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.
Cheers,
Mikko
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.
