Re: Usb to can driver

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

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On 06/26/2013 08:55 PM, Max S. wrote:
>>> My firmware implements LOOPBACK by default. That is all frames sent by
>>> the can hardware are also returned over usb. I use this effect to call
>>> can_get_echo_skb() during bulk in. To support LOOPBACK mode as per
>>> socketcan; should i also call netif_rx() durring bulk in if
>>> CAN_CTRLMODE_LOOPBACK is
>>> set?
>>
>> Message echo (normally via echo_skbs) and loopback (via
>> CAN_CTRLMODE_LOOPBACK)
>> are somehow different things. The first is a local loopback of sent
>> frames,
>> which is described here:
>>
>>   http://lxr.linux.no/#linux+v3.9.7/Documentation/networking/can.txt#L178
> 
> I was confused because i was not sure if 'sent frame' meant either a)
> 'frame that has left Linux', or b) 'frame that actually made it onto the
> CAN bus'. Appears I can choose either :P

b) is preferred

> For usb_8dev and ems_usb, echo via put_echo_skb happens in the USB write
> callback. (a)
> 
> For esd_usb2 a frame is only recognized as sent when the device reports
> it as sent. (b)
> 
> I also noticed esd_usb2 does: dev->nets[msg->msg.txdone.net]
> Is that safe? msg->msg.txdone.net is never check if its in range...

Can you send a patch? :)

>> Does the hardware communicate a TX done event as well?
> Yes, The controller produces a TX event which happens after successful
> frame transmission. I place successfully TXed frames in the USB IN fifo
> with their corresponding echo_ids.

This means you implement b).

>> CAN_CTRLMODE_LOOPBACK is a real hardware loopback. Normally the message
>> does not even go out to the bus mainly useful for testing. For the software the
>> loopback message should be handled like any other incoming message.
> 
> Ah. so CAN_CTRLMODE_LOOPBACK could be used to test data path from
> application->Linux->driver->can_hardware and back. without the need of
> an actual physical CAN bus.

Yes, this is a usecase.

Marc

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