If you're getting "fifo overruns" then the ring buffer isn't your problem.

fifo overrun means we couldn't service the interrupts from the chip fast 
enough;  the ring buffer is used _after_ the interrupt to store the bytes 
until userland gets a chance to process them.

On Wed, 13 Nov 1996, Steven Campbell wrote:

> Hi.  I have a Quadra 650 that has been clock-chipped to run at 42 MHz.
> Everything works well ( no panics, no unusual messages in /var/log/messages
> ) _except_ the serial ports.
> On an ordinary 040 kernel, and when I try to use the serial ports, I get
> /netbsd: zstty0 ( or 1 ): fifo overrun anytime I try to use either port.
> 
> If I try to start a ppp session with a modem, it can't even get to
> establishment.
> 
> I increased the ZLRB_RING_SIZE to 1024, and the ZLRB_RING_MASK to 1023, and
> I still get fifo overruns, but a ppp session can be established.
> 
> My question is: can I continue to increase the ZLRB_RING variables to
> numbers such as 2048, 4096, 8192, etc until I find a large enough buffer
> size, or is there something else that someone else can think of that would
> fix this problem?
> 
> Thanks,
> Steve
> 
> 
>