Re: [PATCH 08/14] media: sunxi: Add support for the A31 MIPI CSI-2 controller

[Helen Koike <helen.koike@collabora.com>]

On 11/4/20 3:45 PM, Maxime Ripard wrote:
> On Wed, Nov 04, 2020 at 01:38:08PM -0300, Helen Koike wrote:
>>
>>
>> On 11/4/20 8:17 AM, Paul Kocialkowski wrote:
>>> Hi,
>>>
>>> On Mon 02 Nov 20, 10:21, Maxime Ripard wrote:
>>>> On Fri, Oct 30, 2020 at 07:45:18PM -0300, Helen Koike wrote:
>>>>> On 10/23/20 2:45 PM, Paul Kocialkowski wrote:
>>>>>> The A31 MIPI CSI-2 controller is a dedicated MIPI CSI-2 controller
>>>>>> found on Allwinner SoCs such as the A31 and V3/V3s.
>>>>>>
>>>>>> It is a standalone block, connected to the CSI controller on one side
>>>>>> and to the MIPI D-PHY block on the other. It has a dedicated address
>>>>>> space, interrupt line and clock.
>>>>>>
>>>>>> Currently, the MIPI CSI-2 controller is hard-tied to a specific CSI
>>>>>> controller (CSI0) but newer SoCs (such as the V5) may allow switching
>>>>>> MIPI CSI-2 controllers between CSI controllers.
>>>>>>
>>>>>> It is represented as a V4L2 subdev to the CSI controller and takes a
>>>>>> MIPI CSI-2 sensor as its own subdev, all using the fwnode graph and
>>>>>> media controller API.
>>>>>
>>>>> Maybe this is a bad idea, but I was thinking:
>>>>> This driver basically just turn on/off and catch some interrupts for errors,
>>>>> and all the rest of v4l2 config you just forward to the next subdevice
>>>>> on the pipeline.
>>>>>
>>>>> So instead of exposing it as a subdevice, I was wondering if modeling
>>>>> this driver also through the phy subsystem wouldn't be cleaner, so
>>>>> you won't need all the v4l2 subdevice/topology boilerplate code that
>>>>> it seems you are not using (unless you have plans to add controls or
>>>>> some specific configuration on this node later).
>>>>>
>>>>> But this would require changes on the sun6i-csi driver.
>>>>>
>>>>> What do you think?
>>>>
>>>> Eventually we'll need to filter the virtual channels / datatypes I
>>>> guess, so it's definitely valuable to have it in v4l2
>>
>> Which kind of datatypes? 
> 
> MIPI-CSI datatypes. Each packet on the MIPI-CSI bus is assigned a
> virtual channel and data type so that you can multiplex multiple streams
> (like a 3d camera would send for example, through the virtual channels)
> and data types (like frames and metadata) and MIPI-CSI controllers
> usually allow to filter them based on what you want.
> 
>> I ask to know if this shouldn't be configured through the video node
>> instead of subdevice.
> 
> Not really, some setups have a mux that can split the multiple virtual
> channels to multiple video nodes for example.
> 
>> Regarding channels, we had a discussion to implement it through the video
>> node (and not subdevice) [1]. But we discussed about blitters and multi-scalers,
>> so now I'm wondering if we could use the same API for mipi-csi virtual channels
>> in the video entity device, or if it doesn't apply and we need another API
>> for that in a subdevice instead.
>>
>> [1] https://patchwork.linuxtv.org/project/linux-media/cover/20200717115435.2632623-1-helen.koike@collabora.com/
> 
> There's already an API to deal with MIPI-CSI virtual channels:
> https://patchwork.kernel.org/project/linux-renesas-soc/cover/20190328200608.9463-1-jacopo+renesas@jmondi.org/
> 
> Maxime
> 

Thanks for the explanation :)

Helen