Re: [linuxsh-dev] Re: FPU in kernel (SH-4)

[Greg B. <gb...@po...>]

NIIBE Yutaka wrote:
>
> Yes, I had thought that, and have been using -m4-nofpu with su.  But
> looking the code deeply this time, it's SH-3 actually, and __sh3__
> definition seems correct.  It's quite counter-intuitive, and confusing
> though.  Well, I've objdump-ed the .o produced by -m4-nofpu, and found
> that the architecture is set to "SH3" not "SH4".  Ummm...

"M. R. Brown" wrote:
> 
> Well, it's clear-cut what it does in gcc, it generates code for a sh3 and
> below, but acts as if it had sh4 hardware:
> 
>   {"4-nofpu",   SH3_BIT|SH2_BIT|SH1_BIT|HARD_SH4_BIT, "" }
> 
> gcc won't generate code for anything higher than a sh3, and since the sh3
> can't generate fpu code, this is (somewhat) of an alias for a sh4 without
> fp support.  So the __sh3__ predefine is correct, because if you define
> __SH4__ you'll pull in fp code into libgcc, and you'll get link errors.
> 

  Ok, now I'm more confused than I was yesterday.

  Firstly, the gcc I have here (rather old) doesn't define __sh3__
when you give it -m4-nofpu.  This might be because we have one of
Kaz' patches.  What it does define is:

sh-linux-gnu-gcc --verbose -m3 -o foo-m3.o -c foo.c
[...]  -D__SH3__ -D__sh3__ -ml -m3

sh-linux-gnu-gcc --verbose -m4 -o foo-m4.o -c foo.c
[...]  -D__SH4__ -ml -m4

sh-linux-gnu-gcc --verbose -m4-nofpu -o foo-m4-nofpu.o -c foo.c
[...] -D__SH4__ -D__SH4_NOFPU__ -ml -m4-nofpu 

  Which looks pretty sensible to me.  The architecture in the ELF files
is pretty screwed up, but it's *consistently* screwed up so everything
still works.  The -m4-nofpu option uses the same integer division millicode
as -m3, which is correct.



  Secondly, defining __sh3__ is the *wrong* thing to do for the kernel.

  One obvious problem is that with __sh3__ defined, lots of stuff in the
kernel breaks completely because the SH internal module registers have
different addresses for SH3 and SH4.  For example:

#if defined(__sh3__)
#define TMU_TOCR	0xfffffe90	/* Byte access */
#define TMU_TSTR	0xfffffe92	/* Byte access */

#define TMU0_TCOR	0xfffffe94	/* Long access */
#define TMU0_TCNT	0xfffffe98	/* Long access */
#define TMU0_TCR	0xfffffe9c	/* Word access */

#define FRQCR		0xffffff80
#elif defined(__SH4__)
#define TMU_TOCR	0xffd80000	/* Byte access */
#define TMU_TSTR	0xffd80004	/* Byte access */

#define TMU0_TCOR	0xffd80008	/* Long access */
#define TMU0_TCNT	0xffd8000c	/* Long access */
#define TMU0_TCR	0xffd80010	/* Word access */

#define FRQCR		0xffc00000
#endif

  (I'm using an old kernel too)

  What the kernel needs is an option that generates SH4 code with __SH4__
defined, but no FPU code used from libgcc.a.  With the gcc I'm using here,
-m4-nofpu is that option.


  So I'm very confused.  Niibe-san, does your gcc behave differently to mine?

Greg.
-- 
If it's a choice between being a paranoid, hyper-suspicious global
village idiot, or a gullible, mega-trusting sheep, I don't look
good in mint sauce.                      - jd, slashdot, 11Feb2000.