Fwd: DRM MIPI DSI device and I2C device?

[Andrzej Hajda]

On 05.04.2018 12:28, Laurent Pinchart wrote:
> Hi Carsten,
>
> On Wednesday, 4 April 2018 11:41:05 EEST Carsten Behling wrote:
>>> Hi,
>>>
>>> I would like to write a DRM bridge driver that is an I2C device and a DRM
>>> MIPI DSI device.
>>>
>>> It looks like that both - 'i2c-core.c: of_i2c_register_devices' and
>>> 'drm_mipi_dsi.c: mipi_dsi_host_register'  are registering their devices by
>>> iterating over devicetree child nodes with
>>> for_each_available_child_of_node.
>>>
>>> Since I can't make the bridge a child node of both, I don't know how to
>>> resolve it.
>> Found the answer myself. adv7533 driver is a good example. Devicetree
>> exists for qcom apq8016-sbc. No need to make the bridge a MIPI DSI child
>> node.
> This is an issue that has largely been ignored so far in Linux. Both DT and 
> the Linux kernel device model organize devices in a tree structure based on 
> the control buses. Devices that are connected to multiple control buses 
> haven't been taken into account in the design and are thus hard to support.
>
> As you may know, DSI can work in command mode or data mode. In data mode the 
> DSI bus is only use to transfer video data, while in command mode it is also 
> used to control the device (reading and writing registers).

I am not sure what you mean by data and command mode. MIPI DSI specs
says about video and command mode - modes to transfer video data. In
both cases DSI can be used to control the device.

>
> A DSI device operating in data mode and controlled through I2C isn't a 
> problem, as there's a single control bus in that case. The device should be a 
> child of the I2C controller in DT, and will be instantiated through 
> of_i2c_register_devices(). 
> A DSI device operating in command mode without any 
> other control bus isn't a problem either, it will be a child of the DSI master 
> in DT, and will bee instantiated through mipi_dsi_host_register().
>
> A DSI device operating in command mode that also require configuration through 
> a separate control bus (such as I2C, but also SPI) is much more problematic to 
> support. Fortunately those devices are rare. Hopefully your device won't fall 
> in this category. If it does, we can discuss how to best support it.
>
As you have already found adv bridge is a good example. It is not clear
from the emails if DSI is used only to send video, or also to control?
And if to control, is it used to pass only specific commands

or can be used as alternative to i2c interface?


Regards

Andrzej