Re: socket can receive order

[Tom Evans <tom_usenet@optusnet.com.au>]

On 08/09/15 21:46, Wolfgang Grandegger wrote:
>
> Am 08.09.2015 um 13:13 schrieb Marc Kleine-Budde:
>> On 09/08/2015 12:41 PM, Daniel Squires wrote:
>>> On my laptop and Desktop PC I have not seen it happen.
>> I mean what kind of CAN adapter...
>
> "https://github.com/fabiobaltieri/open-usb-can" seems to use a MCP2515
> controller.

http://fabiobaltieri.com/2013/07/23/hacking-into-a-vehicle-can-bus-toyothack-and-socketcan/#more-1419

"my own open hardware USB AVR + MCP2515 interface", "the performances are not 
that good above 250kbps", "It’s tempting to use an SPI controller (the MCP2515 
is very common), but that has terrible performances on highly loaded fast 
busses, and you will end up with problem such as RX buffer underruns and 
out-of-order frames."

He means "overruns". The MCP2515 doesn't have a FIFO. Messages have to be read 
out over a slow SPI bus one bit at a time within one message time or it 
overruns. Or two if the BUKT bit is set, but that risks reading messages in 
the wrong order.

The design uses an ATMEGA32U2 and an MCP2515. I can't see why it shouldn't be 
able to buffer messages from the MCP2515 at relatively high data rates, if the 
code is well written. From my experience though, code for the MCP2515 is 
seldom "well written". It is too easy to fall into a trap and get the message 
arrival order wrong.

This is unlikely to be related to the OP's problem, but just something to be 
aware of.

For anybody still coding and debugging MCP2515 stuff:

http://www.microchip.com/forums/m620741.aspx

 > otherwise its prone to loosing TX packets when loaded.

Do you know about having to do something like the following to stop CAN 
Transmit Drops? The networking stack defaults to DROPPING CAN transmit frames 
before blocking the socket if you don't.

     /bin/echo 256 > /sys/class/net/can0/tx_queue_len
     ...
     int sndbuf = (250 + 8) * 256;
     socklen_t socklen = sizeof(sndbuf);
     /* Minimum socket buffer to try and get it blocking */
     rc = setsockopt(pSkt->skt, SOL_SOCKET, SO_SNDBUF,
                     &sndbuf, sizeof(sndbuf));


http://socket-can.996257.n3.nabble.com/Solving-ENOBUFS-returned-by-write-td2886.html

Tom