Floating point operations in interrupt handler (PowerPC, VxWorks)

4
votes

I haven't found any resources that exactly answer what I am trying to understand with an issue I saw in a piece of software I am working on, so I'll ask the geniuses here!

For starters, I'm running with VxWorks on a PowerPC processor.

In trying to debug a separate issue, I tried throwing some quick and dirty debug code in an interrupt handling routine. It involved a double precision floating point operation to store a value of interest (namely, how long it had been since I saw the last interrupt come in) which I used later outside the handler in my running thread. I didn't see a problem in this (sure, it takes longer, but time-wise I had pleanty; the interrupts aren't coming in too quickly) however VxWorks sure didn't like it. It consistently crashes the when it reaches that code, one of the bad crashes that reboots the system. It took me a bit to track down the double operation as the source of the issue, and I realized it's not even double "operations", even returning a constant double from a routine called in the interrupt failed miserably.

On PowerPC (or other architectures in general) are there generally issues doing floating point operations in interrupt handlers and returning floating point (or other type) values in functions called by an interrupt handler? I'm at a loss for why this would cause a program to crash.

(The workaround was to delay the conversion of "ticks" since last interrupt to "time" since laster interrupt until the code is out of the handler, since it seems to handle long integer operations just fine.)

6
c++embeddedvxworkspowerpcinterrupt-handling
Can't you use a 64-bit fixed-point value instead? Should be big enough... – Kerrek SB
As I mentioned, that was the workaround. In general, I wouldn't use floating point operations in interrupt handlers because they tend to take lots of operations to complete and interrupt handlers need to be quick. My question here is really "why doesn't it work" not "what should I do instead". I understand the later, but the former has me boggled. – Anthony
It should work, nothing magic or special about it, should work as well as any other variable type or structure. I would disassemble what the compiler is generating to understand where it is putting the value and where the other end is picking up the value, compare that to passing an unsigned int or something and see the disconnect. – old_timer
Don't know about PowerPC specifically, but saving floating point state can be expensive and are often not done until needed. The interrupt response time would be affected badly if it was done all the time. – Bo Persson

6 Answers

7
votes

In VxWorks, each task that utilises floating point has to be specified as such in the task creation so that the FP registers are saved during context switches, but only when switching from tasks that use floating point. This allows non-floating point tasks to have faster context switch times.

When an interrupt pre-empts a floating point task however, it is most likely the case that FP registers are not saved. To do so, the interrupt handler would need to determine what task was pre-empted and whether it had been specified as a floating point task; this would make the interrupt latency both higher and variable, which is generally undesirable in a real-time system.

So to make it work any interrupt routine using floating point must explicitly save and restore the FP registers itself. Any task that uses floating point must be specified as such in any case, though you can get away with it if you only have one such task.

If a floating-point task is pre-empted, your interrupt will modify floating point register values in use by that task, the result of this when the FP task resumes is non-deterministic but includes causing a floating point exception - if a previously non-zero register for example, becomes zero, and is subsequently used as the right-hand of a division operation.

It seems to me however that in this case the floating point operation is probably entirely unnecessary. Your "workaround" is in fact the conventional, safest and most deterministic method, and should probably be regarded as a correction of your design rather than a workaround.

5
votes

Does your ISR call the fppSave()/fppRestore() functions?

If it doesn't, then the ISR is stomping on FP registers that might be in use by existing tasks.

Specifically, FP registers are used by the C++ compiler on the PPC architecture (I think dealing with throw/catch).

0
votes

In VxWorks, at least for the PPC architectures, a floating point operation will cause a FP Unavilable Exception. This is because when an interrupt occurs the FP bit in MSR is cleared because VxWorks assumes that there will be no FP operations. This speeds up ISR/Task context switching because the FP registers do not have to saved/restored.

That being said, there was a time when we had some debug code that we needed FP operations in the interrupt context. We changed the VxWorks code that calls the specific ISR to 1) set the MSR[FP], do a fpsave call, call the ISR, do a fprestore call, then clear the MSR[FP]. This got us around the problem.

That being said, I agree with the rest of the folks here that FP operations should not be used in an ISR context because that ISRs should be fast and FP operations at typically not.

0
votes

I have worked with e300 core while developing bare-metal applications and I can say that when an interrupt occurs, core closes the FPU, that you can observe by checking FP bit of MSR. Before doing anything with the floating point registers, you must re-enable FPU by writing 1 to FP bit of MSR. Then you make operations on FPU registers as you want in an ISR.

0
votes

The general assumption in VxWorks is that Floating Point registers don't need to be saved and restored by ISRs. Primarily because ISRs usually don't mess with them. Historically, most real-time tasks didn't do FP either, but that's obviously changed. What's not obvious is that many tasks that don't explicitly use floating point nevertheless use the floating point registers. I believe that any task with code written in C++ uses the floating point registers (at least on some processors/compilers), even though no floating point operations are obvious. Such tasks should be given the FP_? (I forget the exact spelling) task attribute, causing their FP regs to be saved during context switches.

-1
votes

I think you will find this article interesting. Maybe you are getting into a floating point exception.

I never used PowerPC, but I'm good with Google :P