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

[Maxime Ripard]

On Thu, Feb 02, 2023 at 10:22:42PM +0530, Jagan Teki wrote:
> 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.

The logic is sound in your patch

> 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;
> }

It's the implementation that isn't. You cannot use a devm hook to
register a KMS structure, so it's not that you should add a
drmm_add_action call, it's that you shouldn't call
devm_drm_panel_bridge_add in the first place.

So either you use drm_panel_bridge_add and a custom drmm action, or you
add a drmm_panel_bridge_add function and use it.

Maxime
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