Jan Kiszka wrote:
Oliver Hartkopp wrote:
Additionally to the written stuff below (please read that first), i want
to remark:

- Remember that we are talking about a case that is not a standard
operation mode but a (temporary) error condition that normally leads to
a bus-off state and appears only in development and hardware setup phase!
- i would suggest to use some low resolution timestamp (like jiffies)
for this, which is very cheap in CPU usage
- the throttling should be configured as a driver module parameter (e.g.
bei_thr=0 or bei_thr=200 )due to the need of the global use-case. If you
are writing a CAN analysis tool you might want to set bei_thr=0 in other
cases a default of 200ms might be the right thing.

We are falling back to #1, i.e. where we are now already. Your
suggestion doesn't help us to provide a generic RT-stack for Xenomai.


Oliver Hartkopp wrote:
Wolfgang Grandegger wrote:
Jan Kiszka wrote:
Wolfgang Grandegger wrote:
Oliver Hartkopp wrote:

I would tend to reduce the notifications to the user by creating a
timer at the first bus error interrupt. The first BE irq would
lead to a CAN_ERR_BUSERROR and after a (configurable) time
(e.g.250ms) the next information about bus errors is allowed to be
passed to the user. After this time period is over a new
CAN_ERR_BUSERROR may be passed to the user containing the count of
occurred bus errors somewhere in the data[]-section of the Error
Frame. When a normal RX/TX-interrupt indicates a 'working' CAN
again, the timer would be terminated.

Instead of a fix configurable time we could also think about a
dynamic behaviour (e.g. with increasing periods).

What do you think about this?
The question is if one bus-error does provide enough information on
the cause of the electrical problem or if a sequence is better.
Furthermore, I personally regard the use of timers as to heavy. But
the solution is feasible, of course. Any other opinions?

I think Oliver's suggestions points in the right direction. But instead
of only coding a timer into the stack, I still vote for closing the
over the application:

After the first error in a potential series, the related error frame is
queued, listeners are woken up, and BEI is disabled for now. Once some
listener read the error frame *and* decided to call into the stack for
further bus errors, BEI is enabled again.

That way the application decides about the error-related IRQ rate and
can easily throttle it by delaying the next receive call. Moreover,
threads of higher priority will be delayed at worst by one error IRQ.
This mechanism just needs some words in the documentation ("Be warned:
error frames may overwhelm you. Throttle your reception!"), but no
further user-visible config options.
I understand, BEI interrupts get (re-)enabled in recvmsg() if the
socket wants to receive bus errors. There can me multiple readers,
but that's not a problem. Just some overhead in this function. This
would also simplify the implementation as my previous one with
"on-demand" bus error would be obsolete. I start to like this solution.
Hm - to reenable the BEI on user interaction would be a nice thing BUT i
can see several problems:

1. In socketcan you have receive queues into the userspace with a
length >1

Can you explain to me what the problem behind this is? I don't see it yet.

2. How can we handle multiple subscribers (A reads three error frames
and reenables therefore the BEI, B reads nothing in this time). Please
remember: To have multiple applications it a vital idea from socketcan.

Same here, I don't see the issue. A and B will both find the first error
frame in their queues/ring buffers/whatever. If A has higher priority
(or gets an earlier timeslice), it may already re-enable BEI before B
was able to run as well. But that's an application-specific scheduling
issue and not a problem of the CAN stack (often it is precisely what you
want when assigning priorities...).

3. The count of occured BEIs gets lost (maybe this is unimportant)

Agreed, but I also don't consider this problematic.


Regarding (2) the solution could be not to reenable the BEI for a device
until every subscriber has read his error frame. But this collides with
a raw-socket that's bound to 'any' device (ifindex = 0).

That can cause prio-inversion: a low-prio BEI-reader decides about when
a high-prio one gets the next message. No-go for RT.

Regarding (3) we could count the BEIs (which would not reduce the
interrupt load) or we just stop the BEI after the first occurance which
might possibly not enough for some people to implement the CAN
academical correct.

As you may see here a tight coupling of the problems on the CAN bus with
the application(s!) is very tricky or even impossible in socketcan.
Regarding other network devices (like ethernet devices) the notification
about Layer 1/2 problems is unusual. The concept of creating error
frames was a good compromise for this reason.

As i also would like to avoid to create a timer for "bus error
throttling", i got a new idea:

- on the first BEI: create an error frame, set a counter to zero and
save the current timestamp
- on the next BEI:
 - increment the counter
 - check if the time is up for the next error frame (e.g. after 200ms -
 - if so: Send the next error frame (including the number of occured
error frames in this 200ms)

BEI means ONLY to have a BEI (and no other error).

Of course this does NOT reduce the interrupt load but all this
throttling is performed inside the interrupt context. This should not be
that problem, or is it? And we do not need a timer ...

Any comments to this idea?


Well, I may oversee some pitfalls of my suggestion, so please help me to
understand your concerns.

There might be a problem with re-enabling BEI interrupts because we need to read the ECC. OK, I'm going to implement the method as well to check for pitfalls.


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