with multiple ports on a shared interrupt demultiplexed by the puc_intr()
handler.
siointr1() first read as much input as possible and then checked all
possibly-relevant status registers, partly for robustness and partly
for historical reasons. This is very bad if it is called for every
port sharing an interrupt like puc_intr() does. It can spend too long
reading all the input for some ports when the interrupt is for a more
urgent event on another, or just too long checking all the status
registers when there are lots of ports. The inter-character time is
too long for reading all the input even when the interrupt is for a
transmitter interrupt on the same port, and at 921600 bps the inter-char
time is 10.85 usec and was often exceeded with just 2 ports, leaving
the transmitters idle for about 6% of the time.
The tweak is to break out of the read loop after reading 1 char if
output can be done. This avoids most of the idle transmitter time for
2 active ports at 921600 bps bidirectional on the test system. It
also reduces overhead by about 20%. More complete fixes use the
programmable tx low watermark on 16950's and reduce overhead by another
65%.
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