CODEWITHSUNDEEP
clinux-kernel
The B-52s Bassist
The B-52s Bassist

Reputation: 55

Why do some linux header files define a function to return 0 after the declaration?

I am looking at the Linux 4.14 kernel's include/linux/clk.h file, and have noticed that some of the functions are declared, and then later defined to return 0 or NULL.

For example:

struct clk *clk_get(struct device *dev, const char *id);
...
static inline struct clk *clk_get(struct device *dev, const char *id)
{
        return NULL;
}

What is the purpose of doing this? I see multiple C source files that fully define this function and still include linux/clk.h.

Upvotes: 1

Views: 154

Answers (1)

Rachid K.
Rachid K.

Reputation: 5211

Linux kernel comes with lots of configuration parameters. For this particular function, you get the service if CONFIG_HAVE_CLK parameter is defined:

#ifdef CONFIG_HAVE_CLK

/**
 * clk_get - lookup and obtain a reference to a clock producer.
 * @dev: device for clock "consumer"
 * @id: clock consumer ID
 *
 * Returns a struct clk corresponding to the clock producer, or
 * valid IS_ERR() condition containing errno.  The implementation
 * uses @dev and @id to determine the clock consumer, and thereby
 * the clock producer.  (IOW, @id may be identical strings, but
 * clk_get may return different clock producers depending on @dev.)
 *
 * Drivers must assume that the clock source is not enabled.
 *
 * clk_get should not be called from within interrupt context.
 */
struct clk *clk_get(struct device *dev, const char *id);
[...]

#else /* !CONFIG_HAVE_CLK */

static inline struct clk *clk_get(struct device *dev, const char *id)
{
    return NULL;
}
[...]

This parameter is defined in arch/Kconfig as:

config HAVE_CLK
    bool
    help
      The <linux/clk.h> calls support software clock gating and
      thus are a key power management tool on many systems.

Upvotes: 3

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