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

[Maxime Ripard]

On Fri, Feb 03, 2023 at 04:13:49PM +0530, Jagan Teki wrote:
> On Fri, Feb 3, 2023 at 1:56 PM Maxime Ripard <maxime at cerno.tech> wrote:
> >
> > 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.
> 
> I think it is because the remove action helper uses
> drm_panel_bridge_remove instead of devm hook.
> >
> > 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.
> 
> It is not possible to use this helper as it is expecting drm_device

It's definitely possible, just change the prototype of the function to
take a drm_device pointer, just like any other drmm_* function.

Maxime
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