It is plausible that the hardware interrupts a host only when GIS goes
from zero to one. GIS is formed by OR-ing multiple hardware statuses,
so it's possible that a previously cleared status gets set again while
another status has not been cleared yet. Thus, there will be no new
interrupt as GIS always stayed set. If we don't re-examine GIS then we
can leave it set and never get another interrupt again.
Without this change I frequently saw a problem where snd_hda would stop
working. Setting dev.hdac.1.polling=1 would bring it back to life and
afterwards I could set polling back to zero. Sometimes the problem
started right after a boot, sometimes it happened after resuming from
S3, frequently it would occur when sound output and input are active
concurrently (such as during conferencing). I looked at HDAC_INTSTS
while the sound was not working and I saw that both HDAC_INTSTS_GIS and
HDAC_INTSTS_CIS were set, but there were no interrupts.
I have collected some statistics over a period of several days about how
many loops (calls to hdac_one_intr) the new code did for a single
interrupt:
+--------+--------------+
|Loops |Times Happened|
+--------+--------------+
|0 |301 |
|1 |12857746 |
|2 |280 |
|3 |2 |
|4+ |0 |
+--------+--------------+
I believe that previously the sound would get stuck each time we had to loop
more than once.
The tested hardware is:
hdac1: <AMD (0x15e3) HDA Controller> mem 0xfe680000-0xfe687fff at device 0.6 on pci4
hdacc1: <Realtek ALC269 HDA CODEC> at cad 0 on hdac1
No objections: mav
MFC after: 5 weeks
Differential Revision: https://reviews.freebsd.org/D25128
4c7d1ab06d