Re: (Hardware?) problem with denormal values on mipsel

James Cowgill <[email protected]> Wed, 27 May 2015 18:15:32 +0100

Newsgroups	gmane.linux.debian.ports.mips
Message-ID	<[email protected]>

On Wed, 2015-05-27 at 18:13 +0100, Maciej W. Rozycki wrote:
> On Wed, 27 May 2015, James Cowgill wrote:
> 
> > Thanks! The kernel was the important part which was different to my
> > setup.
> > 
> > There's probably still a hardware bug in here somewhere, but before 3.16
> > the kernel was fixing it with the math emulator.
> > 
> > From what I can see:
> > Commit 08a07904e182 in 3.16 ('MIPS: math-emu: Remove most ifdefery.')
> > incorrectly disabled emulation of the sqrt.s instruction on processors
> > only supporting the mips2/mips3 ISA.
> > 
> > Commit 7352c8b13dd9 in 3.18 ('MIPS: Loongson: Set Loongson-3's ISA level
> > to MIPS64R1') worked around this in the Loongson 3s because their ISA
> > level was bumped to mips64r1.
> > 
> > Commit 2d83fea786d7 in 4.1-rc1 ('MIPS: Correct FP ISA requirements')
> > fixed the underlying sqrt.s emulation bug.
> > 
> > It would be good if the 4.1-rc1 patch or at least the 'case fsqrt_op'
> > part (which is the fix for this bug) can be applied to a jessie stable
> > kernel and then installed on the buildds. This should fix the original
> > problem of eso-midas failing to build on mipsel.
> 
>  What's the actual problem here, how can I help?

I just thought I would let you know since it was your patch which was
going to be applied somewhere.

The bug is that Loongson machines relied on the kernel emulating sqrt.s
for them (in certain cases) but between 3.16 and 4.1 it didn't and was
causing things to SIGILL.

Thanks,
James

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