clocksource/drivers/timer-atmel-tcb: Stop using the 32kHz for clockevents

Stop using the slow clock as the clock source for 32 bit counters because
even at 10MHz, they are able to handle delays up to two minutes. This
provides a way better resolution.
Signed-off-by: Alexandre Belloni <alexandre.belloni@bootlin.com>
Signed-off-by: Daniel Lezcano <daniel.lezcano@linaro.org>
Link: https://lore.kernel.org/r/20200710230813.1005150-8-alexandre.belloni@bootlin.com

drivers/clocksource/timer-atmel-tcb.c

@@ -27,9 +27,10 @@
  *   - Some chips support 32 bit counter. A single channel is used for
  *     this 32 bit free-running counter. the second channel is not used.
  *
- *   - The third channel may be used to provide a 16-bit clockevent
- *     source, used in either periodic or oneshot mode.  This runs
- *     at 32 KiHZ, and can handle delays of up to two seconds.
+ *   - The third channel may be used to provide a clockevent source, used in
+ *   either periodic or oneshot mode. For 16-bit counter its runs at 32 KiHZ,
+ *   and can handle delays of up to two seconds. For 32-bit counters, it runs at
+ *   the same rate as the clocksource
  *
  * REVISIT behavior during system suspend states... we should disable
  * all clocks and save the power.  Easily done for clockevent devices,
@@ -47,6 +48,8 @@ static struct
 } tcb_cache[3];
 static u32 bmr_cache;
 
+static const u8 atmel_tcb_divisors[] = { 2, 8, 32, 128 };
+
 static u64 tc_get_cycles(struct clocksource *cs)
 {
 	unsigned long	flags;
@@ -143,6 +146,7 @@ static unsigned long notrace tc_delay_timer_read32(void)
 struct tc_clkevt_device {
 	struct clock_event_device	clkevt;
 	struct clk			*clk;
+	u32				rate;
 	void __iomem			*regs;
 };
 
@@ -151,13 +155,6 @@ static struct tc_clkevt_device *to_tc_clkevt(struct clock_event_device *clkevt)
 	return container_of(clkevt, struct tc_clkevt_device, clkevt);
 }
 
-/* For now, we always use the 32K clock ... this optimizes for NO_HZ,
- * because using one of the divided clocks would usually mean the
- * tick rate can never be less than several dozen Hz (vs 0.5 Hz).
- *
- * A divided clock could be good for high resolution timers, since
- * 30.5 usec resolution can seem "low".
- */
 static u32 timer_clock;
 
 static int tc_shutdown(struct clock_event_device *d)
@@ -183,7 +180,7 @@ static int tc_set_oneshot(struct clock_event_device *d)
 
 	clk_enable(tcd->clk);
 
-	/* slow clock, count up to RC, then irq and stop */
+	/* count up to RC, then irq and stop */
 	writel(timer_clock | ATMEL_TC_CPCSTOP | ATMEL_TC_WAVE |
 		     ATMEL_TC_WAVESEL_UP_AUTO, regs + ATMEL_TC_REG(2, CMR));
 	writel(ATMEL_TC_CPCS, regs + ATMEL_TC_REG(2, IER));
@@ -205,10 +202,10 @@ static int tc_set_periodic(struct clock_event_device *d)
 	 */
 	clk_enable(tcd->clk);
 
-	/* slow clock, count up to RC, then irq and restart */
+	/* count up to RC, then irq and restart */
 	writel(timer_clock | ATMEL_TC_WAVE | ATMEL_TC_WAVESEL_UP_AUTO,
 		     regs + ATMEL_TC_REG(2, CMR));
-	writel((32768 + HZ / 2) / HZ, tcaddr + ATMEL_TC_REG(2, RC));
+	writel((tcd->rate + HZ / 2) / HZ, tcaddr + ATMEL_TC_REG(2, RC));
 
 	/* Enable clock and interrupts on RC compare */
 	writel(ATMEL_TC_CPCS, regs + ATMEL_TC_REG(2, IER));
@@ -256,47 +253,55 @@ static irqreturn_t ch2_irq(int irq, void *handle)
 	return IRQ_NONE;
 }
 
-static int __init setup_clkevents(struct atmel_tc *tc, int clk32k_divisor_idx)
+static int __init setup_clkevents(struct atmel_tc *tc, int divisor_idx)
 {
 	int ret;
 	struct clk *t2_clk = tc->clk[2];
 	int irq = tc->irq[2];
-
-	ret = clk_prepare_enable(tc->slow_clk);
-	if (ret)
-		return ret;
+	int bits = tc->tcb_config->counter_width;
 
 	/* try to enable t2 clk to avoid future errors in mode change */
 	ret = clk_prepare_enable(t2_clk);
-	if (ret) {
-		clk_disable_unprepare(tc->slow_clk);
+	if (ret)
 		return ret;
-	}
-
-	clk_disable(t2_clk);
 
 	clkevt.regs = tc->regs;
 	clkevt.clk = t2_clk;
 
-	timer_clock = clk32k_divisor_idx;
+	if (bits == 32) {
+		timer_clock = divisor_idx;
+		clkevt.rate = clk_get_rate(t2_clk) / atmel_tcb_divisors[divisor_idx];
+	} else {
+		ret = clk_prepare_enable(tc->slow_clk);
+		if (ret) {
+			clk_disable_unprepare(t2_clk);
+			return ret;
+		}
+
+		clkevt.rate = clk_get_rate(tc->slow_clk);
+		timer_clock = ATMEL_TC_TIMER_CLOCK5;
+	}
+
+	clk_disable(t2_clk);
 
 	clkevt.clkevt.cpumask = cpumask_of(0);
 
 	ret = request_irq(irq, ch2_irq, IRQF_TIMER, "tc_clkevt", &clkevt);
 	if (ret) {
 		clk_unprepare(t2_clk);
-		clk_disable_unprepare(tc->slow_clk);
+		if (bits != 32)
+			clk_disable_unprepare(tc->slow_clk);
 		return ret;
 	}
 
-	clockevents_config_and_register(&clkevt.clkevt, 32768, 1, 0xffff);
+	clockevents_config_and_register(&clkevt.clkevt, clkevt.rate, 1, BIT(bits) - 1);
 
 	return ret;
 }
 
 #else /* !CONFIG_GENERIC_CLOCKEVENTS */
 
-static int __init setup_clkevents(struct atmel_tc *tc, int clk32k_divisor_idx)
+static int __init setup_clkevents(struct atmel_tc *tc, int divisor_idx)
 {
 	/* NOTHING */
 	return 0;
@@ -346,8 +351,6 @@ static void __init tcb_setup_single_chan(struct atmel_tc *tc, int mck_divisor_id
 	writel(ATMEL_TC_SYNC, tcaddr + ATMEL_TC_BCR);
 }
 
-static const u8 atmel_tcb_divisors[] = { 2, 8, 32, 128 };
-
 static struct atmel_tcb_config tcb_rm9200_config = {
 	.counter_width = 16,
 };
@@ -472,7 +475,7 @@ static int __init tcb_clksrc_init(struct device_node *node)
 		goto err_disable_t1;
 
 	/* channel 2:  periodic and oneshot timer support */
-	ret = setup_clkevents(&tc, ATMEL_TC_TIMER_CLOCK5);
+	ret = setup_clkevents(&tc, best_divisor_idx);
 	if (ret)
 		goto err_unregister_clksrc;