Re: PCAN and SocketCAN Questions

[Marc Kleine-Budde <mkl@pengutronix.de>]

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On 11.08.2022 13:17:47, David Boles wrote:
> Thank you Marc and Tom for your suggestions! I have a few follow-ups:

I'm missing Tom's mail, seems it didn't make it to the mailing list.

> # Question 1: Silently losing received frames if the socket isn't read
> from frequently enough
> 
> > See candump's "-d" command line option.
> 
> Unfortunately it seems like this is happening at the layer of
> individual sockets. Candump was already printing out messages that I
> was "losing", and adding the -d flag doesn't affect its printout
> (presumably since from its perspective, they haven't been dropped).

See the code that the "-d" command line options activates :)

| https://github.com/linux-can/can-utils/blob/master/candump.c#L650

... and add this to your app.

> > Run your program at a lower NICE value

You can try to receive several CAN frames with recvmmsg().

> Good idea!
> 
> > Make the queue as big as you can.

I the socket drops CAN frames, this means there's not enough CPU time
for your app to receive them all. You should look at your system and
find out where the CPU time is lost.

> Yep, that's what I'm currently doing. It's probably an acceptable
> solution though not ideal; I'd like to be able to detect this
> happening and fail noisily.
> 
> # Question 2: Behavior of one-shot and presume-ack
> 
> It sounds like one-shot doesn't retry in the event of other problems
> (like losing arbitration) so that isn't what I want. It sounds like
> presume-ack is what I wanted but is only supported by very few
> devices, so I probably don't want to use it either.
> 
> # New: Question 4
> 
> I think the right course of action is to detect (either via
> looped-back frames or maybe error frames, or something else) that my
> transceiver is the only one on the bus and to stop sending messages
> temporarily. I'd like to avoid sending excessively stale frames (e.g.
> if a new transceiver joins the bus five minutes later, it shouldn't
> receive a five minute old frame from me) and from my testing with
> PCAN-USB dongles they seem to try resending a frame endlessly.

We currently don't have an API for stopping/aborting CAN frames, the
only thing you can do is ifdown/ifup the interface.

> I'd
> also like to avoid missing receiving frames transmitted by any new
> transceivers on the bus. Is there any universal/reliable API for
> clearing all transmit buffers for an interface, down to the device
> itself, without affecting its ability to receive frames?

No - currently there's only the down/up method.

Marc

-- 
Pengutronix e.K.                 | Marc Kleine-Budde           |
Embedded Linux                   | https://www.pengutronix.de  |
Vertretung West/Dortmund         | Phone: +49-231-2826-924     |
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