On an i.MX6Q based system using the imx-UART in rs485 mode the following
could be observed: When a character is supposed to be sent,
imx_start_tx() enables the Transmit Complete Interrupt (UCR4_TCEN) and
then enables DMA. If then the Transmit Complete Interrupt triggers
before the DMA request starts to send characters the irq handler
disables the Transmitter Empty Interrupt (UCR1_TXMPTYEN) in
imx_transmit_buffer() but not Transmit Complete Interrupt and so fails
to remove this irq source which prevents the DMA request to start and
completely locks up the system.
To prevent this lock-up also disable UCR4_TCEN when starting DMA and
expand the comment accordingly.
Fixes: 17b8f2a3fdca ("serial: imx: add support for half duplex rs485")
Signed-off-by: Uwe Kleine-König <u.kleine-koenig@xxxxxxxxxxxxxx>
---
Hello,
The only changed compared to (implicit) v1 is that I rebased this patch
on top of tty-testing.
Best regards
Uwe
drivers/tty/serial/imx.c | 11 +++++++----
1 file changed, 7 insertions(+), 4 deletions(-)
diff --git a/drivers/tty/serial/imx.c b/drivers/tty/serial/imx.c
index 3fdd322b9096..58b585c0465f 100644
--- a/drivers/tty/serial/imx.c
+++ b/drivers/tty/serial/imx.c
@@ -512,11 +512,14 @@ static inline void imx_uart_transmit_buffer(struct imx_port *sport)
}
if (sport->dma_is_enabled) {
- u32 ucr1;
+ u32 ucr1, ucr4;
/*
- * We've just sent a X-char Ensure the TX DMA is enabled
- * and the TX IRQ is disabled.
- **/
+ * Ensure the TX IRQs are disabled when enabling DMA. Otherwise
+ * the triggering TX IRQ might prevent to start the DMA request.
+ */
+ ucr4 = imx_uart_readl(sport, UCR4);
+ if (ucr4 & UCR4_TCEN)
+ imx_uart_writel(sport, ucr4 & ~UCR4_TCEN, UCR4);
ucr1 = imx_uart_readl(sport, UCR1);
ucr1 &= ~UCR1_TXMPTYEN;
if (sport->dma_is_txing) {
--
2.16.1
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