linux-kernel - Re: [PATCH] drm/dp_mst: Add ddc i2c device links for DP MST connectors

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Message-ID: <aafb77646e6f6efdc35789d3654dab14891c7f48.camel@redhat.com>
Date:   Mon, 24 Aug 2020 13:42:38 -0400
From:   Lyude Paul <lyude@...hat.com>
To:     imre.deak@...el.com, Sam McNally <sammc@...omium.org>,
        Wayne Lin <Wayne.Lin@....com>
Cc:     LKML <linux-kernel@...r.kernel.org>,
        Daniel Vetter <daniel@...ll.ch>,
        David Airlie <airlied@...ux.ie>,
        Maarten Lankhorst <maarten.lankhorst@...ux.intel.com>,
        Maxime Ripard <mripard@...nel.org>,
        Thomas Zimmermann <tzimmermann@...e.de>,
        dri-devel@...ts.freedesktop.org
Subject: Re: [PATCH] drm/dp_mst: Add ddc i2c device links for DP MST
 connectors

On Thu, 2020-08-20 at 21:03 +0300, Imre Deak wrote:
> On Thu, Aug 20, 2020 at 12:27:03PM +1000, Sam McNally wrote:
> > > > [...]
> > > > diff --git a/drivers/gpu/drm/drm_dp_mst_topology.c
> > > > b/drivers/gpu/drm/drm_dp_mst_topology.c
> > > > index 1ac874e4e7a1..73a2299c2faa 100644
> > > > --- a/drivers/gpu/drm/drm_dp_mst_topology.c
> > > > +++ b/drivers/gpu/drm/drm_dp_mst_topology.c
> > > > @@ -2161,11 +2161,23 @@ static void build_mst_prop_path(const struct
> > > > drm_dp_mst_branch *mstb,
> > > >  int drm_dp_mst_connector_late_register(struct drm_connector *connector,
> > > >                                      struct drm_dp_mst_port *port)
> > > >  {
> > > > +     int ret;
> > > >       DRM_DEBUG_KMS("registering %s remote bus for %s\n",
> > > >                     port->aux.name, connector->kdev->kobj.name);
> > > > 
> > > >       port->aux.dev = connector->kdev;
> > > > -     return drm_dp_aux_register_devnode(&port->aux);
> > > > +     ret = drm_dp_aux_register_devnode(&port->aux);
> > > > +     if (ret)
> > > > +             return ret;
> > > > +
> > > > +     if (port->pdt != DP_PEER_DEVICE_NONE &&
> > > > +         drm_dp_mst_is_end_device(port->pdt, port->mcs)) {
> > > 
> > > How can we get here when drm_dp_mst_is_end_device(port) is not true?
> > > AFAICS that's only case where we should create a connector and an i2c
> > > device. (IOW we don't create them for branch ports.)
> > 
> > I'm not sure what you mean. Wouldn't this condition be checked during
> > the registration of any MST connector? This follows the pattern used
> > in drm_dp_mst_port_add_connector() [0], which seems like it's invoked
> > in the same cases as drm_dp_mst_connector_late_register(), modulo
> > early outs for errors.
> 
> Re-reading the code, a DRM connector is created whenever the MST port is
> an output port, so even in the case of an output branch port.
> 
> I'm still not sure why we can't register/unregister the I2C bus whenever
> creating/removing the DRM connector. That's also the scope of the AUX
> bus, which is what I2C depends on, and if a port doesn't support I2C
> messaging then the corresponding AUX messages would be NAKed.

FWIW - I'm totally fine with this, as long as it works :)
> 
> --Imre
> 
> > [0] 
> > https://cgit.freedesktop.org/drm-tip/tree/drivers/gpu/drm/drm_dp_mst_topology.c?id=1939e049a8ec6cef03a098f7cc99cb0bbcff21c6#n2188
> > 
> > 
> > 
> > > > +             ret = sysfs_create_link(&port->connector->kdev->kobj,
> > > > +                                     &port->aux.ddc.dev.kobj, "ddc");
> > > > +             if (ret)
> > > > +                     drm_dp_aux_unregister_devnode(&port->aux);
> > > > +     }
> > > > +     return ret;
> > > >  }
> > > >  EXPORT_SYMBOL(drm_dp_mst_connector_late_register);
> > > > 
> > > > @@ -5490,6 +5502,7 @@ static int drm_dp_mst_register_i2c_bus(struct
> > > > drm_dp_mst_port *port)
> > > >  {
> > > >       struct drm_dp_aux *aux = &port->aux;
> > > >       struct device *parent_dev = port->mgr->dev->dev;
> > > > +     int ret;
> > > > 
> > > >       aux->ddc.algo = &drm_dp_mst_i2c_algo;
> > > >       aux->ddc.algo_data = aux;
> > > > @@ -5504,7 +5517,17 @@ static int drm_dp_mst_register_i2c_bus(struct
> > > > drm_dp_mst_port *port)
> > > >       strlcpy(aux->ddc.name, aux->name ? aux->name :
> > > > dev_name(parent_dev),
> > > >               sizeof(aux->ddc.name));
> > > > 
> > > > -     return i2c_add_adapter(&aux->ddc);
> > > > +     ret = i2c_add_adapter(&aux->ddc);
> > > > +     if (ret)
> > > > +             return ret;
> > > > +
> > > > +     if (port->connector && port->connector->kdev) {
> > > > +             ret = sysfs_create_link(&port->connector->kdev->kobj,
> > > > +                                     &port->aux.ddc.dev.kobj, "ddc");
> > > > +             if (ret)
> > > > +                     i2c_del_adapter(&port->aux.ddc);
> > > > +     }
> > > > +     return ret;
> > > >  }
> > > > 
> > > >  /**
> > > > @@ -5513,6 +5536,8 @@ static int drm_dp_mst_register_i2c_bus(struct
> > > > drm_dp_mst_port *port)
> > > >   */
> > > >  static void drm_dp_mst_unregister_i2c_bus(struct drm_dp_mst_port *port)
> > > >  {
> > > > +     if (port->connector && port->connector->kdev)
> > > > +             sysfs_remove_link(&port->connector->kdev->kobj, "ddc");
> > > >       i2c_del_adapter(&port->aux.ddc);
> > > >  }
> > > > 
> > > > --
> > > > 2.28.0.rc0.142.g3c755180ce-goog
> > > > 
-- 
Sincerely,
      Lyude Paul (she/her)
      Software Engineer at Red Hat