kern/51982: sio1: interrupt-level buffer overflows

[Ian Freislich]

Bruce Evans wrote:
> On Mon, 5 May 2003, Ian Freislich wrote:
> 
> > >Description:
> > 	Transferring data at "high" speed 115200bps over the serial
> > 	line (even though the actual incoming line stream is about
> > 	37000bps according to the modem LCD panel) results in the
> > 	following messages on the console at a rate of about 1 log
> > 	line every 10 to 15 seconds.
> >
> > 	These buffer overruns have gradually become more frequent
> > 	from about 3 lines and 24 overruns a day around September
> > 	2002 (when I started running Current - 4.x does not suffer
> > 	from this) to the current flurry.
> 
> -current has excessive interrupt latency caused by Giant locking almost
> everything.
> 
> Try changing this line in sio.c:
> 
> 	cp4ticks = speed / 10 / hz * 4;
> 
> to something like:
> 
> 	cp4ticks = speed / 10 / hz * 40;

Thanks.  Making this change seems to have fixed the problem.  Will
this be committed or do I need to maintain this privately until the
larger interrupt latency problem is fixed?

> Transient interrupt latency problems are supposed to be made harmless
> by using rts flow control.  There is a PR (maybe from you?) about rts
> flow control apparently not working for one modem.

Not me.

Ian