Re: Fwd: Bug#561891: Please support sh4

[Jens Axboe <jens.axboe@oracle.com>]

On Tue, Dec 22 2009, Paul Mundt wrote:
> On Mon, Dec 21, 2009 at 09:57:21PM +0100, Jens Axboe wrote:
> > On Tue, Dec 22 2009, Paul Mundt wrote:
> > > On Tue, Dec 22, 2009 at 05:47:46AM +0900, Nobuhiro Iwamatsu wrote:
> > > > +#define nop             __asm__ __volatile__ ("nop": : :"memory")
> > > > +#define read_barrier()  __asm__ __volatile__ (" " : : : "memory")
> > > > +#define write_barrier() __asm__ __volatile__ (" " : : : "memory")
> > > 
> > > This is true for SH-4, but not for SH-4A, which wants a synco for these.
> > > See how the kernel does it.
> > > 
> > > The sooner we stop pretending like SH-4A and SH-4 are the same thing the
> > > sooner we stop introducing subtle bugs all over the place.
> > 
> > Does sh-a4 support the synco instruction, or will it sigill?
> > 
> SH-4A supports synco, SH-4 does not. SH-4A is based on the SH-4, but has
> a lot of new instructions and so forth. Folks like to treat them the same
> so as to keep the same ABI, but there are a lot of architectural
> differences that are ideally not just glossed over. We have been bitten
> by these in the past.
> 
> Linux reports different uname values for all of SH-4, SH-4A, and
> SH4AL-DSP largely for these reasons, and gcc also has different
> preprocessor tokens for each. In this case, an __SH4A__ test can be done
> for synco with the above definitions sufficing for all other variants.
> 
> Where this silliness started was with FPU-less variants of SH-4 defining
> __SH3__ because the gcc folks decided that an SH-4 without FPU
> instructions was "close enough" to the SH-3 ISA, which while generally
> true at the time, also ignored the fact that they have completely
> different caches -- SH-4/SH-4A/SH4AL-DSP parts end up having to do
> I-cache block invalidations from the libc, while SH-3 does not. SH-4 and
> SH-4A preprocessor tokens on the other hand are sensibly abstracted, so
> there is no reason to pretend like everything is SH-4.

Thanks for the clarification, Paul. One last question... Will __SH4A__
cover sh5 as well, or does it need a __SH5__?

-- 
Jens Axboe