[PATCH v2 1/5] printk: add macros to simplify handling struct va_format

[Andrzej Hajda <andrzej.hajda@intel.com>]

struct va_format is used to facilitate implementation of
variadic functions. Most of variadic users do not parse
arguments one by one, they just forward them to some
helper via va_format pointer. Introduced helpers simplifies
it more.

Signed-off-by: Andrzej Hajda <andrzej.hajda@intel.com>
---
 include/linux/printk.h | 30 ++++++++++++++++++++++++++++++
 1 file changed, 30 insertions(+)

diff --git a/include/linux/printk.h b/include/linux/printk.h
index 45c663124c9b..6fd817ce29a6 100644
--- a/include/linux/printk.h
+++ b/include/linux/printk.h
@@ -87,6 +87,36 @@ struct va_format {
 	va_list *va;
 };
 
+/* Special macro used only by callers of va_format_call to position va_format argument */
+#define va_format_arg (&vaf)
+
+/**
+ * va_format_call() - forward variable arguments to va_format aware function
+ * @_fmt: format used by @_func function
+ * @_func: function which should be called
+ * @_args: arguments passed to @_func. Exactly one of them must be va_format_arg,
+ *         to indicate position of *va_format type arg in arguments of @_func.
+ *
+ * Many variadic functions just forwards their variadic arguments to some helper
+ * function via va_format pointer. It involves few common steps encapsulated in
+ * this macro. The macro is accompanied by va_format_arg macro, described above.
+ * Sample implementation of common helper:
+ * void dev_err(struct device *dev, const char* fmt, ...)
+ * {
+ *    va_format_call(fmt, pr_err, "%s %s: %pV", dev_driver_string(dev),
+ *                   dev_name(dev), va_format_arg);
+ * }
+ */
+#define va_format_call(_fmt, _func, _args...)			\
+({								\
+	va_list ap;						\
+	struct va_format vaf = { .fmt = _fmt, .va = &ap };	\
+								\
+	va_start(ap, _fmt);					\
+	_func(_args);						\
+	va_end(ap);						\
+})
+
 /*
  * FW_BUG
  * Add this to a message where you are sure the firmware is buggy or behaves

-- 
2.43.0