FP register corruption on new exception processing PPC BSPs

[gregory.menke at gsfc.nasa.gov]

Till Straumann writes:
 > powerpc/shared, eth_comm, mbx8xx, mpc8260ads and
 > possibly other 'new exception processing' style BSPs do not
 > save the floating point (FP) registers across dispatching
 > the user ISR (for performance reasons). Nevertheless, the code
 > always re-enables the FPU when entering the user ISR. Hence, the ISR
 > can silently corrupt the FPregs if it uses the FPU. Unfortunately,
 > FPU usage may be the implicit result of compiler optimization
 > (e.g., gcc uses FPregs for copying 'long long' [integer] variables)
 > or the explicit result of bad coding practice.
 > 
 > I just filed PR#774 which makes sure the FPU is disabled across
 > the user ISR which means that an exception will be raised if
 > the ISR uses the FPU making it much easier to catch illegal FPU usage.
 > 
 > Note that this problem is independent of whether 'lazy FP context
 > switching' is used or not.
 > 

We had essentially the same problem with the MIPS bsp's and applied
that solution as well.  Thanks for finding it!

GRegm