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