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

Takashi Iwai
2015-12-04T13:12:17Z

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