linux-kernel - Re: [PATCH 1/2] drm: Only #define DEBUG if CONFIG_DYNAMIC

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Message-ID: <20181206161011.hkjjzdtfzd2hthos@holly.lan>
Date:   Thu, 6 Dec 2018 16:10:11 +0000
From:   Daniel Thompson <daniel.thompson@...aro.org>
To:     Michel Dänzer <michel@...nzer.net>
Cc:     Joe Perches <joe@...ches.com>,
        "Zhang, Jerry(Junwei)" <Jerry.Zhang@....com>,
        Christian Koenig <christian.koenig@....com>,
        Huang Rui <ray.huang@....com>,
        Maarten Lankhorst <maarten.lankhorst@...ux.intel.com>,
        Maxime Ripard <maxime.ripard@...tlin.com>,
        Sean Paul <sean@...rly.run>, David Airlie <airlied@...ux.ie>,
        Chris Wilson <chris@...is-wilson.co.uk>,
        linux-kernel@...r.kernel.org, dri-devel@...ts.freedesktop.org
Subject: Re: [PATCH 1/2] drm: Only #define DEBUG if CONFIG_DYNAMIC_DEBUG is
 disabled

On Thu, Dec 06, 2018 at 03:41:16PM +0100, Michel Dänzer wrote:
> On 2018-12-06 1:23 p.m., Joe Perches wrote:
> > On Thu, 2018-12-06 at 12:52 +0100, Michel Dänzer wrote:
> >> In contrast to the 2b case, the pr_debug output isn't visible by default
> >> with 1b, so the latter doesn't fit "always produce output" either.
> > 
> > I think you are mistaken here.
> 
> Still puzzled as to what you're hoping to achieve with that kind of
> language. None of the confusion about this patch has been on my part. :)
> 
> 
> > Adding #define DEBUG as Chris did enables pr_debug output
> > and is your 1b.
> > 
> > Perhaps your default console logging level is set to a
> > non-default value.
> 
> I have CONFIG_DYNAMIC_DEBUG enabled in my kernels. The problem addressed
> by this patch is that messages from drm_debug_printer are visible by
> default (case 2b), whereas they shouldn't be (case 2a, like 1b).

When enabled (either dynamically or statically) pr_debug() will emit
output at KERN_DEBUG level regardless of whether CONFIG_DYNAMIC_DEBUG
is defined or not.

Thus unless you change additional settings (either dynamically or
statically) then debug messages should not be shown on the console
because the default settings filter KERN_DEBUG messages. However they
are available via dmesg and system loggers (syslogd, journal, etc).

The patch proposed will change the behaviour of the debug messages
w.r.t. system loggers based on whether the user has enabled
CONFIG_DYNAMIC_DEBUG or not, violating the principle of least surprise.


Daniel.