ARM: 7563/1: SMP_TWD: make setup()/stop() reentrant

It has been brought to my knowledge that the .setup()/.stop()
function pair in the SMP TWD is going to be called from atomic
contexts for CPUs coming and going, and then the
clk_prepare()/clk_unprepare() calls cannot be called
on subsequent .setup()/.stop() iterations. This is however
just the tip of an iceberg as the function pair is not
designed to be reentrant at all.

This change makes the SMP_TWD clock .setup()/.stop() pair reentrant
by splitting the .setup() function in three parts:

- One COMMON part that is executed the first time the first CPU
  in the TWD cluster is initialized. This will fetch the TWD
  clk for the cluster and prepare+enable it. If no clk is
  available it will calibrate the rate instead.

- One part that is executed the FIRST TIME a certain CPU is
  brought on-line. This initializes and sets up the clock event
  for a certain CPU.

- One part that is executed on every subsequent .setup() call.
  This will re-initialize the clock event. This is augmented
  to call the clk_enable()/clk_disable() pair properly.

Cc: Shawn Guo <shawn.guo@linaro.org>
Reported-by: Peter Chen <peter.chen@freescale.com>
Reviewed-by: Santosh Shilimkar <santosh.shilimkar@ti.com>
Tested-by: Shawn Guo <shawn.guo@linaro.org>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>
Signed-off-by: Russell King <rmk+kernel@arm.linux.org.uk>

1 files changed, 37 insertions, 5 deletions

diff --git a/arch/arm/kernel/smp_twd.c b/arch/arm/kernel/smp_twd.c
index 780b05706364..a2e74375945a 100644
--- a/arch/arm/kernel/smp_twd.c
+++ b/arch/arm/kernel/smp_twd.c
@@ -31,6 +31,8 @@ static void __iomem *twd_base;
 
 static struct clk *twd_clk;
 static unsigned long twd_timer_rate;
+static bool common_setup_called;
+static DEFINE_PER_CPU(bool, percpu_setup_called);
 
 static struct clock_event_device __percpu **twd_evt;
 static int twd_ppi;
@@ -264,15 +266,45 @@ static struct clk *twd_get_clock(void)
 static int __cpuinit twd_timer_setup(struct clock_event_device *clk)
 {
         struct clock_event_device **this_cpu_clk;
+        int cpu = smp_processor_id();
 
-        if (!twd_clk)
+        /*
+         * If the basic setup for this CPU has been done before don't
+         * bother with the below.
+         */
+        if (per_cpu(percpu_setup_called, cpu)) {
+                __raw_writel(0, twd_base + TWD_TIMER_CONTROL);
+                clockevents_register_device(*__this_cpu_ptr(twd_evt));
+                enable_percpu_irq(clk->irq, 0);
+                return 0;
+        }
+        per_cpu(percpu_setup_called, cpu) = true;
+
+        /*
+         * This stuff only need to be done once for the entire TWD cluster
+         * during the runtime of the system.
+         */
+        if (!common_setup_called) {
                 twd_clk = twd_get_clock();
 
-        if (!IS_ERR_OR_NULL(twd_clk))
-                twd_timer_rate = clk_get_rate(twd_clk);
-        else
-                twd_calibrate_rate();
+                /*
+                 * We use IS_ERR_OR_NULL() here, because if the clock stubs
+                 * are active we will get a valid clk reference which is
+                 * however NULL and will return the rate 0. In that case we
+                 * need to calibrate the rate instead.
+                 */
+                if (!IS_ERR_OR_NULL(twd_clk))
+                        twd_timer_rate = clk_get_rate(twd_clk);
+                else
+                        twd_calibrate_rate();
+
+                common_setup_called = true;
+        }
 
+        /*
+         * The following is done once per CPU the first time .setup() is
+         * called.
+         */
         __raw_writel(0, twd_base + TWD_TIMER_CONTROL);
 
         clk->name = "local_timer";