PM: clk: make PM clock layer compatible with clocks that must sleep

The clock API splits its interface into sleepable ant atomic contexts:

 - clk_prepare/clk_unprepare for stuff that might sleep

 - clk_enable_clk_disable for anything that may be done in atomic context

The code handling runtime PM for clocks only calls clk_disable() on
suspend requests, and clk_enable on resume requests. This means that
runtime PM with clock providers that only have the prepare/unprepare
methods implemented is basically useless.

Many clock implementations can't accommodate atomic contexts. This is
often the case when communication with the clock happens through another
subsystem like I2C or SCMI.

Let's make the clock PM code useful with such clocks by safely invoking
clk_prepare/clk_unprepare upon resume/suspend requests. Of course, when
such clocks are registered with the PM layer then pm_runtime_irq_safe()
can't be used, and neither pm_runtime_suspend() nor pm_runtime_resume()
may be invoked in atomic context.

For clocks that do implement the enable and disable methods then
everything just works as before.

A note on sparse:
According to https://lwn.net/Articles/109066/ there are things
that sparse can't cope with. In particular, pm_clk_op_lock() and
pm_clk_op_unlock() may or may not lock/unlock psd->lock depending on
some runtime condition. To work around that we tell it the lock is
always untaken for the purpose of static analisys.

Thanks to Naresh Kamboju for reporting issues with the initial patch.

Signed-off-by: Nicolas Pitre <npitre@baylibre.com>
Tested-by: Naresh Kamboju <naresh.kamboju@linaro.org>
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>

1 files changed, 23 insertions, 1 deletions

diff --git a/include/linux/clk.h b/include/linux/clk.h
index 31ff1bf1b79f..a4a86aa8b11a 100644
--- a/include/linux/clk.h
+++ b/include/linux/clk.h
@@ -238,6 +238,7 @@ static inline bool clk_is_match(const struct clk *p, const struct clk *q)
 
 #endif
 
+#ifdef CONFIG_HAVE_CLK_PREPARE
 /**
  * clk_prepare - prepare a clock source
  * @clk: clock source
@@ -246,10 +247,26 @@ static inline bool clk_is_match(const struct clk *p, const struct clk *q)
  *
  * Must not be called from within atomic context.
  */
-#ifdef CONFIG_HAVE_CLK_PREPARE
 int clk_prepare(struct clk *clk);
 int __must_check clk_bulk_prepare(int num_clks,
 				  const struct clk_bulk_data *clks);
+
+/**
+ * clk_is_enabled_when_prepared - indicate if preparing a clock also enables it.
+ * @clk: clock source
+ *
+ * Returns true if clk_prepare() implicitly enables the clock, effectively
+ * making clk_enable()/clk_disable() no-ops, false otherwise.
+ *
+ * This is of interest mainly to the power management code where actually
+ * disabling the clock also requires unpreparing it to have any material
+ * effect.
+ *
+ * Regardless of the value returned here, the caller must always invoke
+ * clk_enable() or clk_prepare_enable()  and counterparts for usage counts
+ * to be right.
+ */
+bool clk_is_enabled_when_prepared(struct clk *clk);
 #else
 static inline int clk_prepare(struct clk *clk)
 {
@@ -263,6 +280,11 @@ clk_bulk_prepare(int num_clks, const struct clk_bulk_data *clks)
 	might_sleep();
 	return 0;
 }
+
+static inline bool clk_is_enabled_when_prepared(struct clk *clk)
+{
+	return false;
+}
 #endif
 
 /**