add the ability for userland to be notified of changes on gpio pins via
a select(2)/read(2) interface.
Change the interrupt handler from filtered to threaded.
Because of the uiomove() calls in the new interface, change locking from
standard mutex to sx.
Add / restore the at91_gpio_high_z() function.
Reviewed by: imp (long ago)
Introduce framework to configure the multiplexed pins on boot.
Since the USART supprots RS-485 multidrop mode, it allows the
TX pins to float. However, for RS-232 operations, we don't
want these pins to float. Instead, they should be pulled up
to avoid mismatches. Linux does something similar when it
configures the TX lines. This implies that we also allow the
RX lines to float rather than be in the state they are left in
by the boot loader. Since they are input pins, I think that
this is the right thing to do.
Plus minor for our board.
