> -----Original Message----- > From: gregkh@xxxxxxxxxxxxxxxxxxx <gregkh@xxxxxxxxxxxxxxxxxxx> > Sent: Monday, 29 April, 2019 17:19 > To: Rautkoski Kimmo EXT <ext-kimmo.rautkoski@xxxxxxxxxxx> > Cc: linux-serial@xxxxxxxxxxxxxxx > Subject: Re: [PATCH] serial: 8250: Fix TX interrupt handling condition > > On Fri, Apr 26, 2019 at 12:06:13PM +0000, Rautkoski Kimmo EXT wrote: > > Interrupt handler checked THRE bit (transmitter holding register > > empty) in LSR to detect if TX fifo is empty. > > In case when there is only receive interrupts the TX handling > > got called because THRE bit in LSR is set when there is no > > transmission (FIFO empty). TX handling caused TX stop, which in > > RS-485 half-duplex mode actually resets receiver FIFO. This is not > > desired during reception because of possible data loss. > > > > The fix is to use IIR instead of LSR to detect the TX fifo status. > > This ensures that TX handling is only called when there is really > > an interrupt for THRE and not when there is only RX interrupts. > > > > Signed-off-by: Kimmo Rautkoski <ext-kimmo.rautkoski@xxxxxxxxxxx> > > --- > > drivers/tty/serial/8250/8250_port.c | 2 +- > > 1 file changed, 1 insertion(+), 1 deletion(-) > > > > diff --git a/drivers/tty/serial/8250/8250_port.c > b/drivers/tty/serial/8250/8250_port.c > > index d2f3310..91ca0ca 100644 > > --- a/drivers/tty/serial/8250/8250_port.c > > +++ b/drivers/tty/serial/8250/8250_port.c > > @@ -1875,7 +1875,7 @@ int serial8250_handle_irq(struct uart_port *port, > unsigned int iir) > > status = serial8250_rx_chars(up, status); > > } > > serial8250_modem_status(up); > > - if ((!up->dma || up->dma->tx_err) && (status & UART_LSR_THRE)) > > + if ((!up->dma || up->dma->tx_err) && (iir & UART_IIR_THRI)) > > serial8250_tx_chars(up); > > This feels wrong to me, can someone else test this to verify that it > really does work properly? I don't have access to any 8250 devices at > the moment :( > > thanks, > > greg k-h Thanks for checking this. There is indeed a problem with the patch. Interrupt ID in Interrupt Identification register is actually 3 bits, so the check should be different: ((iir & UART_IIR_ID) == UART_IIR_THRI) I'll send v2 of the patch soon. BR, Kimmo
