Change build to turn ON FPU by default

[kwilsonpdx]

The STM32F4 has a very nice FPU...this option appears to be turning it off:

-DCORTEX_USE_FPU=FALSE

[ThirteenFish]

I'm not sure we want to? None of our projects currently use the FPU and turning it on, among other things, adds overhead to thread context switches (See ChibiOS/os/ports/ARMCMx/chcore_v7m.c _port_switch()).

That said, if we have projects that want floating point we should absolutely remember to turn it on. The option is setting USE_FPU to yes on line 47 of the project makeflies. Setting that brings in -DCORTEX_USE_FPU=TRUE -mfloat-abi=softfp -mfpu=fpv4-sp-d16 -fsingle-precision-constant which looks right except maybe we want -mfloat-abi=hard instead.

[kwilsonpdx]

From the Cortex-M4 Technical Reference Manual: VPUSH number of cycles is: 1+N, where N is the number of registers and VPOP is the same

So the cost is 34 cycles every thread switch. At 168Mhz thats...6ns*34 = 204nS

I'd rather leave it ON by default but if none of our applications are using FP then I guess it doesn't really matter.

From chcore_v7m.c:

240 #if CORTEX_USE_FPU
241   asm volatile ("vpush   {s16-s31}" : : : "memory");
242 #endif
243 
244   asm volatile ("str     sp, [%1, #12]                          \n\t"                                           
245                 "ldr     sp, [%0, #12]" : : "r" (ntp), "r" (otp));
246 
247 #if CORTEX_USE_FPU
248   asm volatile ("vpop    {s16-s31}" : : : "memory");
249 #endif

[ThirteenFish]

We discussed this issue at the meeting and and arrived upon the best solution to this being having our build tools generate a warning or a message if it's actually generating any soft float code. This way the developer would be reminded to set USE_FPU when needed.

Unfortunately it's not immediately obvious how to get GCC to do this for us.

[ThirteenFish]

We're not going to make the build tool changes before the next launch so I'm closing this for now. It can be reopened later.