Re: [RFC PATCH 21/23] powerpc64 is not necessarily BigEndian anymore! :)

[Andrey Borzenkov]

В Thu, 3 Apr 2014 10:33:36 -0700
Ram Pai <address@hidden> пишет:

> On Tue, Apr 01, 2014 at 10:22:10PM +0200, Vladimir 'φ-coder/phcoder' 
> Serbinenko wrote:
> > 
> > > 
> > > For the sake of bisectability this really should be moved earlier;
> > > otherwise at least patch "fix parameter to firmware calls" would
> > > be wrong.
> > > 
> > Even bigger problem is whether we want to run in LE mode at all. From
> > what I understand (correct if I'm wrong) firmware calls remain
> > big-endian and you need to switch back and forth between LE and BE when
> > doing firmware calls.
> 
> Yes. firmware runs in 32bit BE mode. And there is a constant switch from
> 64bit LE to 32bit BE and vice-versa for each firmware call.
> 
> > doing firmware calls. Byteswapping for the purpose of firmware calls is
> > to be avoided as bugs are easy to slip through (in fact the
> > byte-swapping isn't complete in proposed patches.
> > (correct me if I'm wrong) 
> 
> Is that true? maybe you are right. I might have missed something.
> However please hint me what i have missed. I will look into some
> other arch code that support the same ieee platform.
> 
> 
> > these new patches cover a subset of already
> > supported machines and don't add any user-visible feature and no new
> > kernel type (LE kernel can be loaded from BE GRUB).
> > Cross-compiling to BE from LE is easy (TARGET_CFLAGS=-EL).
> 

Hmm ... gcc docs mention this only for MIPS, not for PowerPC; for
PowerPC it says -mbig.

> Well. that is the issue.  Various distros have varied support for
> cross-compilation (multi-arch support). If the distro does not 
> have 32bit BE libraries natively installed (out-of-the-box), they
> wont be able to generate a 32bit BE grub loader.

We speak only about target code that runs at boot time. This code does
not use any library. It only needs compiler support. GRUB does not
support anything besides gcc and recently some clang support was added.
Do you have real life example of distribution which does not support
-mbig gcc option to produce big-endian *code*?

It is perfectly legal to have different architectures for host and
target. So you will have little-endian 64 bit host tools and big-endan
32 bit target. If this works for you this of course is much better
solution (at the end it requires just a single line in configure.ac).

>                                                 These set of patches
> overcomes the deficiency by generating a working native executable 
> on LE systems.
> 
> > So it looks like this patch series adds a new high-maintenance-cost port
> > covering only already supported machines and already supportred features.
> 
> It does add maintainence; I agree. But than it does overcome some
> deficiences aswell.
> 

Do you mean there are some problems with big-endian code at boot time?
Could you give more details?

> 
> Thanks for your comments. And sorry for the delay. I had not seen this
> email, since i was not on copy. I will watch the mailing list regularly.
> RP
> 
> 
> 
> 
> > 
> 
> 
> 
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> 
>