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Date:	Wed, 3 Feb 2010 08:23:17 +0100
From:	christian pellegrin <>
To:	Wolfgang Grandegger <>
Subject: Re: [PATCH net-next-2.6 v2] can: mcp251x: Move to threaded interrupts 
	instead of workqueues.

On Tue, Feb 2, 2010 at 11:00 AM, Wolfgang Grandegger <> wrote:
> Hi Christian,


> The MERR should not come when the device is in bus-off. Nevertheless, it

yes, sorry it's in error-passive state!

> space via error messages. For exactly that reason we do not disable bus
> errors on other CAN controllers, like the SJA1000 or the AT91, even if
> they may produce high load. But we may discuss some generic interface to
> disable bus-errors somehow, maybe configurable via ctrlmode. What do you
> think?

The transition of CAN error states is handled by the ERR interrupt,
the MERR means "message error" and is fired when a transmission or
receptions leads to an error. The problems with this interrupt are:

1) it's impossible to know if it was a TX or RX error.

2) if the error is accounted (for example) to TX the user will see
ton's of TX errors even if he sent just one packet (this happens in
error-passive mode for example) which could be confusing.

3) I guess that microchip has a specific use of this interrupt in mind
which explains it's drawbacks. From the data sheet:

"7.4        Message Error Interrupt
When an error occurs during transmission or reception
of a message the message error flag (CAN-
INTF.MERRF) will be set and, if the CANINTE.MERRE
bit is set, an interrupt will be generated on the INT pin.
This is intended to be used to facilitate baud rate deter-
mination when used in conjunction with listen-only

I think that a much more useful information to somehow export to the
user are the TEC and REC counters. I checked other CAN drivers but no
one seems to do this. Anyway let me know what do you think so I could
prepere the final patch now that OSM problems where sorted out.

Christian Pellegrin, see
"Real Programmers don't play tennis, or any other sport which requires
you to change clothes. Mountain climbing is OK, and Real Programmers
wear their climbing boots to work in case a mountain should suddenly
spring up in the middle of the computer room."
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