Insanely high baud rates

[H. Peter Anvin]

[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 Â2 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 â 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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