netdev - Re: [PATCH net-next-2.6 v2] can: mcp251x: Move to threaded interrupts instead of workqueues.

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Date:	Wed, 3 Feb 2010 08:23:17 +0100
From:	christian pellegrin <chripell@...e.org>
To:	Wolfgang Grandegger <wg@...ndegger.com>
Cc:	socketcan-core@...ts.berlios.de, netdev@...r.kernel.org
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 <wg@...ndegger.com> wrote:
> Hi Christian,
>

Hi,

>
> 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
mode."

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 http://www.evolware.org/chri/
"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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