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

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Date:   Fri, 30 Jun 2017 10:48:27 +0200
From:   Andrzej Hajda <a.hajda@...sung.com>
To:     Eric Anholt <eric@...olt.net>,
        Archit Taneja <architt@...eaurora.org>,
        dri-devel@...ts.freedesktop.org,
        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 29.06.2017 17:22, Eric Anholt wrote:
> Andrzej Hajda <a.hajda@...sung.com> 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.

No, drm master will wait till all components are present.

>
> I *think* what you're thinking is moving DSI host register to probe, 

yes, this way you will allow to create dsi devices.

> and
> then panel lookup stays in bind. 

no, panel lookup will be performed in dsi_host attach callback, and
component_add will be called also there, if all required panels/bridges
are get.

>  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.

I think it is irrelevant, with the current design only transactions
between prepare/unprepare callbacks have chances to succeed.

Andrzej