Prior to this commit IRQs on STM32F4 could be lost because SR is cleared by
reading SR then reading DR. For example, if both RXNE and IDLE IRQs were
active upon entry to the IRQ handler, then IDLE is lost because the code
that handles RXNE comes first and accidentally clears SR (by reading SR
then DR to get the incoming character).
This commit fixes this problem by making the IRQ handler more atomic in the
following operations:
- get current IRQ status flags
- deal with RX character
- clear remaining status flags
- call user handler
On the STM32F4 it's very hard to get this right because the only way to
clear IRQ status flags is to read SR then DR, but the read of DR may read
some data which should remain in the register until the user wants to read
it. And it won't work to cache the read because RTS/CTS flow control will
then not work. So instead the new code disables interrupts if the DR is
full and waits for the user to read it before reenabling the interrupts.
Fixes issue mentioned in #4599 and #6082.
Signed-off-by: Damien George <damien@micropython.org>
83827e8e63