Received-SPF: pass (google.com: domain of linux-kernel-owner@vger.kernel.org designates 23.128.96.18 as permitted sender) client-ip=23.128.96.18;
MIME-Version: 1.0
References: <20201204035318.332419-1-jim.cromie@gmail.com> <20201204035318.332419-2-jim.cromie@gmail.com>
 <X8pY/H512EMR3bUX@intel.com>
In-Reply-To: <X8pY/H512EMR3bUX@intel.com>
From:   jim.cromie@gmail.com
Date:   Fri, 4 Dec 2020 12:20:21 -0700
Message-ID: <CAJfuBxysOxnm846u71f0rQCH9NX-7AMe4brmiD0vqfGeQKuVOw@mail.gmail.com>
Subject: Re: [RFC PATCH 1/2] drm: RFC add choice to use dynamic debug in drm-debug
To:     =?UTF-8?B?VmlsbGUgU3lyasOkbMOk?= <ville.syrjala@linux.intel.com>
Cc:     dri-devel@lists.freedesktop.org,
        LKML <linux-kernel@vger.kernel.org>,
        David Airlie <airlied@linux.ie>,
        Jason Baron <jbaron@akamai.com>,
        Thomas Zimmermann <tzimmermann@suse.de>
Content-Type: text/plain; charset="UTF-8"
Content-Transfer-Encoding: quoted-printable
Precedence: bulk

On Fri, Dec 4, 2020 at 8:42 AM Ville Syrj=C3=A4l=C3=A4
<ville.syrjala@linux.intel.com> wrote:
>
> On Thu, Dec 03, 2020 at 08:53:17PM -0700, Jim Cromie wrote:
> > drm's debug system uses distinct categories of debug messages, mapped
> > to bits in drm.debug.  Currently, code does a lot of unlikely bit-mask
> > checks on drm.debug (in drm_debug_enabled), we can use dynamic debug
> > instead, and get all that jump_label goodness.
>
> whatis jump_label?

Sorry, I should have at least capitalized that, and spelled it differently

kernel/Makefile
118:obj-$(CONFIG_JUMP_LABEL) +=3D jump_label.o

it is the hot-patching substrate underneath it all.
static-key, static-call, etc?
dynamic-debug uses static-key directly.


>
> One thing that bugs me about the current drm_dbg() stuff is that
> it's a function, and thus we pay the cost of setting up the
> arguments even when debugs are not enabled. I played around a bit
> with making it a macro again with the unlikely bit check moved
> into the macro. That did seem to make some of the asm a bit nicer
> where the debug stuff got shoved out the main codepath, but
> it did result in a slight net increase in code size. What I didn't
> have time to do is check if this has any measurable speed effect
> on eg. TEST_ONLY commits.
>
> And while doing that I started to ponder if we could use something
> like the alternate() instruction stuff to patch the code at runtime
> in order to turn all those debug checks into nops when debugging
> is not enabled. But I couldn't immediately find any generic
> infrastructure for it. So now I wonder if this jump_label is something
> like that?
>

this is the droid youre looking for ;-)

> >

> --
> Ville Syrj=C3=A4l=C3=A4
> Intel