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    Subject[PATCH 5.15 595/779] serial: 8250_fsl: Dont report FE, PE and OE twice
    Date
    From: Uwe Kleine-König <u.kleine-koenig@pengutronix.de>

    [ Upstream commit 9d3aaceb73acadf134596a2f8db9c451c1332d3d ]

    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@linux.intel.com>
    Signed-off-by: Uwe Kleine-König <u.kleine-koenig@pengutronix.de>
    Link: https://lore.kernel.org/r/20220704085119.55900-1-u.kleine-koenig@pengutronix.de
    Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
    Signed-off-by: Sasha Levin <sashal@kernel.org>
    ---
    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 fc65a2293ce9..af74f82ad782 100644
    --- a/drivers/tty/serial/8250/8250_fsl.c
    +++ b/drivers/tty/serial/8250/8250_fsl.c
    @@ -81,7 +81,7 @@ 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);

    --
    2.35.1


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