CAN_RAW_RECV_OWN_MSGS and filters

CAN_RAW_RECV_OWN_MSGS and filters

[Erik Flodin]

Hi all,

I've been using a raw CAN socket with RECV_OWN_MSGS enabled to get a
transmission confirmation when a frame has been sent. Today when I
tried to enable filters to limit what I receive to only those frames
that I'm interested in I noticed that the filters also affected
reception of my own frames. So in order to receive all frames that I
send I must also add a filter that matches all frames that I send.

Is this the expected behavior? I can see why it works the way it does,
but I had expected that setting the RECV_OWN_MSGS flag would give me
all frames sent on the socket. Is there a way to get my expected
behavior without having to set a filter that matches all frames that I
plan to send?

// Erik

Re: CAN_RAW_RECV_OWN_MSGS and filters

[Marc Kleine-Budde]

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On 14.04.2021 21:22:50, Erik Flodin wrote:
> I've been using a raw CAN socket with RECV_OWN_MSGS enabled to get a
> transmission confirmation when a frame has been sent. Today when I
> tried to enable filters to limit what I receive to only those frames
> that I'm interested in I noticed that the filters also affected
> reception of my own frames. So in order to receive all frames that I
> send I must also add a filter that matches all frames that I send.
> 
> Is this the expected behavior?

At least it's implemented this way.

On TX complete of a CAN frame it's pushed into the RX path of the
networking stack, along with the information of the originating socket.

Then the CAN frame is delivered into AF_CAN, where it is passed on to
all registered receivers depending on filters. One receiver is the
sending socket in CAN_RAW. Then in CAN_RAW the it is checked if the
sending socket has RECV_OWN_MSGS enabled.

> I can see why it works the way it does, but I had expected that
> setting the RECV_OWN_MSGS flag would give me all frames sent on the
> socket. Is there a way to get my expected behavior without having to
> set a filter that matches all frames that I plan to send?

I don't think there's a way to do that without modifying the kernel
code. One option is to add a new sockopt RECV_OWN_MSGS_ALL or
RECV_OWN_MSGS_UNFILTERED. The CAN_RAW can then register another receiver
at AF_CAN for own messages only.

regards,
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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Re: CAN_RAW_RECV_OWN_MSGS and filters

[Erik Flodin]

On Thu, 15 Apr 2021 at 10:32, Marc Kleine-Budde <mkl@pengutronix.de> wrote:
>
> On 14.04.2021 21:22:50, Erik Flodin wrote:
> > I've been using a raw CAN socket with RECV_OWN_MSGS enabled to get a
> > transmission confirmation when a frame has been sent. Today when I
> > tried to enable filters to limit what I receive to only those frames
> > that I'm interested in I noticed that the filters also affected
> > reception of my own frames. So in order to receive all frames that I
> > send I must also add a filter that matches all frames that I send.
> >
> > Is this the expected behavior?
>
> At least it's implemented this way.
>
> On TX complete of a CAN frame it's pushed into the RX path of the
> networking stack, along with the information of the originating socket.
>
> Then the CAN frame is delivered into AF_CAN, where it is passed on to
> all registered receivers depending on filters. One receiver is the
> sending socket in CAN_RAW. Then in CAN_RAW the it is checked if the
> sending socket has RECV_OWN_MSGS enabled.

Thanks for the explanation. I've sent a patch to clarify in the
documentation that the socket's own frames are also subject to
filtering.

> I don't think there's a way to do that without modifying the kernel
> code. One option is to add a new sockopt RECV_OWN_MSGS_ALL or
> RECV_OWN_MSGS_UNFILTERED. The CAN_RAW can then register another receiver
> at AF_CAN for own messages only.

I'll see if I can figure something out. Unless someone beats me to it :)

// Erik