Re: [REGRESSION] breakage in sun6i-csi media api

["Linux regression tracking (Thorsten Leemhuis)" <regressions@leemhuis.info>]

Hi, Thorsten here, the Linux kernel's regression tracker. Top-posting
for once, to make this easily accessible to everyone.

Paul, what happened to this? It looks like this fall through the cracks,
but maybe I'm missing something, that's why I ask.

Ciao, Thorsten (wearing his 'the Linux kernel's regression tracker' hat)
--
Everything you wanna know about Linux kernel regression tracking:
https://linux-regtracking.leemhuis.info/about/#tldr
If I did something stupid, please tell me, as explained on that page.

#regzbot poke

On 02.06.23 12:03, Laurent Pinchart wrote:
> On Fri, Jun 02, 2023 at 11:39:54AM +0200, Paul Kocialkowski wrote:
>> (Re-adding folks from the original email, adding Laurent and Hans.)
>> On Fri 02 Jun 23, 11:24, Martijn Braam wrote:
>>>
>>> That's basically it yes. My software doesn't expect the bridge block,
>>> because it wasn't there. The pipeline always worked "automatically".
>>>
>>> This is the workaround that's been made now it run on newer kernels:
>>> https://gitlab.com/postmarketOS/megapixels/-/merge_requests/31
>>>
>>> I'm pretty sure format propagation would fix this issue.
>>
>> Okay that's good to know.
>>
>> To be honest it's still not very clear to me if in-driver format propagation is
>> a "nice to have" feature or something that all media pipeline drivers are
>> supposed to implement.
> 
> For MC-based drivers, in-kernel propagation *inside* subdevs is
> mandatory, in-kernel propagration *between* subdevs is not allowed. The
> latter is the responsibility of userspace.
> 
> For traditional (I'd say legacy, but I know not everybody likes that
> term :-)) drivers that only expose video device nodes and do not expose
> subdev nodes to userspace, the driver is responsible for configuring the
> full pipeline internally based on the S_FMT call on the video nodes
> only. This isn't applicable to the sun6i-csi driver, as it exposes
> subdev nodes to userspace.
> 
>> Anyway I feel like this is not really a regression but a result of the driver
>> being converted to a newer API.
>>
>> Also there's a V4L2_CAP_IO_MC flag which should indicate that the video device
>> must be controlled via the media controller API instead of being
>> video-device-centric, but I've seen comments asking not to set the flag even
>> when MC is used so I'm a bit confused here.
> 
> Would you have pointers to those comments ?
> 
>> Perhaps the flag is only required when there is no automatic format
>> propagation?
> 
> The flag is more or less required when you expose subdev nodes to
> userspace.
> 
>> If anyone has solid answers on these points I'd be happy to read some
>> clarification (and act accordingly).
>>
>>> On 6/2/23 11:16, Paul Kocialkowski wrote:
>>>> Hi Martijn,
>>>>
>>>> On Thu 01 Jun 23, 23:19, Martijn Braam wrote:
>>>>> It seems like this commit:
>>>>>
>>>>> media: sun6i-csi: Add bridge v4l2 subdev with port management
>>>>>
>>>>> Has changed the way the media pipeline on a64 devices, in my case the PINE64
>>>>> PinePhone works. Since this is an API towards userspace and there's active
>>>>> applications that use it I think this counts as a regression.
>>>> Do you have more details on what changed specifically?
>>>>
>>>> The commit added a bridge subdev in addition to the video node, which is
>>>> generally a better description of the CSI hardware and also a necessity
>>>> to support the ISP data flow.
>>>>
>>>> Maybe your userspace application is not configuring the bridge media block with
>>>> the right format, which results in a mismatch?
>>>>
>>>> Some work was started to achieve automatic format propagation, perhaps it
>>>> would be enough to solve your issue.
>>>>
>>>> Cheers,
>>>>
>>>> Paul
>>>>
>>>>> #regzbot introduced: 0d2b746b1bef73de62d2d311e594a7ffed4ca43
>