ubb-vga: use a free-running counter instead of resetting it for each line

Two benefits:
1) We don't accumulate errors from the delay between the timer reset and
   the deadline preceding it
2) In the future, we may use WAIT to wait for timer expiration, which
   should cause less bus activity and is should also reduce jitter

- regs4740.h (TFR. TFSR, TFCR, TDHR): added more timer registers
- ubb-vga.c (until): renamed to "delay" and changed to measure relative
  to the last deadline
- ubb-vga.c (line, hdelay, frame): replaced "until" with "delay"
- ubb-vga.c (hdelay, frame, session): reset the timer only once, at
  the beginning of the session
- ubb-vga.c (frame): we didn't wait for the horizontal back porch of
  the last image line

ubb-vga/regs4740.h

@@ -50,8 +50,12 @@
 #define	TSCR	_TCU(0x3c)		/* Timer STOP clear */
 #define	TESR	_TCU(0x14)		/* Timer counter enable set */
 #define	TECR	_TCU(0x18)		/* Timer counter enable clear */
+#define	TFR	_TCU(0x20)		/* Timer flag */
+#define	TFSR	_TCU(0x24)		/* Timer flag set */
+#define	TFCR	_TCU(0x28)		/* Timer flag clear */
 #define	TCSR(n)	_TCU(0x4c+0x10*(n))	/* Timer control */
 #define	TDFR(n)	_TCU(0x40+0x10*(n))	/* Timer data full */
+#define	TDHR(n)	_TCU(0x44+0x10*(n))	/* Timer data half */
 #define	TCNT(n)	_TCU(0x48+0x10*(n))	/* Timer counter */
 
 #define	MSC_STRPCL	_MSC(0x00)	/* Start/stop MMC/SD clock */

ubb-vga/ubb-vga.c

@@ -233,9 +233,14 @@ static void cleanup(void)
 /* ----- Delay logic ------------------------------------------------------- */
 
 
-static void until(uint16_t cycles)
+static void delay(uint16_t cycles)
 {
-	while ((TCNT(TIMER) & 0xffff) < cycles);
+	static uint16_t next = 0;
+
+	next += cycles;
+	TDHR(TIMER) = next;
+	TFCR = 1 << (TIMER+16);
+	while (!(TFR & (1 << (TIMER+16))));
 }
 
 
@@ -292,7 +297,7 @@ static void line(unsigned long line)
 	DTA(DMA) = REG_PADDR(MSC_TXFIFO);	/* MUST set this each time */
 	DTC(DMA) = (mode->xres+63) >> 6;
 
-	until(mode->hback_cycles);
+	delay(mode->hback_cycles);
 
 	/* HSYNC */
 
@@ -313,7 +318,7 @@ static void line(unsigned long line)
 	    (1 << 31) |         /* no descriptor */
 	    1;           
 
-	until(mode->hback_cycles+mode->hsync_cycles);
+	delay(mode->hsync_cycles);
 
 //	MSC_TXFIFO = 0xffffffff;
 
@@ -323,7 +328,7 @@ static void line(unsigned long line)
 	PDDATS = HSYNC;
 	PDFUNC = CMD;
 
-	until(mode->line_cycles);
+	delay(mode->line_cycles-mode->hback_cycles-mode->hsync_cycles);
 	MSC_TXFIFO = 0;
 	if (MSC_STAT & 3)
 		bad++;
@@ -333,11 +338,10 @@ static void line(unsigned long line)
 static void hdelay(int cycles)
 {
 	while (cycles--) {
-		TCNT(TIMER) = 0;
 		PDDATC = HSYNC;
-		until(mode->hsync_cycles);
+		delay(mode->hsync_cycles);
 		PDDATS = HSYNC;
-		until(mode->line_cycles);
+		delay(mode->line_cycles-mode->hsync_cycles);
 	}
 }
 
@@ -359,22 +363,15 @@ static void frame(const unsigned long *f)
 	 * 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(TIMER) = 0;
 	PDDATC = HSYNC;
-	until(mode->hsync_cycles);
+	delay(mode->hsync_cycles);
 	PDDATS = HSYNC;
-	until(mode->line_cycles-mode->hback_cycles);
+	delay(mode->line_cycles-mode->hback_cycles-mode->hsync_cycles);
 
-	/*
-	 * Note: resetting the timer just before calling "line" isn't enough.
-	 * We have t reset it before the loop and right after returning from
-	 * "line".
-	 */
-	TCNT(TIMER) = 0;
-	for (p = f; p != f+mode->yres; p++) {
+	for (p = f; p != f+mode->yres; p++)
 		line(*p);
-		TCNT(TIMER) = 0;
-	}
+
+	delay(mode->hback_cycles);
 
 	/* Back porch */
 	hdelay(mode->vback_lines);
@@ -401,6 +398,7 @@ static void session(void (*gen)(void **fb, int xres, int yres), int frames)
 
 	setup_noirq();
 
+	TCNT(TIMER) = 0;
 	for (i = 0; !frames || i != frames; i++) {
 		frame(f_phys);
 		if (DTC(DMA)) {