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.
So after a spike on the data line (which is correctly recognized as a
frame error) the following valid character is thrown away, because the
driver reports a frame error for that one, 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.")
Acked-by: Ilpo Järvinen <ilpo.jarvinen@xxxxxxxxxxxxxxx>
Signed-off-by: Uwe Kleine-König <u.kleine-koenig@xxxxxxxxxxxxxx>
---
Hello,
changes since (implicit) v1:
- Add Ilpo's Ack
- Add a paragraph to commit log to better describe the user visible
problem.
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: 089866061428ec9bf67221247c936792078c41a4
--
2.36.1
