linux-kernel - Re: [PATCH] drm: Reduce object size of DRM_ERROR and DRM

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Date:   Thu, 15 Mar 2018 17:37:34 +0200
From:   Ville Syrjälä <ville.syrjala@...ux.intel.com>
To:     Joe Perches <joe@...ches.com>
Cc:     Maarten Lankhorst <maarten.lankhorst@...ux.intel.com>,
        Gustavo Padovan <gustavo@...ovan.org>,
        Sean Paul <seanpaul@...omium.org>,
        David Airlie <airlied@...ux.ie>,
        Jani Nikula <jani.nikula@...ux.intel.com>,
        Joonas Lahtinen <joonas.lahtinen@...ux.intel.com>,
        Rodrigo Vivi <rodrigo.vivi@...el.com>,
        intel-gfx@...ts.freedesktop.org, linux-kernel@...r.kernel.org,
        dri-devel@...ts.freedesktop.org
Subject: Re: [PATCH] drm: Reduce object size of DRM_ERROR and DRM_DEBUG uses

On Thu, Mar 15, 2018 at 08:17:53AM -0700, Joe Perches wrote:
> On Thu, 2018-03-15 at 17:05 +0200, Ville Syrjälä wrote:
> > On Thu, Mar 15, 2018 at 03:04:52PM +0100, Maarten Lankhorst wrote:
> > > Op 15-03-18 om 14:30 schreef Ville Syrjälä:
> > > > On Tue, Mar 13, 2018 at 03:02:15PM -0700, Joe Perches wrote:
> > > > > drm_printk is used for both DRM_ERROR and DRM_DEBUG with unnecessary
> > > > > arguments that can be removed by creating separate functins.
> > > > > 
> > > > > Create specific functions for these calls to reduce x86/64 defconfig
> > > > > size by ~20k.
> > > > > 
> > > > > Modify the existing macros to use the specific calls.
> > > > > 
> > > > > new:
> > > > > $ size -t drivers/gpu/drm/built-in.a | tail -1
> > > > > 1876562	  44542	    995	1922099	 1d5433	(TOTALS)
> > > > > 
> > > > > old:
> > > > > $ size -t drivers/gpu/drm/built-in.a | tail -1
> > > > > 1897565	  44542	    995	1943102	 1da63e	(TOTALS)
> > > > > 
> > > > > Miscellanea:
> > > > > 
> > > > > o intel_display requires a change to use the specific calls.
> > > > 
> > > > How much would we lose if we move the (drm_debug&FOO) outside the
> > > > functions again?
> 
> again?

We used to do that. Someone changed it a while back, unintentially
I believe.

> 
> > > >  I'm somewhat concerned about all the function call
> > > > overhead when debugs aren't even enabled.
> 
> Perhaps better to have compilation elimination
> of the entire debug output instead.

That would require every bug reporter to recompile the kernel first.
So this is not a solution we would ever seriously consider.

Not sure if it would be possible to use the alternatives thing to
eliminate the function calls unless the user boots wih drm.debug!=0?

> 
> I think you are discussing a different issue and
> this discussion should not block this patch as
> this patch has no impact other than code size
> reduction.

But what is the goal of the code size reduction? I assume the main
goal is to make better use of the instruction cache to make the
code faster. If there's a tradeoff between smaller and slightly
faster vs. larger and a singificantly faster I tend to think we
should go for the latter option.

-- 
Ville Syrjälä
Intel OTC