samples: usb: uac2: explicit: Fix SOF offset integer conversion glitch

Rework the formula used to convert timer CC values to SOF offset to
eliminate a "division glitch" happening around SOF crossing, i.e. when
the framestart shifts from being captured shortly before SOF to being
captured shortly after SOF.

When audio samples are consumed faster than nominal, i.e. when there
is +1 samples every now and then, the reported SOF offset at High-Speed
was going 10, 9, 8, 7, 6, 5, 4, 3, 2, 1, 0, "glitch" 7, 6, 5, 4, 3, 2,
1, 0, -1, -2, ...

With the fix the SOF offset goes as expected from 0 to -1, i.e. there is
no jump from 0 to 7 around the SOF crossing and the reported SOF offset
goes linearly (until it is correclty bumped from negative values back to
positive values when +1 sample packet is transmitted on I2S).

Signed-off-by: Tomasz Moń <[email protected]>

Author: tmon-nordic

samples/subsys/usb/uac2_explicit_feedback/src/feedback_nrf.c

@@ -272,7 +272,7 @@ static void update_sof_offset(struct feedback_ctx *ctx, uint32_t sof_cc,
 	int sof_offset;
 
 	if (!IS_ENABLED(CONFIG_APP_USE_I2S_LRCLK_EDGES_COUNTER)) {
-		uint32_t clks_per_edge;
+		uint32_t nominator;
 
 		/* Convert timer clock (independent from both Audio clock and
 		 * USB host SOF clock) to fake sample clock shifted by P values.
@@ -282,18 +282,17 @@ static void update_sof_offset(struct feedback_ctx *ctx, uint32_t sof_cc,
 		 * when regulated and therefore the relative clock frequency
 		 * discrepancies are essentially negligible.
 		 */
-		clks_per_edge = sof_cc / ctx->counts_per_sof;
-		sof_cc /= MAX(clks_per_edge, 1);
-		framestart_cc /= MAX(clks_per_edge, 1);
+		nominator = MIN(sof_cc, framestart_cc) * ctx->counts_per_sof;
+		sof_offset = nominator / MAX(sof_cc, 1);
+	} else {
+		sof_offset = framestart_cc;
 	}
 
 	/* /2 because we treat the middle as a turning point from being
 	 * "too late" to "too early".
 	 */
-	if (framestart_cc > ctx->counts_per_sof/2) {
-		sof_offset = framestart_cc - ctx->counts_per_sof;
-	} else {
-		sof_offset = framestart_cc;
+	if (sof_offset > ctx->counts_per_sof/2) {
+		sof_offset -= ctx->counts_per_sof;
 	}
 
 	/* The heuristic above is not enough when the offset gets too large.