Transaction timing out on 586 embedded CPU

[Bjørn Mork]

Kai Scharwies <kai at scharwies.de> writes:

> I saw the line "read(3, 0xbfdfbd6c, 16)                 = -1 EAGAIN
> (Resource temporarily unavailable)" in strace. Could that be the
> problem?

That looks normal to me.  Just means that there were no data available
for the non blocking read.


> eventfd2(0, O_NONBLOCK|O_CLOEXEC)       = 3

So fd 3 is the eventfd

> open("/dev/cdc-wdm0", O_RDWR|O_EXCL|O_NOCTTY|O_NONBLOCK) = 4

and fd 4 is the character device

> write(4, "\1\17\0\0\0\0\0\1\"\0\4\0\1\1\0\2", 16) = 16

writing first QMI message

> poll([{fd=3, events=POLLIN}, {fd=4, events=POLLIN}], 2, 10625) = 1
> ([{fd=4, revents=POLLIN}])

the driver signals data ready 

> read(3, 0xbfdfbd6c, 16)                 = -1 EAGAIN (Resource

don't understand why this is done, but it should be harmless

> read(4, "\1\27\0\200\0\0\1\1\"\0\f\0\2\4\0\0\0\0\0\1\2\0\2\7", 2048) = 24

successfully read the QMI message from the driver

> write(4, "\1\f\0\0\2\7\0\1\0%\0\0\0", 13) = 13

writing next QMI message to the driver

> poll([{fd=3, events=POLLIN}, {fd=4, events=POLLIN}], 2, 10611) = 0 (Timeout)
> read(3, 0xbfdfbd6c, 16)                 = -1 EAGAIN (Resource
> temporarily unavailable)
> clock_gettime(CLOCK_MONOTONIC, {77050, 15159036}) = 0
> poll([{fd=3, events=POLLIN}, {fd=4, events=POLLIN}], 2, 0) = 0 (Timeout)
> write(2, "error: operation failed: Transac"..., 47error: operation
> failed: Transaction timed out
> ) = 47

Then just silence from the driver until the poll times out.  Strange.
Polling worked above, so why not here too?

Unless you are doing the same I usually do whenever I play with
NetworkManager and ModemManager: Do you have some other process running
and reading from /dev/cdc-wdm0? lsof will tell.  It's perfectly possible
to have multiple processes reading from the device at the same time, but
only one of them will see each QMI message from the driver. This results
in failures like yours when some other process has "stolen" the QMI
message you are waiting for.

I have considered ways to avoid this by driver modifications (copying
all messages to all readers, or making open exclusive), but rejected
them.  This is not possible without changing the userspace API and
that's a no-no.

But let's verify if this indeed is the problem first.



Bjørn