Hi,

On Thu, Apr 15, 2021 at 08:18:52AM +0800, Kevin Tang wrote:
> Maxime Ripard <maxime@cerno.tech> 于2021年3月24日周三 下午7:10写道：
> > > +static struct sprd_dpu *sprd_crtc_init(struct drm_device *drm,
> > > +                      struct drm_plane *primary)
> > > +{
> > > +     struct device_node *port;
> > > +     struct sprd_dpu *dpu;
> > > +
> > > +     /*
> > > +      * set crtc port so that drm_of_find_possible_crtcs call works
> > > +      */
> > > +     port = of_parse_phandle(drm->dev->of_node, "ports", 0);
> > > +     if (!port) {
> > > +             drm_err(drm, "find 'ports' phandle of %s failed\n",
> > > +                       drm->dev->of_node->full_name);
> > > +             return ERR_PTR(-EINVAL);
> > > +     }
> > > +     of_node_put(port);
> >
> > The YAML binding should already make sure that your binding is sane, and
> > if you still get a DT that doesn't follow it, you have a whole lot of
> > other issues than whether ports is there :)
> >
> > > +     dpu = drmm_crtc_alloc_with_planes(drm, struct sprd_dpu, base,
> > > +                                     primary, NULL,
> > > +                                     &sprd_crtc_funcs, NULL);
> > > +     if (IS_ERR(dpu)) {
> > > +             drm_err(drm, "failed to init crtc.\n");
> > > +             return dpu;
> > > +     }
> > > +
> > > +     dpu->base.port = port;
> >
> > But you're still referencing it here, while you called of_node_put on it
> > already? You should only call it once you're done with it.
> 
>  of_node_put should be called after done with it, this maybe indeed be a bug.
> i will fix it.
> >
> >
> > I'm not really sure why you would need drm_of_find_possible_crtcs to
> > work then if you don't follow the OF-Graph bindings.
> 
> it scan all endports of encoder, if a matching crtc is found by
> OF-Graph bindings
> and then genarate the crtc mask, here is description:
> 41  /**
> 42   * drm_of_find_possible_crtcs - find the possible CRTCs for an encoder port
> 43   * @dev: DRM device
> 44   * @port: encoder port to scan for endpoints
> 45   *
> 46   * Scan all endpoints attached to a port, locate their attached CRTCs,
> 47   * and generate the DRM mask of CRTCs which may be attached to this
> 48   * encoder.
> 49   *
> if we don't follow the OF-Graph bindings, crtc can't attched to encoder.

Yeah, what I'm actually confused about is why you would need the
of_parse_phandle call. You usually call drm_of_find_possible_crtcs with
the encoder device node, so from your MIPI-DSI driver in your case, and
with it's device->of_node pointer and it should work perfectly fine?

Maxime