DMA support for half-duplex RS-485 never worked correctly on i.MX6Q/D due to an undiscovered SMP-related bug, instead of the real data being sent out, the rest of the transmit buffer is sent (xmit->tail jumps over xmit->head in imx_transmit_buffer and UART_XMIT_SIZE bytes are sent out) More details: https://lkml.org/lkml/2017/1/4/579 Disable it for that configuration until the bug is found and fixed. We need to know at probe time if we can enable DMA. (RS-485 could be enabled after that). Let's therefore only enable DMA if it is not an i.MX6Q/D UART with uart-has-rtscts in the DT and CONFIG_SMP enabled. Fixes: 17b8f2a3fdca ("serial: imx: add support for half duplex rs485") Cc: <stable@xxxxxxxxxxxxxxx> Signed-off-by: Clemens Gruber <clemens.gruber@xxxxxxxxxxxx> --- drivers/tty/serial/imx.c | 10 ++++++++-- 1 file changed, 8 insertions(+), 2 deletions(-) diff --git a/drivers/tty/serial/imx.c b/drivers/tty/serial/imx.c index bbefddd92bfe..000949f686a4 100644 --- a/drivers/tty/serial/imx.c +++ b/drivers/tty/serial/imx.c @@ -1277,8 +1277,14 @@ static int imx_startup(struct uart_port *port) writel(temp & ~UCR4_DREN, sport->port.membase + UCR4); - /* Can we enable the DMA support? */ - if (!uart_console(port) && !sport->dma_is_inited) + /* + * Can we enable the DMA support? + * RS-485 DMA is broken on i.MX6D/Q SMP systems. Do not use DMA on + * UARTs with RTS/CTS to prevent misbehavior until the bug is fixed. + */ + if (!uart_console(port) && !sport->dma_is_inited && + !(sport->have_rtscts && is_imx6q_uart(sport) && + IS_ENABLED(CONFIG_SMP))) imx_uart_dma_init(sport); spin_lock_irqsave(&sport->port.lock, flags); -- 2.13.1