linux-kernel - [PATCH] tty: serial: 8250_omap: fix TX with DMA for am33xx

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Message-ID: <20250514072035.2757435-1-jirislaby@kernel.org>
Date: Wed, 14 May 2025 09:20:35 +0200
From: "Jiri Slaby (SUSE)" <jirislaby@...nel.org>
To: gregkh@...uxfoundation.org
Cc: linux-serial@...r.kernel.org,
	linux-kernel@...r.kernel.org,
	"Jiri Slaby (SUSE)" <jirislaby@...nel.org>,
	Mans Rullgard <mans@...sr.com>,
	stable@...r.kernel.org
Subject: [PATCH] tty: serial: 8250_omap: fix TX with DMA for am33xx

Commit 1788cf6a91d9 ("tty: serial: switch from circ_buf to kfifo")
introduced an error in the TX DMA handling for 8250_omap.

When the OMAP_DMA_TX_KICK flag is set, the "skip_byte" is pulled from
the kfifo and emitted directly in order to start the DMA. While the
kfifo is updated, dma->tx_size is not decreased. This leads to
uart_xmit_advance() called in omap_8250_dma_tx_complete() advancing the
kfifo by one too much.

In practice, transmitting N bytes has been seen to result in the last
N-1 bytes being sent repeatedly.

This change fixes the problem by moving all of the dma setup after the
OMAP_DMA_TX_KICK handling and using kfifo_len() instead of the DMA size
for the 4-byte cutoff check. This slightly changes the behaviour at
buffer wraparound, but it still transmits the correct bytes somehow.

Now, the "skip_byte" would no longer be accounted to the stats. As
previously, dma->tx_size included also this skip byte, up->icount.tx was
updated by aforementioned uart_xmit_advance() in
omap_8250_dma_tx_complete(). Fix this by using the uart_fifo_out()
helper instead of bare kfifo_get().

Based on patch by Mans Rullgard <mans@...sr.com>

Fixes: 1788cf6a91d9 ("tty: serial: switch from circ_buf to kfifo")
Reported-by: Mans Rullgard <mans@...sr.com>
Cc: stable@...r.kernel.org

---
The same as for the original patch, I would appreaciate if someone
actually tests this one on a real HW too.

A patch to optimize the driver to use 2 sgls is still welcome. I will
not add it without actually having the HW.
---
 drivers/tty/serial/8250/8250_omap.c | 25 ++++++++++---------------
 1 file changed, 10 insertions(+), 15 deletions(-)

diff --git a/drivers/tty/serial/8250/8250_omap.c b/drivers/tty/serial/8250/8250_omap.c
index c9b1c689a045..bb23afdd63f2 100644
--- a/drivers/tty/serial/8250/8250_omap.c
+++ b/drivers/tty/serial/8250/8250_omap.c
@@ -1151,16 +1151,6 @@ static int omap_8250_tx_dma(struct uart_8250_port *p)
 		return 0;
 	}
 
-	sg_init_table(&sg, 1);
-	ret = kfifo_dma_out_prepare_mapped(&tport->xmit_fifo, &sg, 1,
-					   UART_XMIT_SIZE, dma->tx_addr);
-	if (ret != 1) {
-		serial8250_clear_THRI(p);
-		return 0;
-	}
-
-	dma->tx_size = sg_dma_len(&sg);
-
 	if (priv->habit & OMAP_DMA_TX_KICK) {
 		unsigned char c;
 		u8 tx_lvl;
@@ -1185,18 +1175,22 @@ static int omap_8250_tx_dma(struct uart_8250_port *p)
 			ret = -EBUSY;
 			goto err;
 		}
-		if (dma->tx_size < 4) {
+		if (kfifo_len(&tport->xmit_fifo) < 4) {
 			ret = -EINVAL;
 			goto err;
 		}
-		if (!kfifo_get(&tport->xmit_fifo, &c)) {
+		if (!uart_fifo_out(&p->port, &c, 1)) {
 			ret = -EINVAL;
 			goto err;
 		}
 		skip_byte = c;
-		/* now we need to recompute due to kfifo_get */
-		kfifo_dma_out_prepare_mapped(&tport->xmit_fifo, &sg, 1,
-				UART_XMIT_SIZE, dma->tx_addr);
+	}
+
+	sg_init_table(&sg, 1);
+	ret = kfifo_dma_out_prepare_mapped(&tport->xmit_fifo, &sg, 1, UART_XMIT_SIZE, dma->tx_addr);
+	if (ret != 1) {
+		ret = -EINVAL;
+		goto err;
 	}
 
 	desc = dmaengine_prep_slave_sg(dma->txchan, &sg, 1, DMA_MEM_TO_DEV,
@@ -1206,6 +1200,7 @@ static int omap_8250_tx_dma(struct uart_8250_port *p)
 		goto err;
 	}
 
+	dma->tx_size = sg_dma_len(&sg);
 	dma->tx_running = 1;
 
 	desc->callback = omap_8250_dma_tx_complete;
-- 
2.49.0