64 bit memory access - again ...

Dan Malek dan at embeddededge.com
Tue Dec 4 04:01:44 EST 2001

Shie Erlich wrote:

> i also know that floating point in the kernel is a big NO, but if no other
> choice is found, i'll have to do it. my questions are:

If possible, I would mmap() the device from user space and perform
the access there.

> a) what is the recommended way of doing a 64bit access in the linux kernel ?

I would first try to do it with a cache line burst access.  Put it in
copyback mode, zero the line, write the line, flush the line, invalidate
the line.

> b) if the fpu is the way to go, what do i need to do so that the kernel does
> not
>     trap my floating point access ?

You can't trap and emulate, that defeats the purpose.  The biggest challenge
when using floating point in the kernel is context switching the FPU.
I would probably force the kernel to always context switch the FPU (as
it does on SMP) so you would always have a clean context to use in
the kernel.  I would then disable interrupts (to avoid other FPU context
problems), enable the FPU, perform the access, ensure the context is
reloaded for the switched-in thread, disable the FPU, enable interrupts.

Any system performance gains you thought you may have by using 64-bit
I/O are likely to be significantly reduced by the amount of software
overhead needed to manage this.

Depending upon what you are using for a bridge/memory controller you
may have a DMA controller that could perform this access as well.

	-- Dan

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