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 loop
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

Yes, and they are still used to receive normal and other error messages.

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.

My idea was to re-enable (or trigger) one BEI at the beginning of recvmsg in selected. Then all listening sockets will receive all bus error messages, e.g. A and B will receive up to the sum of the recvmsg calls form A and B. Well, this mixture is not really nice but it will downscale the bus error rate to a "digestible" level.

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

I do not regard this as a problem. The purpose of this option is to downscale the BEI rate.

----

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).

We cannot do that.

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 -
configurable?)
 - 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 ...

It's about reducing the very high BEI interrupt rate. A rate of 15kHz is _really_ heavy on mid and low end systems and especially for real-time.

Any comments to this idea?

Well, till now I do not see a 100% satisfactory solution and I do not want to make the implementation too sophisticated. Currently I'm in favor of my proposes solution with a low default value (10 or even 1):

  config XENO_DRIVERS_CAN_SJA1000_BUS_ERR_LIMIT
        depends on XENO_DRIVERS_CAN_SJA1000
        int "Maximum number of successive bus errors"
        range 0 255
        default 10
        help

        CAN bus errors are very useful for analyzing electrical problems
        but they can come at a very high rate resulting in interrupt
        flooding with bad impact on system performance and real-time
        behavior. This option, if greater than 0, will limit the amount
        of successive bus error interrupts. If the limit is reached, an
        error message with "can_id = CAN_ERR_BUSERR_FLOOD" is sent and
        the bus error interrutps get disabled. They get re-enabled on
        restart of the device and on any successful message transmission
        or reception. Be aware that bus error interrupts are only
        enabled if at least one socket is listening on bus errors.

The user has still all choices. What is you preferred solution?

Wolfgang


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