Date:   Mon, 17 Jul 2017 09:56:49 +0530
From:   Archit Taneja <architt@...eaurora.org>
To:     Eric Anholt <eric@...olt.net>, 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.



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?

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:

	if (dsi->host_node) == host->dev->of_node) {
		...
		...
	}

Since Thierry also reviews drm_mipi_dsi.c changes, it would nice to
get some comments from him too.

Thanks,
Archit

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