ubb-vga2.c: code cleanup and some fine-tuning

ubb-vga/ubb-vga2.c

@@ -267,12 +267,12 @@ static void line(const uint32_t *line, volatile const uint32_t *prefetch)
 	*msc_strpcl = 1 << 3;	/* reset the MSC */
 //	while (*msc_stat & (1 << 15));
 
-	/* HSYNC */
 	*msc_txfifo = *p++;
-//	*msc_txfifo = *p++;
 
 	until(US(0.79));
 
+	/* HSYNC */
+
 	*pddatc = HSYNC;
 	*msc_strpcl = 2;	/* start MMC clock output */
 
@@ -284,18 +284,28 @@ static void line(const uint32_t *line, volatile const uint32_t *prefetch)
 
 	*msc_strpcl = 4;	/* START_OP */
 
-#if 1
+	until(US(0.79+3.77-0.3));
-//	*msc_txfifo = *p++;
-#else
-	for (p = line; p != line+1; p++)
-	for (p = line; p != line+1; p++)
-		*msc_txfifo = *p;//0xf0f0f0f0;//*p;
-#endif
-
-	until(US(0.79+3.77-0.2));
 	/*
-	 * Without the -0.2, the XEN-1510 only detects the signal in only about
+	 * Adjustment value tests with the XEN-1510:
-	 * 20% of all cases.
+	 *
+	 * Adjustment	Tries	Good	Jam	FIFO jitter
+	 *						Quick load
+	 * -0.0		10	3	7	0	n
+	 * -0.1		10	5	5	0	n
+	 * -0.2		10	6	4	0	n
+	 * -0.3		10	7	3	0	n
+	 *		10	5	5	0	y
+	 * -0.4		10	1	0	9	n
+	 * 		10	5	0	5	n	repeat
+	 *		10	5	0	5	y
+	 * -0.5		10	3	0	7	n
+	 * 		10	7	0	3	y
+	 * -1.0		 5	0	5	0
+	 *
+	 * Good = image is stable
+	 * Jam = does not detect the signal properly, loss of HSYNC, artefacts,
+	 *       or no image at all
+	 * FIFO jitter = some lines get shifted by a "digital" amount
 	 */
 
 	/* Front porch */
@@ -303,32 +313,32 @@ static void line(const uint32_t *line, volatile const uint32_t *prefetch)
 	*pdfuns = CMD;
 	*pddats = HSYNC;
 	*pdfunc = CMD;
-//	*msc_txfifo = *p++;
-//	for (; p != line+4; p++)
-//		*msc_txfifo = *p;//0xf0f0f0f0;//*p;
-//	until(US(0.79+3.77+1.79-0.1));
 
-//	while (p != line+32) { //line_words) {
+	/*
+	 * We don't wait for the end of the front porch because the beginning
+	 * of pixel data is determined by the MSC. Instead, we make good use
+	 * of the delay to shovel bits into the MSC's FIFO.
+	 */
+
+#if 0	/* quick load */
+	*msc_txfifo = *p++;
+	*msc_txfifo = *p++;
+#endif
 	while (p != line+line_words) {
 		uint8_t st;
 		do {
 			st = *msc_stat;
 			if (st & 3) {
-			//	printf("st 0x%04x\n", st);
+				bad++;
-bad++;
+				goto fail;
-goto fail;
-	//			return;
 			}
 		}
 		while (st & (1 << 7));
-		*msc_txfifo = *p;
+		*msc_txfifo = *p++;
-		//*msc_txfifo = 0xf0f0f0f0;
-		p++;
 	}
 
 fail:
-//(void) *(volatile uint32_t *) (line+line_words);
+	(void) *prefetch;
-(void) *prefetch;
 	until(line_cycles);
 }
 
@@ -356,16 +366,17 @@ static void frame(const uint32_t *f)
 
 	/* Front porch */
 
-#if 0
-	hdelay(32);
-#else
 	hdelay(31);
+
+	/*
+	 * The horizontal back porch of the previous line is handled inside
+	 * "line", so we have to wait for less than a full line here.
+	 */
 	*tcnt = 0;
 	*pddatc = HSYNC;
 	until(US(3.77));
 	*pddats = HSYNC;
 	until(line_cycles-US(0.79));
-#endif
 
 	for (p = f; p != f+240*line_words; p += line_words) {
 		line(p, p);