Hi Mikko, Il giorno Thu, 5 May 2022 12:42:08 +0300 Mikko Perttunen <cyndis@xxxxxxxx> ha scritto: > On 4/29/22 19:49, Luca Ceresoli wrote: > > Hello, > > Hi! > > > > > 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? > > There isn't much documentation, apart from what exists in the TRMs. > But basically, syncpoints are an array of 32-bit unsigned integers in > hardware, with each 32-bit value having the following fundamental > operations: > > * Increment by one, wrapping (i.e. mod 2^32) > * Wait for value to reach threshold, such that if you consider the 32 > bit unsigned integers a circle, then the half-circle preceding > (integer-wise) the threshold value are considered to be in the past > and the half-circle succeeding are considered to be in the future. > For example, if we specify threshold 0xffffffff then 0x0 is > considered to be one increment in the future. Waits can be done > either by HW stalling host1x channels (similar to DMA) or CPU > interrupts. > > > > > 2. Is the VIP (parallel input) section of VI (video capture) for > > Tegra20 documented anywhere? > > If not in the TRM, then no. > > > > > 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? > > The old code used to have hard-coded syncpoint IDs for each use, but > now we allocate dynamically. This is a global namespace so you need > to call host1x_syncpt_request to get an ID that is not colliding with > anyone else. > > > > > 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? > > - request/put -- allocate and free syncpoint > - incr/wait -- as described before > - incr_max -- when syncpoint is used with host1x channels, used to > indicate to job tracking code how many times the job will increment > the syncpoint. when not using channels, should not be used -- looks > like the tegra210 code calls this a couple of times but it probably > shouldn't be doing that. > > > > > 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? > > This means that the syncpoint's value did not reach the specified > threshold. Either the specified threshold is incorrect, or the engine > did not increment the syncpoint (or did not increment sufficient > number of times), either because the syncpoint increment was not > programmed correctly, or the condition for the increment was not > fulfilled (i.e. the engine got stuck for whatever reason). > > Apart from the wait, you can use > /sys/kernel/debug/tegra-host1x/status to track syncpoint value. Apart > from that, if you're not using channels, you'll just have to look at > engine registers to see why it's not processing. Thank you very much! I appreciate a lot your feedback and it will be helpful for my advancement. I will probably come back with more questions later on. Best regards. -- Luca Ceresoli, Bootlin Embedded Linux and Kernel engineering https://bootlin.com
