PPC floating point in the kernel?

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heynow567 Contact options for registered users 's Avatar (by Gravatar) posted on
December 7, 2004, 8:39 pm
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I am using a PPC 7455 and running Monta Vista Linux 2.6.  I have heard
that there are issues with using floating point in the kernel.  I have
hardware floating point support in the PPC 7455.  Should I use the
hardware floating point or should I use software floating point?
Thanks,

Andy


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Roger Larsson Contact options for registered users 's Avatar (by Gravatar) posted on
December 7, 2004, 11:33 pm
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Re: PPC floating point in the kernel?

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No.

Longer answer:

Software FP: you need to add FP libraries to the kernel. Usually quite big
        and "slow"... Compared to integer/fixed point math.
Hardware FP: the kernel or you will need to keep track on FP registers all
        the time, since the kernel tries to be lazy about them
         - only save/restore when needed and only when in use.
         - Preemptive kernel? (Monta Vista) then you will need to disable/enable
           preemptions around the floating point code
                 - see linux/Documentation/preempt-locking.txt
                   "RULE #2: CPU state must be protected."
Any FP: Portability?

Summary: Don't use it if you find any way to avoid it.
         For what would you want to use it anyway?
         Interrupt driver with floating point?

/RogerL



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