[RESEND PATCH v11 02/18] drm: bridge: panel: Add devm_drm_of_dsi_get_bridge helper

[Jagan Teki]

Hi Maxime,

On Mon, Jan 30, 2023 at 6:28 PM Maxime Ripard <maxime at cerno.tech> wrote:
>
> On Thu, Jan 26, 2023 at 08:48:48PM +0530, Jagan Teki wrote:
> > On Thu, Jan 26, 2023 at 5:42 PM Maxime Ripard <maxime at cerno.tech> wrote:
> > >
> > > Hi,
> > >
> > > On Mon, Jan 23, 2023 at 08:41:56PM +0530, Jagan Teki wrote:
> > > > Add devm OF helper to return the next DSI bridge in the chain.
> > > >
> > > > Unlike general bridge return helper devm_drm_of_get_bridge, this
> > > > helper uses the dsi specific panel_or_bridge helper to find the
> > > > next DSI device in the pipeline.
> > > >
> > > > Helper lookup a given child DSI node or a DT node's port and
> > > > endpoint number, find the connected node and return either
> > > > the associated struct drm_panel or drm_bridge device.
> > >
> > > I'm not sure that using a device managed helper is the right choice
> > > here. The bridge will stay longer than the backing device so it will
> > > create a use-after-free. You should probably use a DRM-managed action
> > > here instead.
> >
> > Thanks for the comments. If I understand correctly we can use
> > drmm_panel_bridge_add instead devm_drm_panel_bridge_add once we found
> > the panel or bridge - am I correct?
>
> It's not that we can, it's that the devm_panel_bridge_add is unsafe:
> when the module is removed the device will go away and all the devm
> resources freed, but the DRM device sticks around until the last
> application with a fd open closes that fd.

Would you please check this, Here I'm trying to do

1. find a panel or bridge
2. if panel add it as a panel bridge
3. add DRM-managed action with the help of bridge->dev after step 2.

Didn't test the behavior, just wanted to check whether it can be a
possibility to use bridge->dev as this dev is assigned with
encoder->dev during the bridge attach the chain. Please check and let
me know.

struct drm_bridge *devm_drm_of_dsi_get_bridge(struct device *dev,
                                              struct device_node *np,
                                              u32 port, u32 endpoint)
{
        struct drm_bridge *bridge;
        struct drm_panel *panel;
        int ret;

        ret = drm_of_dsi_find_panel_or_bridge(np, port, endpoint,
                                              &panel, &bridge);
        if (ret)
                return ERR_PTR(ret);

        if (panel)
                bridge = devm_drm_panel_bridge_add(dev, panel);

        if (IS_ERR(bridge))
                return bridge;

        ret = drmm_add_action_or_reset(bridge->dev,
drmm_drm_panel_bridge_release,
                                       bridge);
        if (ret)
                return ERR_PTR(ret);

        return bridge;
}

Thanks,
Jagan.