linux-kernel - Re: Dynamic debug on by default?

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Date:	Wed, 14 Aug 2013 09:51:54 -0700
From:	Joe Perches <joe@...ches.com>
To:	Sarah Sharp <sarah.a.sharp@...ux.intel.com>
Cc:	Xenia Ragiadakou <burzalodowa@...il.com>,
	Jason Baron <jbaron@...mai.com>,
	Greg KH <gregkh@...uxfoundation.org>,
	linux-usb@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: Dynamic debug on by default?

On Wed, 2013-08-14 at 09:40 -0700, Sarah Sharp wrote:
> Hi Xenia,
> 
> I'm a bit confused.  I thought that debugging messages would be turned
> off by default for a module if CONFIG_DYNAMIC_DEBUG was turned on.  When
> I tested your patch to remove the CONFIG_USB_XHCI_HCD_DEBUGGING and just
> use dev_dbg, the messages from the xHCI driver appeared in dmesg by
> default.
> 
> That generates a lot of log spew.  We can have distros add a boot
> parameter option to turn off debug messages, but that boot parameter is
> limited to 1023 characters.  I'm concerned that if more drivers add
> dynamic debugging, the distros will eventually run out of space in the
> dynamic debugging boot parameter.  I know Greg was ripping out debugging
> config options in other USB drivers, so this is a bit concerning.
> 
> Jason, is there a way within the xHCI driver to say that dynamic
> debugging should be off by default?  I've looked through the
> documentation, and I can't find anything like that documented.

#undefine DEBUG

> I've attached my .config file, in case I have something misconfigured.

Because of:

drivers/usb/host/Makefile:ccflags-$(CONFIG_USB_DEBUG) := -DDEBUG

you probably want to turn this off

CONFIG_USB_DEBUG=y

when dynamic_debug is on.

CONFIG_DYNAMIC_DEBUG=y

This will cause all pr_debug/dev_dbg statements to be
emitted without specific enabling via the dynamic_debug
control file.

(from include/linux/dynamic_debug.h)

#if defined DEBUG
#define _DPRINTK_FLAGS_DEFAULT _DPRINTK_FLAGS_PRINT
#else
#define _DPRINTK_FLAGS_DEFAULT 0
#endif


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