[PATCH 1/2] powerpc: split the math emulation into two parts

[Scott Wood]

On 07/22/2013 09:36:05 AM, Kumar Gala wrote:
> 
> On Jul 16, 2013, at 6:57 AM, Kevin Hao wrote:
> 
> > For some SoC (such as the FSL BookE) even though there does have
> > a hardware FPU, but not all floating point instructions are
> > implemented. Unfortunately some versions of gcc do use these
> > unimplemented instructions. Then we have to enable the math  
> emulation
> > to workaround this issue. It seems a little redundant to have the
> > support to emulate all the floating point instructions in this case.
> > So split the math emulation into two parts. One is for the SoC which
> > doesn't have FPU at all and the other for the SoC which does have  
> the
> > hardware FPU and only need some special floating point instructions  
> to
> > be emulated.
> >
> > Signed-off-by: Kevin Hao <haokexin at gmail.com>
> > ---
> > arch/powerpc/Kconfig           | 20 ++++++++++++++++++++
> > arch/powerpc/math-emu/Makefile | 24 ++++++++++++------------
> > arch/powerpc/math-emu/math.c   | 20 ++++++++++++++------
> > 3 files changed, 46 insertions(+), 18 deletions(-)
> 
> why make the split, what harm is there in just turning on the full  
> emulation code to handle the unimplemented cases?

My main motivation in requesting it was to contain the increase in  
build time -- math-emu always stuck out to me as something that took a  
noticeable amount of time to build.  It also reduces the increase in  
kernel image size.

> who says what some other implementation doesn't need something that  
> you have in CONFIG_MATH_EMULATION_FULL?

The point is to include any instructions that are known to be missing  
in any chip's FPU (excluding chips that don't have an FPU at all).  If  
it is discovered that some chip is missing an instruction that we  
didn't account for, then we'd move that instruction from one list to  
the other.

> Is the kernel code size really an issue?

It can be when you're storing it on flash -- especially when the growth  
is out of control because of the need to justify pruning low-hanging  
fruit such as this.

-Scott