linux-kernel - Re: [PATCH 6/8] drm: Allow DSI devices to be registered before the host registers.

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Message-ID: <87mv81bj1r.fsf@eliezer.anholt.net>
Date:   Tue, 18 Jul 2017 13:13:04 -0700
From:   Eric Anholt <eric@...olt.net>
To:     Archit Taneja <architt@...eaurora.org>,
        dri-devel@...ts.freedesktop.org,
        Andrzej Hajda <a.hajda@...sung.com>,
        Laurent Pinchart <Laurent.pinchart@...asonboard.com>,
        Thierry Reding <thierry.reding@...il.com>
Cc:     linux-kernel@...r.kernel.org,
        Boris Brezillon <boris.brezillon@...e-electrons.com>
Subject: Re: [PATCH 6/8] drm: Allow DSI devices to be registered before the host registers.

Archit Taneja <architt@...eaurora.org> writes:

> On 07/15/2017 04:28 AM, Eric Anholt wrote:
>> Archit Taneja <architt@...eaurora.org> writes:
>> 
>>> On 06/28/2017 01:28 AM, Eric Anholt wrote:
>>>> When a mipi_dsi_host is registered, the DT is walked to find any child
>>>> nodes with compatible strings.  Those get registered as DSI devices,
>>>> and most DSI panel drivers are mipi_dsi_drivers that attach to those nodes.
>>>>
>>>> There is one special case currently, the adv7533 bridge, where the
>>>> bridge probes on I2C, and during the bridge attach step it looks up
>>>> the mipi_dsi_host and registers the mipi_dsi_device (for its own stub
>>>> mipi_dsi_driver).
>>>>
>>>> For the Raspberry Pi panel, though, we also need to attach on I2C (our
>>>> control bus), but don't have a bridge driver.  The lack of a bridge's
>>>> attach() step like adv7533 uses means that we aren't able to delay the
>>>> mipi_dsi_device creation until the mipi_dsi_host is present.
>>>>
>>>> To fix this, we extend mipi_dsi_device_register_full() to allow being
>>>> called with a NULL host, which puts the device on a queue waiting for
>>>> a host to appear.  When a new host is registered, we fill in the host
>>>> value and finish the device creation process.
>>>
>>> This is quite a nice idea. The only bothering thing is the info.of_node usage
>>> varies between child nodes (mipi_dsi_devs) and non-child nodes (i2c control
>>> bus).
>>>
>>> For DSI children expressed in DT, the of_node in info holds the DT node
>>> corresponding to the DSI child itself. For non-DT ones, this patch assumes
>>> that info.of_node stores the DSI host DT node. I think it should be okay as
>>> long as we mention the usage in a comment somewhere. The other option is to
>>> have a new info.host_node field to keep a track of the host DT node.
>> 
>> I think maybe you misread the patch?  We're using
>> of_get_parent(dsi->dev.node), which came from info->node, to compare to
>> host->dev->of_node().
>
> I think I did misread it.
>
> Although, I'm not entirely clear what we should be setting info.node to.
> In patch #8, info.node is set by:
>
> 	endpoint = of_graph_get_next_endpoint(dev->of_node, NULL);
> 	info.node = of_graph_get_remote_port(endpoint);
>
> Looking at the dt bindings in patch #7, it looks like info.node is set
> to the 'port' device node in dsi@...00000, is that right?

Yeah.


> I suppose 'port' here seems like a reasonable representation of
> dsi->dev.node, I wonder how it would work if the dsi host had multiple
> ports underneath it. I.e:
>
> dsi@...00000 {
> 	...
> 	...
> 	ports {
> 		port@0 {
> 			...
> 			dsi_out_port: endpoint {
> 				remote-endpoint = <&panel_dsi_port>;
> 			};
> 		};
> 		port@1 {
> 			...
> 			...
> 		};
> 	};
> };
>
> Here, we would need to set info.node to the 'ports' node, so that
> of_get_parent(dsi->dev.of_node) equals host->dev->of_node. That doesn't
> seem correct.
>
> Ideally, a dev's 'of_node' should be left to NULL if we don't have a
> corresponding OF node. We're sort of overriding it here since we don't
> have any other place to store this information in the mipi_dsi_device
> struct.
>
> Maybe we could add a 'host_node' entry in mipi_dsi_device itself, which
> is exclusively used cases where the DSI device doesn't have a DT node.
> Our check in mipi_dsi_host_register() could then be something like:

I think instead of extending the struct, we can just walk to the parent
similarly to how of_graph_get_remove_port_parent() does.  And fix some
node refcounting at the same time:

diff --git a/drivers/gpu/drm/drm_mipi_dsi.c b/drivers/gpu/drm/drm_mipi_dsi.c
index ed3d505dc203..77d439254da6 100644
--- a/drivers/gpu/drm/drm_mipi_dsi.c
+++ b/drivers/gpu/drm/drm_mipi_dsi.c
@@ -313,7 +313,12 @@ int mipi_dsi_host_register(struct mipi_dsi_host *host)
         * connect our host to it and probe them now.
         */
        list_for_each_entry_safe(dsi, temp, &unattached_device_list, list) {
-               if (of_get_parent(dsi->dev.of_node) == host->dev->of_node) {
+               struct device_node *host_node = of_get_parent(dsi->dev.of_node);
+
+               if (!of_node_cmp(host_node->name, "ports"))
+                       host_node = of_get_next_parent(host_node);
+
+               if (host_node == host->dev->of_node) {
                        dsi->host = host;
                        dsi->dev.parent = host->dev;
                        device_initialize(&dsi->dev);
@@ -321,6 +326,8 @@ int mipi_dsi_host_register(struct mipi_dsi_host *host)
                        mipi_dsi_device_add(dsi);
                        list_del_init(&dsi->list);
                }
+
+               of_node_put(host_node);
        }
        mutex_unlock(&host_lock);
 

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