[Libre-soc-bugs] [Bug 855] add libre-soc to kestrel

[bugzilla-daemon at libre-soc.org]

https://bugs.libre-soc.org/show_bug.cgi?id=855

--- Comment #16 from tpearson at raptorengineering.com ---
Very quick update in advance of a more detailed update tomorrow...

Tracked the main Zephyr problem down to a specific section of mis-executed
assembler:

subf    r9,r29,r31
addi    r9,r9,999
lis     r30,8388
ori     r30,r30,39845
rldicr  r30,r30,32,31
oris    r30,r30,58195
ori     r30,r30,63439
mulhd   r30,r9,r30
sradi   r10,r30,7
sradi   r30,r9,63
subf    r30,r30,r10
clrldi  r30,r30,32

What this is supposed to do is basically:

ceiling_fraction(deadline - now, MSEC_PER_SEC);

where ceiling_fraction is defined as:

#define ceiling_fraction(numerator, divider) \
        (((numerator) + ((divider) - 1)) / (divider))

r31 is "deadline", set to 8480 decimal
r29 is "now", set to 480 decimal

The correct output would be 8 decimal, stored in in r30.  What's actually
returned is zero, which causes all manner of chaos in the timing systems of
Zephyr.

I've verified this mis-execution on real hardware and in simulation.  I'm
working on reducing the test case down and identifying the actual broken
instruction.

I'll also be uploading the requested branches tomrorow, just wanted to get this
out there in case it sparks some ideas in the interim.

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