Re: [PATCH v3 5/6] dt-bindings: add the rockchip, dual-channel for dw-mipi-dsi

[Brian Norris <briannorris@chromium.org>]

Hi Archit,

I'm a relative n00b here, but I'm trying to follow along and I have some
questions:

On Fri, Dec 01, 2017 at 06:29:04PM +0530, Archit Taneja wrote:
> On 11/30/2017 11:02 PM, Nickey Yang wrote:
> >I try to follow as you suggested,use
> >
> >mipi_dsi: mipi@ff960000 {
> >     ...
> >     ...
> >     clock-master;    /* implies that this DSI instance drivers the clock
> >              * for both the DSIs.
> >              */
> >     ports {
> >         mipi_in: port {
> >             ...
> >             ...
> >         };
> >         /* add extra output ports for both DSIs */
> >         mipi_out: port {
> >             mipi_panel_out: endpoint {
> >                 remote-endpoint = <&panel_in_channel0>;
> >             };
> >         };
> >     };
> >     panel {
> >         ...
> >         ...
> >         /*
> >          * panel node can describe its input ports, if both the DSIs output
> >          * ports are connected to the same device (i.e, the same DSI panel),
> >          * we can assume that the DSIs need to operate in dual DSI mode
> >          */
> >         ports {
> >             ...
> >             port@0 {
> >                 panel_in_channel0: endpoint {
> >                     remote-endpoint = <&mipi_panel_out>;
> >                 };
> >             };
> >             port@1 {
> >                 panel_in_channel1: endpoint {
> >                     remote-endpoint = <&mipi1_panel_out>;
> >                 };
> >
> >             };
> >         };
> >     };
> >};
> >
> >mipi_dsi1: mipi@ff968000 {
> >     ...
> >     ...
> >     ports {
> >         mipi1_in: port {
> >             ...
> >             ...
> >         };
> >         mipi1_out: port {
> >             mipi1_panel_out: endpoint {
> >                 remote-endpoint = <&panel_in_channel1>;
> >             };
> >         };
> >     };
> >}
> >
> >But it seems we can not use of_drm_find_panel(like below)
> >
> >/*
> >         port = of_graph_get_port_by_id(dev->of_node, 1);
> >         if (port) {
> >                 endpoint = of_get_child_by_name(port, "endpoint");
> >                 of_node_put(port);
> >                 if (!endpoint) {
> >                         dev_err(dev, "no output endpoint found\n");
> >                         return -EINVAL;
> >                 }
> >                 panel_node = of_graph_get_remote_port_parent(endpoint);
> >                 of_node_put(endpoint);
> >                 if (!panel_node) {
> >                         dev_err(dev, "no output node found\n");
> >                         return -EINVAL;
> >                 }
> >                 panel = of_drm_find_panel(panel_node);
> >                 of_node_put(panel_node);
> >                 if (!panel)
> >                         return -EPROBE_DEFER;
> >         }
> >*/
> >to get DSI1 outputs,because of_drm_find_panel need compare
> >
> >if (panel->dev->of_node == np)
> >
> >in dsi_panel driver innolux->base.dev = &innolux->link->dev;
> >dsi->dev
> 
> Yes, we should only have 1 drm_panel in the global panel list.
> Shouldn't it be possible to modify the dsi driver such that dsi1
> doesn't care whether it has a drm_panel for it or not, if we are
> in dual dsi mode?
> 
> I imagine a sequence like this:
> 
> 1. dsi0 probes, parses the of-graph, finds the panel and saves its device
> node.

Does this mean we depend on probe order? Or would we be able to
-EPROBE_DEFER or similar if dsi1 binds first?

> 2. dsi1 probes, parses the of-graph, find the panel's device node
>   - dsi1 checks if it is the same as the panel attached to dsi0.
>   - If so, it just takes the drm_panel pointer from dsi0.
>   - If not, it tries a of_drm_find_panel() on the panel's device node.

So, that all means we'd need a new variant of
drm_of_find_panel_or_bridge() for "dual" drivers like this? Or else
open-code this logic in dw-mipi-dsi.c?

> A dual DSI panel driver would also be a bit different. It will be a
> mipi_dsi_driver with the master DSI (dsi0) as the mipi_dsi_device. Using
> the of-graph helpers, we would get the device node of dsi1 using
> of_find_mipi_dsi_host_by_node(), and create another DSI device using
> mipi_dsi_device_register_full(). Then, we call mipi_dsi_attach() on
> both the dsi devices.

That seems...interesting. I guess that sounds like it could work, but
someone would have to play with that a bit more.

I assume one wouldn't want to do all this in every dual DSI driver that
needs this, right?

> >struct innolux_panel {
> >         struct drm_panel base;
> >         struct mipi_dsi_device *link;
> >};
> >It means one panel can only be found in his dsi node,(like dsi0 above).
> >
> >I'm doubting about it, Or  may we follow tegra_dsi_ganged_probe
> >(drivers/gpu/drm/tergra/dsi.c) method.
> 
> This method will add a new binding similar to "nvidia,ganged-mode", which
> is something we don't want to do.

It's unfortunate we have the anti-pattern already merged :(

Brian