Re: [PATCH v2 1/9] drm/i915: Remove superfluous NULL check

From: Takashi Iwai <tiwai@suse.de>
To: "Ville Syrjälä" <ville.syrjala@linux.intel.com>
Cc: alsa-devel@alsa-project.org,
	Mengdong Lin <mengdong.lin@linux.intel.com>,
	Vinod Koul <vinod.koul@intel.com>,
	intel-gfx@lists.freedesktop.org,
	Daniel Vetter <daniel.vetter@intel.com>,
	David Henningsson <david.henningsson@canonical.com>
Subject: Re: [PATCH v2 1/9] drm/i915: Remove superfluous NULL check
Date: Fri, 04 Dec 2015 14:12:14 +0100	[thread overview]
Message-ID: <s5hpoymz7bl.wl-tiwai@suse.de> (raw)
In-Reply-To: <20151204130703.GX4437@intel.com>

On Fri, 04 Dec 2015 14:07:03 +0100,
Ville Syrjälä wrote:
> 
> On Fri, Dec 04, 2015 at 01:54:56PM +0100, Takashi Iwai wrote:
> > On Fri, 04 Dec 2015 13:16:46 +0100,
> > Ville Syrjälä wrote:
> > > 
> > > On Tue, Dec 01, 2015 at 05:09:50PM +0100, Takashi Iwai wrote:
> > > > to_intel_crtc() always returns a non-NULL pointer.
> > > 
> > > Eh? to_intel_crtc(NULL) should return NULL or we might have tons of
> > > breakage on our hands. Or maybe the atomic work has gotten rid of that
> > > assumption, but at least we used to depend on that heavily.
> > 
> > Well, to_intel_crtc() has been always container_of() since the very
> > beginning of universe:
> > 
> > commit 79e539453b34e35f39299a899d263b0a1f1670bd
> > Author: Jesse Barnes <jbarnes@virtuousgeek.org>
> > Date:   Fri Nov 7 14:24:08 2008 -0800
> > 
> >     DRM: i915: add mode setting support
> >     
> > --- /dev/null
> > +++ b/drivers/gpu/drm/i915/intel_drv.h
> > ....
> > +#define to_intel_crtc(x) container_of(x, struct intel_crtc, base)
> > ....
> 
> Yes, but 
> struct intel_crtc {
> 	struct drm_crtc base;
> 	...
> }
> 
> So offsetof(struct intel_crtc, base)==0 and NULL-0 is still NULL.
> 
> I once suggested that someone should add a compile time assert to
> make sure this always holds for us, but no one took the bait.

Argh, only from the usage of container_of(), no one expects this
implicit assumption :-<  Indeed intel code has lots of these silent
rules, e.g. calling kfree() for the base class object.

Daniel, could you please take my patch back?

thanks,

Takashi

> 
> > 
> > 
> > Takashi
> > 
> > > 
> > > > 
> > > > Signed-off-by: Takashi Iwai <tiwai@suse.de>
> > > > ---
> > > >  drivers/gpu/drm/i915/intel_audio.c | 4 ----
> > > >  1 file changed, 4 deletions(-)
> > > > 
> > > > diff --git a/drivers/gpu/drm/i915/intel_audio.c b/drivers/gpu/drm/i915/intel_audio.c
> > > > index 4dccd9b003a1..0c38cc6c82ae 100644
> > > > --- a/drivers/gpu/drm/i915/intel_audio.c
> > > > +++ b/drivers/gpu/drm/i915/intel_audio.c
> > > > @@ -656,10 +656,6 @@ static int i915_audio_component_sync_audio_rate(struct device *dev,
> > > >  		intel_dig_port = enc_to_dig_port(&intel_encoder->base);
> > > >  		if (port == intel_dig_port->port) {
> > > >  			crtc = to_intel_crtc(intel_encoder->base.crtc);
> > > > -			if (!crtc) {
> > > > -				DRM_DEBUG_KMS("%s: crtc is NULL\n", __func__);
> > > > -				continue;
> > > > -			}
> > > >  			pipe = crtc->pipe;
> > > >  			break;
> > > >  		}
> > > > -- 
> > > > 2.6.3
> > > 
> > > -- 
> > > Ville Syrjälä
> > > Intel OTC
> > > 
> 
> -- 
> Ville Syrjälä
> Intel OTC
> 
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