linux-kernel - [PATCH 1/3] printk: add macros to simplify handling struct va

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Message-Id: <20251126-va_format_call-v1-1-8a604a331730@intel.com>
Date: Wed, 26 Nov 2025 12:35:49 +0100
From: Andrzej Hajda <andrzej.hajda@...el.com>
To: Petr Mladek <pmladek@...e.com>, Steven Rostedt <rostedt@...dmis.org>, 
 John Ogness <john.ogness@...utronix.de>, 
 Sergey Senozhatsky <senozhatsky@...omium.org>, 
 Greg Kroah-Hartman <gregkh@...uxfoundation.org>, 
 "Rafael J. Wysocki" <rafael@...nel.org>, Danilo Krummrich <dakr@...nel.org>, 
 Andrew Morton <akpm@...ux-foundation.org>, Vlastimil Babka <vbabka@...e.cz>, 
 Suren Baghdasaryan <surenb@...gle.com>, Michal Hocko <mhocko@...e.com>, 
 Brendan Jackman <jackmanb@...gle.com>, Johannes Weiner <hannes@...xchg.org>, 
 Zi Yan <ziy@...dia.com>, Christoph Lameter <cl@...two.org>, 
 David Rientjes <rientjes@...gle.com>, 
 Roman Gushchin <roman.gushchin@...ux.dev>, Harry Yoo <harry.yoo@...cle.com>
Cc: linux-kernel@...r.kernel.org, linux-mm@...ck.org, 
 Andrzej Hajda <andrzej.hajda@...el.com>
Subject: [PATCH 1/3] printk: add macros to simplify handling struct
 va_format

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@...el.com>
---
 include/linux/printk.h | 30 ++++++++++++++++++++++++++++++
 1 file changed, 30 insertions(+)

diff --git a/include/linux/printk.h b/include/linux/printk.h
index 45c663124c9bd3b294031d839f1253f410313faa..6fd817ce29a66b33a804b660c2b770efa110ee7e 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