select statement in a thread

[Folknology]

Agreed this is good stuff, you should add a page to the wiki about select using your work above Bianco.

[bsmithyman]

Would someone be willing to check me on this? My servo library uses a couple of nested selects to iterate over the servo lines on the output port. Originally I'd thought that using a select with a "tmr when timerafter :> blah" would prevent it from blocking. However, it seems like in the second select statement a default case is needed to force it to be non-blocking (and otherwise a waiteu is issued).

In my for loop, if the default case doesn't exist in the select statement, would all the later loop iterations be waiting on the first timer?

Code: Select all

void servo_handler (out port servo_port, chanend cmd_line) {
	
	timer tmr;
	unsigned int time, durations[8];
	unsigned char servo_state = 0;
	unsigned int i;
	servo_command instructions;
	
	
	for (i = 0; i < 8; i++) {
		durations[i] = SERVO_CENTRE_DURATION;
	}
	
	tmr :> time;
	
	while (1) {
		select {
		case cmd_line :> instructions:
			parse_instructions(instructions, durations);
			break;
		
		case tmr when timerafter(time + SERVO_REFRESH_PERIOD) :> void:
			time += SERVO_REFRESH_PERIOD;
			servo_state = 0xff;
			servo_port <: servo_state;
			break;
		
		default:
			for (i = 0; i < 8; i++) {
				select {
				case tmr when timerafter(time+durations[i]) :> void:
					servo_state &= !(1 << i);
					servo_port <: servo_state;
					break;

				default:		// <-- This block is what's in question.
					break;	// <-- i.e. Whether or not to include the default case.
				}
			}
			break;
		}	
	}

Thanks!

[Folknology]

It looks correct to me, the second default is required to avoid it waiting on whatever duration[0] is and hence blocking duration[1..7].

If you had the durations sorted lowest first you could use a blocking (no default) select perhaps..

P.S. I love your servo technique, its neat
regards
Al

[bsmithyman]

It looks correct to me, the second default is required to avoid it waiting on whatever duraction[0] is and hence blocking duration[1..7].

If you had the durations sorted lowest first you could use a blocking (no default) select perhaps..

P.S. I love your servo technique, its neat
regards
Al

Thanks Al,

Actually, I might have to try the sorting technique. I'd have to see how quickly I can do the math in the instruction parsing phase, so that none of the servos missed a timing signal, but it would significantly reduce the processing overhead from the infinite loop. Although, that would come with more latency on the command thread waiting to be acknowledged, perhaps as much as 2 ms I suppose.

Cheers,
Brendan