Jan Kiszka wrote:
Wolfgang Grandegger wrote:
Oliver Hartkopp wrote:
Wolfgang Grandegger wrote:
Wolfgang Grandegger wrote:
But flooding can still occur and we
are thinking about a better way of downscaling or temporarily disabling
them. Socket-CAN currently restarts the controller after 200 bus errors.
My preferred solution for RT-Socket-CAN currently is to stop the CAN
controller after a kernel configurable amount of successive bus errors.
More clever ideas and comments are welcome?
What do you think about the following method?
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 20
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. The
bus error counter gets reset 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.
Hi Wolfgang,
what would be the wanted behaviour, after the discussed problem of bus
error flooding occurred?
Well, I think the bus error rate should be downscaled without loosing
vital information concerning the cause of the problem and it should
require as little user intervention as possible. Treating it like a bus
error as currently done in Socket-CAN is a bit to strong in my mind.
Can the Controller be assumed to be 'slightly dead', or what? Is there
any chance that the bus heals by itself (=> no more bus errors) and can
be used in a normal way? Or is a user interaction recommended or _required_?
Yes, if you plug the cable, the bus errors might go away and the TX done
interrupt will arrive or you get a bus-off (I have seen both).
Indeed the slow down of bus errors is a reasonable approach, but your
suggested method leaves too many questions open for the user :-/
What questions?
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.
Well, and if there is no thread listening on bus errors, but we want
stats to be updated once in a while, a slow low-prio timer to re-enable
BEI might still be created in the stack like Oliver suggested. For
Xenomai, you could consider pending an rtdm_nrtsig to keep the impact on
the RT domain low. But that's a minor implementation detail. The
important point is to avoid uncontrolled error bursts, even over a short
period (20 bus errors at 1 MBit/s already last for > 1 ms).
I think the above solution is enough. Let's go for it?
Wolfgang.
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