Re: What are you doing if the TX buffer overflows?

[Wolfgang Grandegger <wg@grandegger.com>]

On 09/18/2012 02:34 PM, Marc Kleine-Budde wrote:
> On 09/18/2012 02:14 PM, Wolfgang Grandegger wrote:
>> On 09/17/2012 09:19 PM, Oliver Hartkopp wrote:
>>> Hello Heinz,
>>>
>>> On 17.09.2012 15:58, Heinz-Jürgen Oertel wrote:
>>>
>>>>
>>>> is there a way to empty the tx buffer ?
>>>
>>>
>>> Usually the buffer does not get empty due to a problem of the CAN controller
>>> and/or the CAN network.
>>>
>>> So even if you could flush the queue the CAN controller is probably still
>>> stuck with his processed frame.
>>>
>>> Setting the interface to DOWN and UP again re-initializes the CAN controller
>>> and flushes all the queues.
>>>
>>> This would work - but all open sockets would get a notification of the
>>> interface went down.
>>>
>>> Btw. IIRC there's a IFLA_CAN_RESTART functionality to kick the CAN controller
>>> if it got stuck. It can by triggered by a netlink message. This is the same
>>> configuration interface that's used to set the bittiming:
>>>
>>> http://lxr.linux.no/#linux+v3.5.4/include/linux/can/netlink.h#L103
>>>
>>> But i have currently no source code example as i always use the 'ip' tool from
>>> the iproute2 package to configure my CAN interfaces:
>>>
>>> http://lxr.linux.no/#linux+v3.5.4/Documentation/networking/can.txt#L646
>>>
>>> Following that documentation
>>>
>>> 	ip link set can0 type can restart
>>>
>>> should do it for 'can0'.
>>
>> No, this does only works if a bus-off is pending. See:
>>
>> http://lxr.linux.no/#linux+v3.5.4/drivers/net/can/dev.c#L413
>>
>> Well, the name IFLA_CAN_RESTART is not really well chosen.
> 
> Hmmm, it corresponds somewhat to the restart-ms property.

Yes, for automatic restart after bus-off.

> We have several customers who asked how to abort pending TX messages,
> too. Which involves:
> a) clear the TX-queue in Linux
> b) clear queue in hardware
> c) abort currently transmitting CAN frame
> 
> I think c) would be a usecase of its own, too.

I think you need c) for b), at least for some controllers. These
features, especially b) and c), are not yet available because they are
not easy to implement (hardware-dependent) and there was no request so
far. Anyway, clearing the TX queue should be rather common and might
already be available for other protocols.

Wolfgang.