linux-kernel - [PATCH 8/8] serial: fsl_lpuart: disable DMA for console and fix sysrq

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Message-Id: <20210511200148.11934-9-michael@walle.cc>
Date:   Tue, 11 May 2021 22:01:48 +0200
From:   Michael Walle <michael@...le.cc>
To:     linux-serial@...r.kernel.org, linux-kernel@...r.kernel.org
Cc:     Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        Jiri Slaby <jirislaby@...nel.org>,
        Angelo Dureghello <angelo.dureghello@...esys.com>,
        Fugang Duan <fugang.duan@....com>,
        Philippe Schenker <philippe.schenker@...adex.com>,
        Michael Walle <michael@...le.cc>
Subject: [PATCH 8/8] serial: fsl_lpuart: disable DMA for console and fix sysrq

SYSRQ doesn't work with DMA. This is because there is no error
indication whether a symbol had a framing error or not. Actually,
this is not completely correct, there is a bit in the data register
which is set in this case, but we'd have to read change the DMA access
to 16 bit and we'd need to post process the data, thus make the DMA
pointless in the first place.

Signed-off-by: Michael Walle <michael@...le.cc>
---
Please note, that there is already sysrq/break support in the 8 bit
version. But I think there is a race between the hardware DMA controller
and the ISR in this driver. I'm not sure though and can't test it.

Angelo, maybe you could test it, I'd presume with this patch you don't need
the special handling in the ISR anymore.

 drivers/tty/serial/fsl_lpuart.c | 6 ++++++
 1 file changed, 6 insertions(+)

diff --git a/drivers/tty/serial/fsl_lpuart.c b/drivers/tty/serial/fsl_lpuart.c
index 5e66f628e895..bf869c8d0897 100644
--- a/drivers/tty/serial/fsl_lpuart.c
+++ b/drivers/tty/serial/fsl_lpuart.c
@@ -1587,6 +1587,9 @@ static void lpuart_tx_dma_startup(struct lpuart_port *sport)
 	u32 uartbaud;
 	int ret;
 
+	if (uart_console(&sport->port))
+		goto err;
+
 	if (!sport->dma_tx_chan)
 		goto err;
 
@@ -1616,6 +1619,9 @@ static void lpuart_rx_dma_startup(struct lpuart_port *sport)
 	int ret;
 	unsigned char cr3;
 
+	if (uart_console(&sport->port))
+		goto err;
+
 	if (!sport->dma_rx_chan)
 		goto err;
 
-- 
2.20.1