Commit

flicker_investigation_continued.html

@@ -252,16 +252,53 @@ transient that completes within one poll interval. Specifically:</p>
       hardware-level events that do not change any register I am polling
       (e.g. a brief PHY FIFO timing slip with no associated interrupt)
       are also outside what this instrumentation can detect.</li>
-  <li><strong>Why I did not poll faster.</strong> The DSIM register reads
-      go through the <code>memtool</code> userspace tool via subprocess
-      calls, which has non-trivial overhead. Pushing the wide poll much
-      below ~100 ms started loading the device CPU and impacting the I²C
-      poll loop. I did not try to optimise this further.</li>
+  <li><strong>Why I did not push the wide poll faster.</strong> The
+      <code>memtool</code> subprocess calls have non-trivial overhead and
+      pushing the wide poll much below ~100 ms started loading the device CPU
+      and impacting the I²C poll loop.</li>
 </ul>
 
-<p>So the correct way to read section 4 is: at the resolutions I could poll,
-nothing showed up. That is consistent with — but not proof of — the fault
-being below register visibility (e.g. a sub-frame transient timing event).</p>
+<h3>Follow-up trial: high-rate /dev/mem mmap poll</h3>
+
+<p>To try to see below the 500 ms ceiling I added a second poller that
+bypasses <code>memtool</code> by mmap-ing <code>/dev/mem</code> directly in
+Python and reading the DSIM registers via load instructions, aiming for
+~1 ms resolution. A short captured window showed:</p>
+
+<ul class="tight">
+  <li><code>DSIM_CLKCTRL</code> (offset 0x008) <em>is</em> active at sub-
+      millisecond timescales — it flips from its steady-state value of
+      <code>0x02</code> to <code>0x10</code>, <code>0x100</code> or
+      <code>0x01</code> in 1–2 ms wide pulses, occurring roughly every
+      200 ms during normal operation.</li>
+  <li>The pulses appear at a very regular cadence and are not visibly
+      aligned with the video switches I was driving. The pattern looks
+      consistent with routine per-frame clock-enable management by the
+      kernel driver rather than fault-correlated events — but I am
+      <em>not</em> claiming to have shown that, only that the activity I
+      saw was regular.</li>
+  <li>The mmap poll at that rate destabilised the device — the full unit
+      (not just the userspace service) crashed/rebooted within seconds of
+      starting, on multiple attempts at 1 ms target rate. I therefore could
+      not run it long enough to mark visible flicker events and compare
+      timestamps against the CLKCTRL transitions.</li>
+</ul>
+
+<p>So the most accurate position on register-level diagnostics for this
+investigation is:</p>
+
+<ul class="tight">
+  <li>At the resolutions I could safely sustain (500 ms wide poll, 10 ms PLL
+      poll), nothing showed up during a confirmed flicker burst.</li>
+  <li>At higher rate (1 ms) the registers are demonstrably busy, but the
+      activity I captured looks like normal driver behaviour, and I could
+      not keep the device alive long enough to test correlation with
+      flicker timing.</li>
+  <li>The combined evidence is consistent with — but not proof of — the
+      fault being below register-level visibility on this rig, e.g. a
+      sub-frame transient timing event that does not change any register
+      I am able to read safely.</li>
+</ul>
 
 <h2>5.  Reproducer without GStreamer or video</h2>