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

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Message-ID: <8d40ab2b-2d1d-5d6d-0bab-afa536bf6a32@codeaurora.org>
Date:   Mon, 17 Jul 2017 19:09:04 +0530
From:   Archit Taneja <architt@...eaurora.org>
To:     Eric Anholt <eric@...olt.net>, Andrzej Hajda <a.hajda@...sung.com>,
        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 07/15/2017 04:31 AM, Eric Anholt wrote:
> Archit Taneja <architt@...eaurora.org> writes:
> 
>> On 06/29/2017 04:09 PM, Andrzej Hajda wrote:
>>> 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.
>>>
>>> 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?
>>
>> This seems like it would work, but would require KMS drivers to restructure
>> themselves around this approach. For KMS drivers that don't even use the
>> component stuff, it might be asking too much.
>>
>> We could maybe consider Eric's patch as an intermediate solution, we should
>> definitely put WARN_ON(!dsi->host) like checks for all the existing
>> mipi_dsi_*() API.
> 
> Could you clarify which entrypoints you'd like a warning on?  Is it just
> "everything that gets the host ops"?

Sorry, all API was a bad suggestion.

I think a warning and early bail in mipi_dsi_attach() should be sufficient.
A panel/bridge DSI device shouldn't use any other API if it hasn't called
mipi_dsi_attach().

Thanks,
Archit

> 

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