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

[Eric Anholt]

Andrzej Hajda <a.hajda@xxxxxxxxxxx> writes:

> On 29.06.2017 07:03, Archit Taneja wrote:
>>
>> 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.
>
> Field abuse is not a biggest issue.
>
> This patch changes totally semantic of mipi_dsi_device_register_full.
> Currently semantic of *_device_register* function is to create and add
> device to existing bus, ie after return we have device attached to bus,
> so it can be instantly used. With this change function can return some
> unattached device, without warranty it will be ever attached - kind of
> hidden deferring. Such change looks for me quite dangerous, even if it
> looks convenient in this case.

It only changes the semantic if you past in a NULL host, from "oops" to
"do something useful".

> As discussed in other thread more appealing solution for me would be:
> 1. host creates dsi bus, but doesn't call component_add as it does not
> have all required resources.
> 2. host waits for all required dsi devs attached, gets registered panels
> or bridges and calls component_add after that.
> 3. in bind phase it has all it needs, hasn't it?
>
> I did not spent much time on this idea, so I cannot guarantee it has not
> fundamental issues :)

If component_add() isn't called during probe, then DSI would just get
skipped during bind as far as I know.

I *think* what you're thinking is moving DSI host register to probe, and
then panel lookup stays in bind.  That seems much more risky to me -- do
we know for sure that no mipi_dsi_device will do any DSI transactions
during its probe?  I would expect some of them to.

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