netdev - Re: [PATCH] Add Support for Freescale FlexCAN CAN controller

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Date:	Mon, 27 Jul 2009 14:12:11 +0200
From:	Wolfgang Grandegger <wg@...ndegger.com>
To:	Oliver Hartkopp <socketcan@...tkopp.net>
CC:	Socketcan-core@...ts.berlios.de,
	Linux Netdev List <netdev@...r.kernel.org>
Subject: Re: [PATCH] Add Support for Freescale FlexCAN CAN controller

Oliver Hartkopp wrote:
> Wolfgang Grandegger wrote:
>> Oliver Hartkopp wrote:
>>> Wolfgang Grandegger wrote:
>>>> Sascha Hauer wrote:
>>>>> On Fri, Jul 24, 2009 at 04:55:02PM +0200, Wolfgang Grandegger wrote:
>>>> [...]
>>>>>>> +	kfree_skb(skb);
>>>>>> Support for echo skb using can_put/get_echo_skb() is missing. It should
>>>>>> not be a big deal to add it.
>>>>> In fact it's not missing, but the hardware is configured to receive its
>>>>> own packets, so this isn't needed.
>>>> But the user may disable IFF_ECHO, which should be handled somehow.
>>> Really?
>>>
>>> IMO IFF_ECHO indicates the capability of the network driver to perform the MAC
>>> layer frame echo on driver level.
>>>
>>> Only when you have a CAN driver that does not have this capability (the 'bad'
>>> non default case) this flag is cleared.
>>>
>>> There is no need to provide a mode that can switch this functionality 'on' and
>>> 'off'. IFF_ECHO is just an indication provided by the driver.
>>>
>>> When the FlexCAN controller can 'receive' a sent frame, so that it reflects
>>> the correct message order on the bus, this is perfect.
>>>
>>> In this case Sascha should set the IFF_ECHO flag and he's done.
>> I thought the user can toggle the IFF_ECHO flag with the "ip" utility,
>> but obviously I'm wrong.
> 
> AFAIK the only driver that currently allows to switch this flag is the vcan
> driver - but this is for testing purposes only and can be configured by the
> module commandline.
> 
> I assume having the echo functionality runtime configurable is only effort and
> complicates the driver ...

Yep.

Wolfgang.
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