Re: [PATCH] lib: xz: add support for bcj filters

[Sascha Hauer <s.hauer@pengutronix.de>]

On Wed, Feb 22, 2017 at 09:15:36AM +0100, Yegor Yefremov wrote:
> On Wed, Feb 22, 2017 at 5:29 AM, Jean-Christophe PLAGNIOL-VILLARD
> <plagnioj@jcrosoft.com> wrote:
> >
> >> On Feb 21, 2017, at 10:47 PM, yegorslists@googlemail.com wrote:
> >>
> >> From: Yegor Yefremov <yegorslists@googlemail.com>
> >>
> >> Add missing configuration options for various bcj filters. Without
> >> these options the lib/xz/xz_dec_bcj.c file will be compiled, but all
> >> filters will be disabled.
> >>
> >> Signed-off-by: Yegor Yefremov <yegorslists@googlemail.com>
> >> ---
> >> lib/Kconfig | 28 ++++++++++++++++++++++++++++
> >> 1 file changed, 28 insertions(+)
> >>
> >> diff --git a/lib/Kconfig b/lib/Kconfig
> >> index f9f25bdef..83dd8e0a4 100644
> >> --- a/lib/Kconfig
> >> +++ b/lib/Kconfig
> >> @@ -22,6 +22,34 @@ config XZ_DECOMPRESS
> >>       bool "include xz uncompression support"
> >>       select UNCOMPRESS
> >>
> >> +if XZ_DECOMPRESS
> >> +
> >> +config XZ_DEC_X86
> >> +        bool "x86 BCJ filter decoder"
> >> +        default y
> > this need to be ARCH dependant
> 
> Why? In Linux kernel it is not arch dependent, because AFAIK it
> describes only compression particularities. This way you can extract
> sqaushfs images on your ARM machine, that were compressed on SPARK
> etc.

So xz uses the Sparc bcj filter even when compressing ARM binaries? That
doesn't sound very mature to me.

It is quite surprising to me that I have to enable XZ_DEC_X86 in barebox
since this is the architecture the xz binary was most likely compressed
on. Also it's surprising that XZ_DEC_X86 won't bring us any gain since
the xz binary will most likely contain ARM instructions on a ARM build,
except that without XZ_DEC_X86 enabled it will not work.

Then in bcj_apply() we have:

#ifdef XZ_DEC_X86
	case BCJ_X86:
		filtered = bcj_x86(s, buf, size);
		break;
#endif

...

	default:
		/* Never reached but silence compiler warnings. */
		filtered = 0;

This is simply not true when any of the ifdefs is not true.

I think we should enable all the XZ_DEC_* options but without making them
visible since if someone ever stumbles upon these options and changes them
he will most likely do it wrong.

Sascha

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