parameters for tsleep(9)

From: Norbert Koch (nkoch_at_gmx.com)
Date: 12/24/04

  • Next message: Robert Watson: "Re: Kernel crash w/o reason" 
    To: <freebsd-hackers@freebsd.org>
Date: Fri, 24 Dec 2004 14:53:45 +0100

    Hello.

    I am just writing a device driver for the i82527 (can-bus) chip.
    For testing I need the driver to poll the chip instead of running
    in interrupt mode.

    My dev_t read function basically looks like this:

    for (;;)
    {
      while (chip_has_data(...))
      {
        read_chip_data(...);
        error = do_uiomove(...);
        if (error || enough_read(...))
        {
          return error;
        }
      };
      if (do_not_block_on_read(...))
      {
        return EWOULDBLOCK;
      }
      error = tsleep (XXX, PCATCH|PWAIT, "canrd", hz / 10);
      if (error != EWOULDBLOCK)
      {
        return error;
      }
    }

    XXX should be 'something' which could be used
    as parameter to wakeup(9), I read in tsleep(9).
    In the kernel source tree I found one
    place where tsleep _only_ sleeps: in sys/isa/ppc.c
    (which already seems to be in the attic [?] but
    still is in my computer's source tree).
    Here, the first parameter was set to NULL.
    Doing this I found, that tsleep immediately
    returns 0 (which means: wakueup was called)
    _without_ waiting. I even crashed or
    froze the kernel by calling tsleep (NULL, ...)
    for a random number of times. After changing
    this to the address of the read-function itself,
    all worked fine. No more crashes.

    Just for my understanding: Is this a bug?
    Does the first parameter have to point to
    something useful?
    Is it allowed to point it to a code position?
    Or should I use some kind of dummy data in
    the softc structure instead?

    What about the second parameter: Is PWAIT
    ok here or should I use PZERO or whatever?

    (And btw, why has ppc.c been removed?)

    Thank you.
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