On 13 July 2017 at 09:26, George Broz <broz...@gmail.com> wrote:
> Hi -
>
> I'm using the UDD driver on Xenomai 3.0.4 / Linux 3.18.20 on an ARM SoC.
>
> I use select() to block on read events. The problem is select() unblocks
> twice in response to one interrupt event. The corresponding read()
> to the second erroneous unblocked select() then blocks until the next
> interrupt arrives.
>
> The psuedo-code in user-space looks like this:
>
> while (active)
> {
> FD_ZERO (&FileDescSet);
> FD_SET (FileDesc, &FileDescSet);
>
> /* block until interrupt or timeout occurs */
> IsInterruptPending = select ((FileDesc + 1), &FileDescSet, NULL,
> NULL, &Timeout);
>
> /* check timeout (0) and error cases (-1) */
> ...
>
> /* otherwise, process */
> if (IsInterruptPending && FD_ISSET(FileDesc, &FileDescSet))
> {
> err = read (FileDesc, &u32EventCount, sizeof (u32EventCount));
> ...
> }
> }
>
>
> I have added debug to the UDD driver to verify udd_notify_event() is called
> exactly once per interrupt event, and that the ur->event is properly
> incremented.
>
> In udd_read_rt(), debug showing ur->event and context->event_count do what
> they should be doing - returning data when they mismatch and then setting
> one equal to the other, or blocking when they are equal. This function would
> be fine if select were working properly.
>
> (One thing to note is that when ur->event and context->event_count are equal,
> it takes two iterations of the for (;;) loop to actually block on
> rtdm_event_wait().)
>
> I added debug to udd_select(), but it doesn't work as expected. I had hoped
> to see debug emitted for each user-space call to select(), but it seems only
> one call is made to udd_select() the first time through the loop.
>
> In an unsuccessful workaround, I tried issuing rtdm_event_init() in an
> ioctl() called just before unmasking interrupts for the next iteration
> of the loop.
However, if I add:
rtdm_event_clear(&ur->pulse);
just before the return statement (return ret ?: sizeof(count);) in
udd_read_rt(),
then things to seem to work properly - select() unblocks only when there
is something to read...
>
> Is there a possible bug in the RTDM select() handling? Is there a patch
> or additional debug someone could suggest? Any input greatly appreciated.
>
>
> Regards,
>
> --George Broz
> Moog Industrial Group
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