Subject: 64 bit memory access

From: Tim Montgomery <[email-masked]>
Date: Fri, 29 Dec 2000 11:08:55 -0600

Hello,

I have a quandary.

My platform is a PowerPC (MPC750) based embedded machine.

I have a device (memory-mapped) that I need to access that lives on an
internal peripheral bus but has the unfortunate constraint of allowing
only 64bit-wide writes.  Any write of a smaller width results in an
exception generated by the memory controller.

Drivers exist for this device for other, lesser OSs that use FP regs to
write the 64bit quantity to the bus (using lfd and stfd).  However, from
what I gather, using the FP regs in kernel drivers under Linux kernel
(2.4.0test12-based) seems to be a no-no.

I am afraid I am at an impasse.

I need to:
1.) figure out a way to perform a 64bit write via other means
OR
2.) make sure I don't screw up user mode FP usage (potential consequence
of dinking with FP while in the kernel?)

Any suggestions/insight would be appreciated.

Regards,
Tim

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