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SubjectRe: [PATCH] serial: Do not treat the IIR register as a bitfield
While you're fixing this, there's a bug in samples/vfio-mdev/mtty.c:

u8 ier = mdev_state->s[index].uart_reg[UART_IER];
*buf = 0;

mutex_lock(&mdev_state->rxtx_lock);
/* Interrupt priority 1: Parity, overrun, framing or break */
if ((ier & UART_IER_RLSI) && mdev_state->s[index].overrun)
*buf |= UART_IIR_RLSI;

/* Interrupt priority 2: Fifo trigger level reached */
if ((ier & UART_IER_RDI) &&
(mdev_state->s[index].rxtx.count ==
mdev_state->s[index].intr_trigger_level))
*buf |= UART_IIR_RDI;

/* Interrupt priotiry 3: transmitter holding register empty */
if ((ier & UART_IER_THRI) &&
(mdev_state->s[index].rxtx.head ==
mdev_state->s[index].rxtx.tail))
*buf |= UART_IIR_THRI;

/* Interrupt priotiry 4: Modem status: CTS, DSR, RI or DCD */
if ((ier & UART_IER_MSI) &&
(mdev_state->s[index].uart_reg[UART_MCR] &
(UART_MCR_RTS | UART_MCR_DTR)))
*buf |= UART_IIR_MSI;

/* bit0: 0=> interrupt pending, 1=> no interrupt is pending */
if (*buf == 0)
*buf = UART_IIR_NO_INT;

It's treating the UART_IIR_* fields as a bitmask which is bad enough,
but in the "Interrupt priority 4" case, UART_IIR_MSI is zero, so
"*buf |= UART_IIR_MSI" is a no-op. And in the case where the modem
status interrupt is the only thing set, *buf will be 0, and UART_IIR_NO_INT
gets set erroneously.

So this is another example of the bug of trying to treat the
UART_IIR_* fields as a bitmask....

Yes, it's only sample code, but best fix it now before it gets copied
elsewhere and metastisizes. :-)

- Ted


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