Subject: Re: [PATCH] CAN: Add Flexcan CAN controller driver

From: Wolfgang Grandegger <[email-masked]>
Date: Thu, 22 Jul 2010 11:44:53 +0200

On 07/22/2010 11:33 AM, Marc Kleine-Budde wrote:
> Wolfgang Grandegger wrote:
>> On 07/21/2010 10:42 PM, Marc Kleine-Budde wrote:
>>> Marc Kleine-Budde wrote:
>>>> Wolfgang Grandegger wrote:
>>>>> I realized a few issues. You can add my "acked-by" when they are fixed.
>>>> thanks for the review.
>>> [...]
>>>
>>>>>> +static void flexcan_poll_err_frame(struct net_device *dev,
>>>>>> +				   struct can_frame *cf, u32 reg_esr)
>>>>>> +{
>>>>>> +	struct flexcan_priv *priv = netdev_priv(dev);
>>>>>> +	int error_warning = 0, rx_errors = 0, tx_errors = 0;
>>>>>> +
>>>>>> +	if (reg_esr & FLEXCAN_ESR_BIT1_ERR) {
>>>>>> +		rx_errors = 1;
>>>>>> +		cf->can_id |= CAN_ERR_PROT | CAN_ERR_BUSERROR;
>>>>>> +		cf->data[2] |= CAN_ERR_PROT_BIT1;
>>>>>> +	}
>>>>>> +
>>>>>> +	if (reg_esr & FLEXCAN_ESR_BIT0_ERR) {
>>>>>> +		rx_errors = 1;
>>>>>> +		cf->can_id |= CAN_ERR_PROT | CAN_ERR_BUSERROR;
>>>>>> +		cf->data[2] |= CAN_ERR_PROT_BIT0;
>>>>>> +	}
>>>>>> +
>>>>>> +	if (reg_esr & FLEXCAN_ESR_FRM_ERR) {
>>>>>> +		rx_errors = 1;
>>>>>> +		cf->can_id |= CAN_ERR_PROT | CAN_ERR_BUSERROR;
>>>>>> +		cf->data[2] |= CAN_ERR_PROT_FORM;
>>>>>> +	}
>>>>>> +
>>>>>> +	if (reg_esr & FLEXCAN_ESR_STF_ERR) {
>>>>>> +		rx_errors = 1;
>>>>>> +		cf->can_id |= CAN_ERR_PROT | CAN_ERR_BUSERROR;
>>>>>> +		cf->data[2] |= CAN_ERR_PROT_STUFF;
>>>>>> +	}
>>>>>> +
>>>>>> +
>>>>>> +	if (reg_esr & FLEXCAN_ESR_ACK_ERR) {
>>>>>> +		tx_errors = 1;
>>>>>> +		cf->can_id |= CAN_ERR_ACK;
>>>>> This is a bus-error as well. Therefore I think it should be:
>>>>>
>>>>> 	if (reg_esr & FLEXCAN_ESR_ACK_ERR) {
>>>>> 		tx_errors = 1;
>>>>> 		cf->can_id |= CAN_ERR_ACK;
>>>>> 		cf->can_id |= CAN_ERR_PROT | CAN_ERR_BUSERROR;
>>>>> 		cf->data[3] |= CAN_ERR_PROT_LOC_ACK; /* ACK slot */
>>>>> 	}
>>>>>
>>>>> I need to check what CAN_ERR_ACK is intended for. Then, cf->can_id could
>>>>> be preset with "CAN_ERR_PROT | CAN_ERR_BUSERROR" at the beginning.
>>> This controller issues an ACK error if there are no other nodes on the
>>> CAN bus to send a ACK that the message has been received. Or all
>>> remaining Nodes are in bus off state.
>>
>> Mainly this bus error can cause the infamous IRQ and message flooding
>> when no cable is connected.
> 
> No cable connected can (if your node doesn't have an activated on baord
> termination) result in no termination at all, and this may result in a
> different error.
> 
> At least it does on the at91. I haven't checked with the flexcan.
> 
> The subtile difference is that the CAN controller isn't allowed to go
> into bus-off with a proper terminated bus when it recevies no ACKs, but
> going to bus off on a not terminated bus is okay.
> 
>>> From the datasheet:
>>> "This bit indicates that an acknowledge (ACK) error has been detected by
>>> the transmitter node; that is, a dominant bit has not been detected
>>> during the ACK SLOT."
>>
>> That's what the above error describes, like on the SJA1000, apart from
>> setting CAN_ERR_ACK. Setting CAN_ERR_ACK is somehow bogus, but just
>> leave it for the time being. I will fix it globally when time permits.
> 
> Now I'm confused. What's the meaning of CAN_ERR_ACK? When should it be used?

The type of the error is already defined via "CAN_ERR_PROT |
CAN_ERR_BUSERROR". The details of "CAN_ERR_PROT" are described in
data[2..3]. Just for the ACK errors we have CAN_ERR_ACK, but not for the
other bus errors and I ask myself why CAN_ERR_ACK was introduced.
If it does not have another meaning, I tend to remove it (only the AT91
driver actually uses it).

Wolfgang.