Insanely high baud rates

H. Peter Anvin
2018-10-09T19:19:04Z

[Resending to a wider audience]

In trying to get the termios2 interface actually implemented in glibc,
the question came up if we will ever care about baud rates in excess of
4 Gbps, even in the relatively remote future.

If this is something we care about *at all*, I would like to suggest
that rather than defining yet another kernel interface, we steal some
bits from the MSB of the speed fields, alternatively one of the c_cc
bytes (all likearchitectures seem to have c_cc[18] free) or some field,
if we can find them, in c_cflags, to indicate an exponent.

With 5 bits from the top of the speed fields, the current values would
be identical up to 248 Gbps, and values up to ~288 Pbps would be
encodable ±2 ppb.

In the short term, all we would have to do in the kernel would be
erroring out on baud rates higher than 0x0fffffff (2^28-1 due to
implicit one aliasing rhe first bit of a 5-bit exponent – less than 2^27
are functionally denorms.) However, I'd like to put the glibc
infrastructure for this now if this is something we may ever be
interested in.

Thoughts?
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