linux-kernel - Subject: [PATCH v2 1/1] Add documentation for proper usage and order of preference of calls to print diagnostic messages.

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Date:	Tue, 4 Mar 2014 22:31:53 +0530
From:	yogesh <mr.yogesh@...il.com>
To:	LKML <linux-kernel@...r.kernel.org>
Cc:	Levente Kurusa <levex@...ux.com>, Joe Perches <joe@...ches.com>,
	rdunlap@...radead.org
Subject: Subject: [PATCH v2 1/1] Add documentation for proper usage and order
 of preference of calls to print diagnostic messages.

This patch adds documentation that clarifies the use of various
diagnostic printing messages. It shows the preference of subsystem_dbg
calls to dev_dbg (whenever possible), as the first preferred format of
logging debug messages.
Signed-off-by: Yogesh Chaudhari <mr.yogesh@...il.com>
---
 Documentation/CodingStyle | 17 +++++++++++++++++
 1 file changed, 17 insertions(+)

diff --git a/Documentation/CodingStyle b/Documentation/CodingStyle
index 7fe0546..cd14cc8 100644
--- a/Documentation/CodingStyle
+++ b/Documentation/CodingStyle
@@ -662,6 +662,23 @@ and driver, and are tagged with the right level:  dev_err(), dev_warn(),
 dev_info(), and so forth.  For messages that aren't associated with a
 particular device, <linux/printk.h> defines pr_debug() and pr_info().
 
+If the subsystem has its own diagnostic macros then they should be used
+instead of dev_dbg calls.
+E.g. If you are using network subsystem, use netdev_dbg;
+if you are using V4L, use v4l_dbg etc.
+This standardises the output format in every subsystem.
+
+Depending on your changes, the following order of precedence
+applies to printing messages:
+1. [subsystem]_dbg() is preferred when the subsystem has its own
+diagnostic macros.
+2. dev_dbg() is preferred when you have a generic struct device object.
+3. pr_debug() should be used when 1 and 2 above are not applicable.
+4. printk() should be avoided.
+
+Note: The above order applies to diagnostic calls of all log levels viz:
+*_emerg, *_alert, *_crit, *_err, *_warn, *_notice, *_info and *_dbg.
+
 Coming up with good debugging messages can be quite a challenge; and once
 you have them, they can be a huge help for remote troubleshooting.  Such
 messages should be compiled out when the DEBUG symbol is not defined (that
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