SYSTIMER device polling

[Matthew Dillon]

:
:Hi all,
:
:Following patch(against HEAD) make device polling utilize seperate
:systimer, instead of hard clock:
:http://leaf.dragonflybsd.org/~sephe/systimer_poll.diff
:
:Tested with fxp(4)
:
:Please review it.
:
:Cheers,
:sephe
:
:--
:Live Free or Die

    Sephe, the patch is good, please commit it any time.  I like the 
    shortcut you used to change the frequency... I wasn't even thinking
    about simply changing the periodic reload value, but that's clearly the
    most effective way to do it.

    We may want to put an upper limit on the frequency to avoid livelocking
    the system... say something like 20000 hz (50uS).  So e.g.:

    if (phz <= 0)
	    return EINVAL;
    if (phz > DEVICE_POLLING_MAX_FREQ)		<<< ADDME
	    phz = DEVICE_POLLING_MAX_FREQ;	<<< ADDME

    And then add a #ifndef/#define/#endif sequence so it can be specified as
    an option.  But it isn't critical.  If you decide to do that, just make
    the changes, test, and commit.

					-Matt
					Matthew Dillon 
					<dillon at xxxxxxxxxxxxx>