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Subject[PATCH 5/7] clocksource/cadence_ttc: Overhaul clocksource frequency adjustment
Date
The currently used method adjusting the clocksource to a changing input
frequency does not work on kernels from 3.11 on.
The new approach is to keep the timer frequency as constant as possible.
I.e.
- due to the TTC's prescaler limitations, allow frequency changes
only if the frequency scales by a power of 2
- adjust the counter's divider on the fly when a frequency change
occurs

When suspending though, the driver should not prevent rate changes in
order to allow the system to enter its low power state. For that
reason a PM notifier is added so rate changes can be ignored during
suspend/resume.

This limits cpufreq to scale by certain factors only.
But we may keep the time base somewhat constant, so that sleep() & co
keep working as expected, while supporting cpufreq and suspend.

Signed-off-by: Soren Brinkmann <soren.brinkmann@xilinx.com>
---
drivers/clocksource/cadence_ttc_timer.c | 142 +++++++++++++++++++++++++++-----
1 file changed, 122 insertions(+), 20 deletions(-)

diff --git a/drivers/clocksource/cadence_ttc_timer.c b/drivers/clocksource/cadence_ttc_timer.c
index 68a336038d8f..421b942dd707 100644
--- a/drivers/clocksource/cadence_ttc_timer.c
+++ b/drivers/clocksource/cadence_ttc_timer.c
@@ -16,12 +16,14 @@
*/

#include <linux/clk.h>
+#include <linux/clk-provider.h>
#include <linux/interrupt.h>
#include <linux/clockchips.h>
#include <linux/of_address.h>
#include <linux/of_irq.h>
#include <linux/slab.h>
#include <linux/sched_clock.h>
+#include <linux/suspend.h>

/*
* This driver configures the 2 16-bit count-up timers as follows:
@@ -52,6 +54,8 @@
#define TTC_CNT_CNTRL_DISABLE_MASK 0x1

#define TTC_CLK_CNTRL_CSRC_MASK (1 << 5) /* clock source */
+#define TTC_CLK_CNTRL_PSV_MASK 0x1e
+#define TTC_CLK_CNTRL_PSV_SHIFT 1

/*
* Setup the timers to use pre-scaling, using a fixed value for now that will
@@ -63,6 +67,8 @@
#define CLK_CNTRL_PRESCALE_EN 1
#define CNT_CNTRL_RESET (1 << 4)

+#define MAX_F_ERR 50
+
/**
* struct ttc_timer - This definition defines local timer structure
*
@@ -82,8 +88,13 @@ struct ttc_timer {
container_of(x, struct ttc_timer, clk_rate_change_nb)

struct ttc_timer_clocksource {
+ int scale_dir;
+ u32 scale_clk_ctrl_reg_old;
+ u32 scale_clk_ctrl_reg_new;
+ int suspending;
struct ttc_timer ttc;
struct clocksource cs;
+ struct notifier_block suspend_nb;
};

#define to_ttc_timer_clksrc(x) \
@@ -228,33 +239,120 @@ static int ttc_rate_change_clocksource_cb(struct notifier_block *nb,
struct ttc_timer_clocksource *ttccs = container_of(ttc,
struct ttc_timer_clocksource, ttc);

+ if (ttccs->suspending)
+ return NOTIFY_OK;
+
switch (event) {
- case POST_RATE_CHANGE:
+ case PRE_RATE_CHANGE:
+ {
+ u32 psv;
+ unsigned long factor, rate_low, rate_high;
+
+ if (ndata->new_rate == ndata->old_rate) {
+ ttccs->scale_dir = 0;
+ return NOTIFY_OK;
+ }
+
+ if (ndata->new_rate > ndata->old_rate) {
+ factor = DIV_ROUND_CLOSEST(ndata->new_rate,
+ ndata->old_rate);
+ ttccs->scale_dir = 1;
+ rate_low = ndata->old_rate;
+ rate_high = ndata->new_rate;
+ } else {
+ factor = DIV_ROUND_CLOSEST(ndata->old_rate,
+ ndata->new_rate);
+ ttccs->scale_dir = -1;
+ rate_low = ndata->new_rate;
+ rate_high = ndata->old_rate;
+ }
+
+ if (!is_power_of_2(factor))
+ return NOTIFY_BAD;
+
+ if (abs(rate_high - (factor * rate_low)) > MAX_F_ERR)
+ return NOTIFY_BAD;
+
+ factor = __ilog2_u32(factor);
+
/*
- * Do whatever is necessary to maintain a proper time base
- *
- * I cannot find a way to adjust the currently used clocksource
- * to the new frequency. __clocksource_updatefreq_hz() sounds
- * good, but does not work. Not sure what's that missing.
- *
- * This approach works, but triggers two clocksource switches.
- * The first after unregister to clocksource jiffies. And
- * another one after the register to the newly registered timer.
- *
- * Alternatively we could 'waste' another HW timer to ping pong
- * between clock sources. That would also use one register and
- * one unregister call, but only trigger one clocksource switch
- * for the cost of another HW timer used by the OS.
+ * store timer clock ctrl register so we can restore it in case
+ * of an abort.
*/
- clocksource_unregister(&ttccs->cs);
- clocksource_register_hz(&ttccs->cs,
- ndata->new_rate / PRESCALE);
- /* fall through */
- case PRE_RATE_CHANGE:
+ ttccs->scale_clk_ctrl_reg_old =
+ __raw_readl(ttccs->ttc.base_addr +
+ TTC_CLK_CNTRL_OFFSET);
+
+ psv = (ttccs->scale_clk_ctrl_reg_old &
+ TTC_CLK_CNTRL_PSV_MASK) >>
+ TTC_CLK_CNTRL_PSV_SHIFT;
+ if (ttccs->scale_dir < 0)
+ psv -= factor;
+ else
+ psv += factor;
+
+ /* prescaler within legal range? */
+ if (psv & ~(TTC_CLK_CNTRL_PSV_MASK >> TTC_CLK_CNTRL_PSV_SHIFT))
+ return NOTIFY_BAD;
+
+ ttccs->scale_clk_ctrl_reg_new = ttccs->scale_clk_ctrl_reg_old &
+ ~TTC_CLK_CNTRL_PSV_MASK;
+ ttccs->scale_clk_ctrl_reg_new |= psv << TTC_CLK_CNTRL_PSV_SHIFT;
+
+
+ /* scale down: adjust divider in post-change notification */
+ if (ttccs->scale_dir < 0)
+ return NOTIFY_DONE;
+
+ /* scale up: adjust divider now - before frequency change */
+ __raw_writel(ttccs->scale_clk_ctrl_reg_new,
+ ttccs->ttc.base_addr + TTC_CLK_CNTRL_OFFSET);
+ break;
+ }
+ case POST_RATE_CHANGE:
+ /* scale up: pre-change notification did the adjustment */
+ if (ttccs->scale_dir >= 0)
+ return NOTIFY_OK;
+
+ /* scale down: adjust divider now - after frequency change */
+ __raw_writel(ttccs->scale_clk_ctrl_reg_new,
+ ttccs->ttc.base_addr + TTC_CLK_CNTRL_OFFSET);
+ break;
+
case ABORT_RATE_CHANGE:
+ /* we have to undo the adjustment in case we scale up */
+ if (ttccs->scale_dir <= 0)
+ return NOTIFY_OK;
+
+ /* restore original register value */
+ __raw_writel(ttccs->scale_clk_ctrl_reg_old,
+ ttccs->ttc.base_addr + TTC_CLK_CNTRL_OFFSET);
+ /* fall through */
default:
return NOTIFY_DONE;
}
+
+ return NOTIFY_DONE;
+}
+
+static int ttc_suspend_notifier_cb(struct notifier_block *nb,
+ unsigned long event, void *data)
+{
+ struct ttc_timer_clocksource *ttccs = container_of(nb,
+ struct ttc_timer_clocksource, suspend_nb);
+
+ switch (event) {
+ case PM_SUSPEND_PREPARE:
+ ttccs->suspending = 1;
+ break;
+ case PM_POST_SUSPEND:
+ ttccs->suspending = 0;
+ break;
+ default:
+ break;
+ }
+
+ return NOTIFY_DONE;
}

static void __init ttc_setup_clocksource(struct clk *clk, void __iomem *base)
@@ -283,6 +381,10 @@ static void __init ttc_setup_clocksource(struct clk *clk, void __iomem *base)
&ttccs->ttc.clk_rate_change_nb))
pr_warn("Unable to register clock notifier.\n");

+ ttccs->suspend_nb.notifier_call = ttc_suspend_notifier_cb;
+ if (register_pm_notifier(&ttccs->suspend_nb))
+ pr_warn("Unable to register PM notifier.\n");
+
ttccs->ttc.base_addr = base;
ttccs->cs.name = "ttc_clocksource";
ttccs->cs.rating = 200;
--
1.8.4.2


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