Hi Jacopo,
On 09/07/2021 18:18, Jacopo Mondi wrote:
Hi Tomi, Laurent,
On Sun, Jun 06, 2021 at 03:06:18AM +0300, Laurent Pinchart wrote:
Hi Hans, Sakari,
We need your feedback on this series, at least on the general approach.
There are quite a few issues to be addressed, and it makes no sense to
invest time in this if you don't think this is a good direction.
If anyone else wants to give feedback, speak now or forever hold your
peace :-)
Since you ask...
Having been involved a bit as the n-th person that tried to bring this
to completion I spent a bit of time trying to recollect how the
previous approach worked and how it compares to this one. Sorry if
this goes in length.
I share Tomi's concern on one part of the previous version:
- The resulting device topology gets complicated in a non-trivial way.
The typical example of having to model one image sensor that sends
embedded data and images with three sub-devices speaks for itself, I
presume.
However in one way, I feel like this is somehow correct and provides
a more accurate representation of the actual sensor architecture.
Splitting a sensor into components would allow to better handle
devices which supports multiple buses (typically CSI-2 and
parallel) through the internal routing tables, and allows
better control of the components of the image sensor. [1]
I'm not sure what kind of setup you mean, but nothing prevents you from
splitting devices into multiple subdevs with the new approach if it
makes sense on your HW.
I have a parallel sensor that provides metadata on a line before the
actual frame. I have hard time understanding why that should be split
into 3 subdevs.
- Multiplexed source pads do not accept a format or any other configuration
like crop/composing. Again this might seem odd, and it might be
worth considering if those pads shouldn't be made 'special' somehow,
but I again think it models a multiplexed bus quite accurately,
doesn't it ? It's weird that the format of, in example, a CSI-2
receiver source pad has to be propagated from the image sensor
entity sink pad, crossing two entities, two routes and one
media link. This makes rather complex to automate format propagation along
pipelines, not only when done by abusing media-ctl like most people do,
but also when done programmatically the task is not easy (I know I'm
contradicting my [1] point here :)
Hmm, but is it easy in the kernel side, then? I didn't feel so with the
previous version. The kernel needed to travel the graph back and forth
"all the time", just to figure out what's going on and where.
If the userspace understands the HW topology (as it more or less must),
and it configures the routes (as it has to), and sets the formats on
certain subdevs, then I don't see that it would have any issues in
propagating the formats.
Also link validation is of course a bit more complex as shown by
731facccc987 ("v4l: subdev: Take routing information into account in link validation")
which was part of the previous series, but it's totally up to the
core..
Moving everything to the pads by adding a 'stream' field basically
makes all pads potentially multiplexed, reducing the problem of format
configuration/validation to a 1-to-1 {pad, stream} pair validation
which allows to collapse the topology and maintain the current one.
Yes. I think I have problem understanding the counter arguments as I
don't really see a difference with a) two subdevs, each with two
non-multiplexed pads, linked 1-to-1 and b) two subdevs, each with one
multiplexed pad, with two routes.
There is one particular issue I had with the previous version, which I
think is a big reason I like the new approach:
I'm using TI CAL driver, which already exists in upstreams and supports
both non-MC and MC-without-streams. Adding support for streams, i.e
supporting non-MC, MC-without-streams and MC-with-streams made the
driver an unholy mess (including a new module parameter to enable
streams). With the new approach, the changes were relatively minor, as
MC with and without streams are really the same thing.
With the previous approach you couldn't e.g. have a CSI2-RX bridge
driver that would support both old, non-multiplexed CSI2 sensor drivers
and multiplexed CSI2 sensor drivers. Unless you had something like the
module parameter mentioned above. Or perhaps a DT property to define
which mode the pad is in.
Also, one problem is that I really only have a single multiplexed HW
setup, which limits my testing and the way I see multiplexed streams.
That setup is "luckily" not the simplest one:
SoC CSI-2 RX <-> FPDLink Deserializer <-> FPDLink Serializer <-> Sensor
4 serializer+sensor cameras can be connected to the deserializer. Each
sensor provides 2 streams (pixel and metadata). So I have 8 streams
coming in to the SoC.
Apart from the concerns expressed by Laurent (which I share but only
partially understand, as the implications of bulk moving the
v4l2-subdev configuration API to be stream-aware are not totally clear
to me yet) what I'm not convinced of is that now cross-entities
"routes" (or "streams") on a multiplexed bus do require a format
assigned, effectively exposing them to userspace, with the consequence
that the format configuration influences the routes setup up to the
point the two have to be kept consistent. The concept
could even be extended to inter-entities routes, as you suggested the
routing tables could even be dropped completely in this case, but I
feel mixing routing and format setup is a bit a layer violation and
forbids, in example, routing two streams to the same endpoint, which I
feel will be required to perform DT multiplexing on the same virtual
channel. The previous version had the multiplexed link configuration
completely hidden from userspace and controlled solely by the routing API,
which seems a tad more linear and offers more flexibility for drivers.
I'm not strongly pushing for one solution over the other, the only use
case I can reason on at the moment is a simple single-stream VC
multiplexing and both solutions works equally fine for that. This one
is certainly simpler regarding the required device topology.
Btw Tomi, do you have examples of drivers ported to your new proposal ?
Yes. They're a bit messy, but I can share them with the next version.
I'm currently fixing a lot of things, and making full use of the new
v4l2_subdev_state.
Tomi
