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

["Ville Syrjälä" <ville.syrjala@linux.intel.com>]

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.

> 
> 
> 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