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

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Message-ID: <154408851245.18057.9814330902486356341@skylake-alporthouse-com>
Date:   Thu, 06 Dec 2018 09:28:32 +0000
From:   Chris Wilson <chris@...is-wilson.co.uk>
To:     "Zhang, Jerry\(Junwei\)" <Jerry.Zhang@....com>,
        Michel Dänzer <michel@...nzer.net>,
        Christian Koenig <christian.koenig@....com>,
        David Airlie <airlied@...ux.ie>, Huang Rui <ray.huang@....com>,
        Maarten Lankhorst <maarten.lankhorst@...ux.intel.com>,
        Maxime Ripard <maxime.ripard@...tlin.com>,
        Sean Paul <sean@...rly.run>
Cc:     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

Quoting Michel Dänzer (2018-12-06 09:21:40)
> On 2018-12-06 10:12 a.m., Chris Wilson wrote:
> > Quoting Zhang, Jerry(Junwei) (2018-12-06 02:40:42)
> >> On 12/6/18 12:56 AM, Michel Dänzer wrote:
> >>> From: Michel Dänzer <michel.daenzer@....com>
> >>>
> >>> The following cases are possible for pr_debug():
> >>>
> >>> 1. CONFIG_DYNAMIC_DEBUG disabled
> >>>     a) DEBUG not defined: pr_debug() translates to no_printk(...), i.e.
> >>>        it never generates any output.
> >>>     b) DEBUG defined: pr_debug() translates to printk(KERN_DEBUG ...),
> >>>        i.e. it generates output which doesn't appear in dmesg by default,
> >>>        can be enabled dynamically.
> >>>
> >>> 2. CONFIG_DYNAMIC_DEBUG enabled: pr_debug() translates to
> >>>     dynamic_pr_debug()
> >>>     a) DEBUG not defined: dynamic_pr_debug() generates no output by
> >>>        default, can be enabled dynamically.
> >>>     b) DEBUG defined: dynamic_pr_debug() generates output by default,
> >>>        can be disabled dynamically.
> >>>
> >>> The intention for drm_debug_printer() is to generate output which
> >>> doesn't appear in dmesg by default, but can be enabled dynamically, i.e.
> >>> cases 1b) and 2a). However, defining DEBUG unconditionally gave us 2b)
> >>> instead of 2a) with CONFIG_DYNAMIC_DEBUG enabled.
> >>>
> >>> Fixes: 79a5ad2fdb3c ("drm: Enable pr_debug() for drm_printer")
> >>> Signed-off-by: Michel Dänzer <michel.daenzer@....com>
> >> Reviewed-by: Junwei Zhang <Jerry.Zhang@....com>
> > 
> > At the cost of 1a? Nah.
> 
> We still #define DEBUG if CONFIG_DYNAMIC_DEBUG isn't defined, so it's
> still 1b), not 1a).

I completely fluffed my reading of ifndef.

#if !IS_ENABLED(CONFIG_DYNAMIC_DEBUG)
-Chris