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

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Date:	Wed, 14 Aug 2013 14:21:10 -0400
From:	Jason Baron <jbaron@...mai.com>
To:	Joe Perches <joe@...ches.com>
CC:	Sarah Sharp <sarah.a.sharp@...ux.intel.com>,
	Xenia Ragiadakou <burzalodowa@...il.com>,
	Greg KH <gregkh@...uxfoundation.org>,
	"linux-usb@...r.kernel.org" <linux-usb@...r.kernel.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: Re: Dynamic debug on by default?

On 08/14/2013 12:51 PM, Joe Perches wrote:
> 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.

They should also be emitted in the case that dynamic_debug is off, but
we have set -DDEBUG, right? That was the whole point of enabling them
for dynamic debug too - to mirror the behavior of !CONFIG_DYNAMIC_DEBUG
case.

-Jason


> 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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