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

Daniel Vetter
2015-12-04T14:55:27Z

On Fri, Dec 04, 2015 at 02:12:14PM +0100, Takashi Iwai wrote:
> 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?

Oops. Patch reverted, sorry for the mess.
-Daniel

>
>
> 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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--
Daniel Vetter
Software Engineer, Intel Corporation