Some Freescale 8250 implementations have the problem that a single long
break results in one irq per character frame time. The code in
fsl8250_handle_irq() that is supposed to handle that uses the BI bit in
lsr_saved_flags to detect such a situation and then skip the second
received character. However it also stores other error bits and so after
a single frame error the character received in the next irq handling is
passed to the upper layer with a frame error, too.
To weaken this problem restrict saving LSR to only the BI bit.
Note however that the handling is still broken:
- lsr_saved_flags is updated using orig_lsr which is the LSR content
for the first received char, but there might be more in the FIFO, so
a character is thrown away that is received later and not necessarily
the one following the break.
- The doubled break might be the 2nd and 3rd char in the FIFO, so the
workaround doesn't catch these, because serial8250_rx_chars() doesn't
handle the workaround.
- lsr_saved_flags might have set UART_LSR_BI at the entry of
fsl8250_handle_irq() which doesn't originate from
fsl8250_handle_irq()'s "up->lsr_saved_flags |= orig_lsr &
UART_LSR_BI;" but from e.g. from serial8250_tx_empty().
- For a long or a short break this isn't about two characters, but more
or only a single one.
Fixes: 9deaa53ac7fa ("serial: add irq handler for Freescale 16550 errata.")
Signed-off-by: Uwe Kleine-König <u.kleine-koenig@xxxxxxxxxxxxxx>
---
Hello,
note this patch is more or less untested because I have an MPC8313 that
doesn't show the behaviour described in the erratum. However the patch
fixes handling of frame errors, partity errors and overflow errors if
for that SoC the fsl8250_handle_irq is still used (which is a different
bug).
drivers/tty/serial/8250/8250_fsl.c | 2 +-
1 file changed, 1 insertion(+), 1 deletion(-)
diff --git a/drivers/tty/serial/8250/8250_fsl.c b/drivers/tty/serial/8250/8250_fsl.c
index 9c01c531349d..71ce43685797 100644
--- a/drivers/tty/serial/8250/8250_fsl.c
+++ b/drivers/tty/serial/8250/8250_fsl.c
@@ -77,7 +77,7 @@ int fsl8250_handle_irq(struct uart_port *port)
if ((lsr & UART_LSR_THRE) && (up->ier & UART_IER_THRI))
serial8250_tx_chars(up);
- up->lsr_saved_flags = orig_lsr;
+ up->lsr_saved_flags |= orig_lsr & UART_LSR_BI;
uart_unlock_and_check_sysrq_irqrestore(&up->port, flags);
base-commit: 3123109284176b1532874591f7c81f3837bbdc17
--
2.35.1
