ARM: ks8695: convert to generic time and clocksource

Old platforms using ancient gettimeoffset() and other arcane APIs are standing in the way of cleaning up the ARM kernel. The gettimeoffset() was also broken: it would try to read out the timer counter value, while this would not work (the counter statically returns the initially programmed value) so the implementation would anyway fall back to a homebrew version of jiffie calculation.

This is an attempt at blind-coding a generic time and clocksource driver for the platform by way of a datasheet and looking at the old code.

Signed-off-by Linus Walleij <[email protected]>

---
ChangeLog v4->v5:
- Don't unconditionally disable timer 1 in the set_mode() function, this reportedly hangs the platform in loop calibration.
ChangeLog v3->v4:
- Skip trying to provide an MMIO clocksource, there is no clocksource to be found on this system, so set up a clockevent and let the timekeeping core fall back to jiffies.
ChangeLog v2->v3:
- Correct the MMIO timer read function: this timer counts DOWN not up - since clocksources shall move forward in time, the ever decreasing clocksource was likely ignored.
ChangeLog v1->v2:
- Corrected the mask when setting clockevents from & TMCON_T1EN to & ~TMCON_T1EN so it doesn't shut down the clock source.

---
  arch/arm/Kconfig            |   3 +-
  arch/arm/mach-ks8695/time.c | 105 ++++++++++++++++++++++++++++----------------
  2 files changed, 69 insertions(+), 39 deletions(-)

arch/arm/Kconfig +2 -1

index 6d6e18f..e9ce038 100644
@@ -652,8 +652,9 @@ config ARCH_KS8695
  	bool "Micrel/Kendin KS8695"
  	select CPU_ARM922T
  	select ARCH_REQUIRE_GPIOLIB
-	select ARCH_USES_GETTIMEOFFSET
  	select NEED_MACH_MEMORY_H
+	select CLKSRC_MMIO
+	select GENERIC_CLOCKEVENTS
  	help
  	  Support for Micrel/Kendin KS8695 "Centaur" (ARM922T) based
  	  System-on-Chip devices.

arch/arm/mach-ks8695/time.c +67 -38

index 6974c23..46c84bc 100644
@@ -25,6 +25,7 @@
  #include <linux/kernel.h>
  #include <linux/sched.h>
  #include <linux/io.h>
+#include <linux/clockchips.h>
 
  #include <asm/mach/time.h>
  #include <asm/system_misc.h>

@@ -53,44 +54,69 @@
  /* Timer0 Timeout Counter Register */
  #define T0TC_WATCHDOG		(0xff)		/* Enable watchdog mode */
 
-/*
- * Returns number of ms since last clock interrupt.  Note that interrupts
- * will have been disabled by do_gettimeoffset()
- */
-static unsigned long ks8695_gettimeoffset (void)
+static void ks8695_set_mode(enum clock_event_mode mode,
+			    struct clock_event_device *evt)
  {
-	unsigned long elapsed, tick2, intpending;
+	u32 tmcon;
 
-	/*
-	 * Get the current number of ticks.  Note that there is a race
-	 * condition between us reading the timer and checking for an
-	 * interrupt.  We solve this by ensuring that the counter has not
-	 * reloaded between our two reads.
-	 */
-	elapsed = readl_relaxed(KS8695_TMR_VA + KS8695_T1TC) + readl_relaxed(KS8695_TMR_VA + KS8695_T1PD);
-	do {
-		tick2 = elapsed;
-		intpending = readl_relaxed(KS8695_IRQ_VA + KS8695_INTST) & (1 << KS8695_IRQ_TIMER1);
-		elapsed = readl_relaxed(KS8695_TMR_VA + KS8695_T1TC) + readl_relaxed(KS8695_TMR_VA + KS8695_T1PD);
-	} while (elapsed > tick2);
-
-	/* Convert to number of ticks expired (not remaining) */
-	elapsed = (CLOCK_TICK_RATE / HZ) - elapsed;
-
-	/* Is interrupt pending?  If so, then timer has been reloaded already. */
-	if (intpending)
-		elapsed += (CLOCK_TICK_RATE / HZ);
-
-	/* Convert ticks to usecs */
-	return (unsigned long)(elapsed * (tick_nsec / 1000)) / LATCH;
+	if (mode == CLOCK_EVT_FEAT_PERIODIC) {
+		u32 rate = DIV_ROUND_CLOSEST(KS8695_CLOCK_RATE, HZ);
+		u32 half = DIV_ROUND_CLOSEST(rate, 2);
+
+		/* Disable timer 1 */
+		tmcon = readl_relaxed(KS8695_TMR_VA + KS8695_TMCON);
+		tmcon &= ~TMCON_T1EN;
+		writel_relaxed(tmcon, KS8695_TMR_VA + KS8695_TMCON);
+
+		/* Both registers need to count down */
+		writel_relaxed(half, KS8695_TMR_VA + KS8695_T1TC);
+		writel_relaxed(half, KS8695_TMR_VA + KS8695_T1PD);
+
+		/* Re-enable timer1 */
+		tmcon |= TMCON_T1EN;
+		writel_relaxed(tmcon, KS8695_TMR_VA + KS8695_TMCON);
+	}
+}
+
+static int ks8695_set_next_event(unsigned long cycles,
+				 struct clock_event_device *evt)
+
+{
+	u32 half = DIV_ROUND_CLOSEST(cycles, 2);
+	u32 tmcon;
+
+	/* Disable timer 1 */
+	tmcon = readl_relaxed(KS8695_TMR_VA + KS8695_TMCON);
+	tmcon &= ~TMCON_T1EN;
+	writel_relaxed(tmcon, KS8695_TMR_VA + KS8695_TMCON);
+
+	/* Both registers need to count down */
+	writel_relaxed(half, KS8695_TMR_VA + KS8695_T1TC);
+	writel_relaxed(half, KS8695_TMR_VA + KS8695_T1PD);
+
+	/* Re-enable timer1 */
+	tmcon |= TMCON_T1EN;
+	writel_relaxed(tmcon, KS8695_TMR_VA + KS8695_TMCON);
+
+	return 0;
  }
 
+static struct clock_event_device clockevent_ks8695 = {
+	.name		= "ks8695_t1tc",
+	.rating		= 300, /* Reasonably fast and accurate clock event */
+	.features	= CLOCK_EVT_FEAT_ONESHOT | CLOCK_EVT_FEAT_PERIODIC,
+	.set_next_event	= ks8695_set_next_event,
+	.set_mode	= ks8695_set_mode,
+};
+
  /*
   * IRQ handler for the timer.
   */
  static irqreturn_t ks8695_timer_interrupt(int irq, void *dev_id)
  {
-	timer_tick();
+	struct clock_event_device *evt = &clockevent_ks8695;
+
+	evt->event_handler(evt);
  	return IRQ_HANDLED;
  }
 

@@ -102,18 +128,22 @@ static struct irqaction ks8695_timer_irq = {
 
  static void ks8695_timer_setup(void)
  {
-	unsigned long tmout = CLOCK_TICK_RATE / HZ;
  	unsigned long tmcon;
 
-	/* disable timer1 */
+	/* Disable timer 0 and 1 */
  	tmcon = readl_relaxed(KS8695_TMR_VA + KS8695_TMCON);
-	writel_relaxed(tmcon & ~TMCON_T1EN, KS8695_TMR_VA + KS8695_TMCON);
-
-	writel_relaxed(tmout / 2, KS8695_TMR_VA + KS8695_T1TC);
-	writel_relaxed(tmout / 2, KS8695_TMR_VA + KS8695_T1PD);
+	tmcon &= ~TMCON_T0EN;
+	tmcon &= ~TMCON_T1EN;
+	writel_relaxed(tmcon, KS8695_TMR_VA + KS8695_TMCON);
 
-	/* re-enable timer1 */
-	writel_relaxed(tmcon | TMCON_T1EN, KS8695_TMR_VA + KS8695_TMCON);
+	/*
+	 * Use timer 1 to fire IRQs on the timeline, minimum 2 cycles
+	 * (one on each counter) maximum 2*2^32, but the API will only
+	 * accept up to a 32bit full word (0xFFFFFFFFU).
+	 */
+	clockevents_config_and_register(&clockevent_ks8695,
+					KS8695_CLOCK_RATE, 2,
+					0xFFFFFFFFU);
  }
 
  static void __init ks8695_timer_init (void)

@@ -126,7 +156,6 @@ static void __init ks8695_timer_init (void)
 
  struct sys_timer ks8695_timer = {
  	.init		= ks8695_timer_init,
-	.offset		= ks8695_gettimeoffset,
  };
 
  void ks8695_restart(char mode, const char *cmd)