Inconsistent RTIO Underflow for KC705 DMA

[pca006132]

Bug Report

One-Line Summary

Unable to measure a consistent slack for preventing DMA RTIO Underflow with KC705.

Issue Details

The measured value seems dependent on whether print is invoked, whether the DMA is played the first time, and possibly other unknown factors.

Steps to Reproduce

from artiq.experiment import *

class DMAPulses(EnvExperiment):
    def build(self):
        self.setattr_device("core")
        self.setattr_device("core_dma")
        self.setattr_device("ttl0")

    @kernel
    def record(self, t):
        with self.core_dma.record("pulse"):
            for i in range(10000):
                self.ttl0.pulse(t*ns)
                delay(t*ns)

    @kernel
    def run(self):
        self.core.reset()
        self.record(62)
        pulse_handle = self.core_dma.get_handle("pulse")
        lower = 0
        upper = 10000
        while upper - lower > 1:
            print('upper: ', upper, ', lower: ', lower)
            mid = (upper + lower) // 2;
            try:
                now = self.core.get_rtio_counter_mu()
                at_mu(now + mid)
                self.core_dma.playback_handle(pulse_handle)
                upper = mid
            except:
                lower = mid
            # tried adding some delay and see if it would be better
            x = self.core.get_rtio_counter_mu()
            self.core.wait_until_mu(x + 10000)
        print('slack: ', self.core.mu_to_seconds(upper)*1000000000)

And tweak some parameters, such as removing the line print('upper: ', upper, ', lower: ', lower), changing upper to 10000, etc. All of these would yield drastically different numbers, such as 2500, 1250, 1562, 3125.

Expected Behavior

Give approximately the same value. For example, the script gives around 1550 for ZC706 consistently.

Actual (undesired) Behavior

The tweaks would give drastically different values.

Your System (omit irrelevant parts)

• Operating System: NixOS
• ARTIQ version: current master.
• Version of the gateware and runtime loaded in the core device: 6.7530.9b39ble3.beta;nist_clock
• Hardware involved: KC705