Re: Tegra20 parallel video capture driver

[Luca Ceresoli <luca.ceresoli@bootlin.com>]

Hi,

Il giorno Fri, 29 Apr 2022 18:49:31 +0200
Luca Ceresoli <luca.ceresoli@bootlin.com> ha scritto:

> Hello,
> 
> I am working to implement a driver for the Tegra20 camera
> interface (VI) and have it merged in mainline Linux. I have already
> done part of the work, but now I'm facing some difficulties,
> especially with respect to interaction with the syncpt feature of
> host1x. I have a few questions that hopefully someone here can reply
> to, but first let me lay some background.
> 
> First, the good news.
> 
> A good thing is I have an old reference image using a vendor kernel
> based on 3.1 which works and I can capture video from a parallel smart
> sensor in YUV. This is on a custom hardware.
> 
> Second, there is a (staging) video capture driver in mainline [0], but
> the huge delta between 3.1 and mainline makes the two drivers not
> trivial to compare.
> 
> Now to the bad news.
> 
> First, that the mainline driver only implements CSI input for
> tegra210 and I only have a parallel camera and a tegra20. The
> tegra20 documentation I have does not document parallel parallel video
> at all.
> 
> Second, the VI is heavily based on the host1x peripheral, and
> especially the Synchronization Point (syncpt) feature. Syncpts are
> also barely documented and I don't have any previous experience with
> this kind of device. And the syncpt (and perhaps host1x too)
> implementation in the vendor kernel vs mainline is very different.
> 
> And here's my status.
> 
> The mainline staging driver is already structured to support different
> SoCs, so I added tegra20 there, initially as a copy of tegra210. I did
> the same to add VIP (parallel input) support in addition to CSI. Then
> I filled the gaps as far as I could and now I have register access in
> the mainline driver that produces pretty much the same writes as the
> vendor kernel does, but it does so using the same driver structure
> that is in mainline, quite cleanly.
> 
> Now I am stuck at trying to understand how to make syncpt code work.
> As said the syncpt APIs are very different from what I see in the old
> vendor kernel. Thus I cannot try to do make the mainline driver behave
> like the old one. Moreover the mainline driver does many more calls to
> syncpt code than the old one does.
> 
> And finally here are some questions.
> 
> 1. Is there any good documentation on host1x, especially syncpt?
> 
> 2. Is the VIP (parallel input) section of VI (video capture) for
>    Tegra20 documented anywhere?
> 
> 3. The old driver accesses the syncpt using fixed IDs from #defines
> (15 for VI, 16 for CSI), while the mainline driver uses IDs obtained
> at runtime from host1x_syncpt_request(). Is it correct that one can
> use any of the 32 available syncpts, and the old driver using fixed
>    values is just an implementation choice?
> 
> 4. I am currently working on a version of
> tegra_channel_capture_frame() [1] for tegra20 VIP and other code
> involving syncpt. The mainline driver calls many different
> host1x_syncpt_*() functions, while the old driver does call very few
> of them. Can anybody clarify the meaning of the various syncpt calls?
> 
> 5. In tegra_channel_capture_frame() the call to host1x_syncpt_wait()
>    always returns -EAGAIN. Where would you start to investigate the
>    reason of this timeout?
> 
> I think it is enough for today, thank you in advance for any feedback!
> 
> [0]
> https://elixir.bootlin.com/linux/latest/source/drivers/staging/media/tegra-video
> 
> [1]
> https://elixir.bootlin.com/linux/v5.17.5/source/drivers/staging/media/tegra-video/tegra210.c#L325

Should it be any helpful, here is a reference to the old driver in the
vendor 3.1 branch that I am using as a reference:

https://github.com/SKIDATA/linux/blob/l4t-3.1.y/drivers/media/video/tegra_v4l2_camera.c

-- 
Luca Ceresoli, Bootlin
Embedded Linux and Kernel engineering
https://bootlin.com