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

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

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On 09/19/2012 08:50 AM, Wolfgang Grandegger wrote:
> On 09/18/2012 10:20 PM, Kurt Van Dijck wrote:
>> On Tue, Sep 18, 2012 at 09:13:48PM +0200, Wolfgang Grandegger wrote:
>>> On 09/18/2012 09:01 PM, Marc Kleine-Budde wrote:
>>>> On 09/18/2012 08:50 PM, Wolfgang Grandegger wrote:
>>>>> On 09/18/2012 03:42 PM, Marc Kleine-Budde wrote:
>>>>>> On 09/18/2012 03:39 PM, Wolfgang Grandegger wrote:
>>>>>>> On 09/18/2012 03:00 PM, Marc Kleine-Budde wrote:
>>>>>>>> On 09/18/2012 02:49 PM, Wolfgang Grandegger wrote:
>>>>>>>> [...]
>>>>>>>>
>>>>>>>>>> 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
>>>>>>>>
>>>>>>>> Yes, if it's a hardware limitation so be it. But if we design an
>>>>>>>> interface it should support "clear everything" (a+b+c), but also just
>>>>>>>> only c.
>>>>>>>
>>>>>>> Yes, that you be nice. The only portable "clear everything" (a+b+c) I
>>>>>>> see is "ifconfig down -> up". This also answers you other related mail.
>>>>>>>
>>>>>>> What do people really want/need and why? This is still not clear to me.
>>>>>>> More input would be nice.
>>>>>>
>>>>>> Heinz-Jürgen uses abort current TX Message on SJA1000, can you give us
>>>>>> more insight? I've talked to customers, e.g. they want to abort the
>>>>>> current frame if it takes "too long" to send it, because the frames CAN
>>>>>> id priority is too low.
>>
>> If "too long" is defined as a period of time where it makes sense to
>> take such actions in software (like > 10msec), and your message still
>> did not get out, but you wanted it to be,
>> then IMO the bus load is too high with regard to the expected time restrictions.
>>
>> Or am I missing something?
>>
>>>>>
>>>>> What we could implement rather easily is a "tx-abort-last" or
>>>>> "tx-abort-all" netlink command. As this command does not make sense when
>>>>> more than one message is pending I'm in favor of "tx-abort-last".
>>>>
>>>> I like to have both commands.
>>>
>>> But then "tx-abort-all" should also clear the tx socket queue. Also be
>>> aware that more than one socket may send messages. Let's wait for more
>>> use-cases.
>>
>> I'm afraid tx-abort-all & tx-abort-last cause more damage than good,
>> especially, but not only, in multi-user environments.
> 
> At least it's incompatible with multi-user (-sender) support and it does
> not fit into the Linux networking principles! And so far I have not
> heard strong arguments for implementing message flush and abort. I still
> believe that a device stop->restart is enough to cure such problems. But
> let's wait and see.

Network stop->start does basically drain all buffers (software +
hardware + frame currently sending). Just remove the currently tx-ing
frame is a valid use case, and ifconfig down; ifconfig  up has some
drawbacks:
- a transceiver (if controlled by the driver) will be switched
  off and on.
- the clocks will be turned off and on
- there might be some "mdelay" or "msleep" loops during hw/sw reset

In case the hardware a clear hw fifo and abort current tx frame there
might be a much faster and cleaner way.

Marc

-- 
Pengutronix e.K.                  | Marc Kleine-Budde           |
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