Changing Vcc on the fly turns out to be pretty complicated: you are
required to power down the socket, and power it up again from scratch,
which takes several seconds. In practice it is essentially never
useful since the card should always be operable at the voltage it
indicated at power-up time.
The pcmcia-cs package and the 2.4 kernel PCMCIA subsystem already do
the right thing. The 2.5 8250_cs driver had been sufficiently altered
(reindented, line breaks moved around, etc) that applying patches is
quite inconvenient and I had not gotten around to going through line
by line and figuring out what changes needed to be applied.
Probably, the best thing would be for cs.c in the kernel to do what
the pcmcia-cs package now does: just print a warning and ignore Vcc
values that don't match the power-up voltage.
The only useful case I could come up with for changing the power-up
voltage, is that some cards (compact flash I know, perhaps more) run
faster at 5V than at 3.3V, and the user might want to choose between
higher performance or better battery life.
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