin/grabserial -v -d /dev/ttyUSB0 -e 15 -t -m "Uncompressing Linux" -i "done," > booting-lzo.log


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