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List:       linux-serial
Subject:    [PATCH v2 2/2] sc16is7xx: Fix wrong tx fifo level read-out by preventing empty writes
From:       Florian Achleitner <achleitner.florian () fronius ! com>
Date:       2015-09-30 14:10:01
Message-ID: 2827862.bPBYSxNhrh () r90b40zn
[Download RAW message or body]

We found that our sc16is7xx on spi reported a TX fifo free space value
(TXLVL_REG) of 255 ocassionally, which is obviously wrong, with a
64 byte fifo (and caused a buffer overrun, which is fixed seperately).

To trigger this, a large write to the tty is sufficient. The fifo fills,
TXLVL_REG reads zero, but the handle_tx function does a zero-data-length
write to the TX fifo anyways through sc16is7xx_fifo_write. The next
TXLVL_REG read then yields 255, for unknown reasons. A subsequent read
is ok.

Prevent zero-data-length writes if the TX fifo is full, because they are
pointless, and because they trigger wrong TXLVL read-outs, and probably
other unspecified effects.

Signed-off-by: Florian Achleitner <achleitner.florian@fronius.com>
---

I think this should not have any negative side-effect. In my setup, it is necessary.

 drivers/tty/serial/sc16is7xx.c | 8 ++++++++
 1 file changed, 8 insertions(+)

diff --git a/drivers/tty/serial/sc16is7xx.c b/drivers/tty/serial/sc16is7xx.c
index df45eb2..f1f6323 100644
--- a/drivers/tty/serial/sc16is7xx.c
+++ b/drivers/tty/serial/sc16is7xx.c
@@ -630,10 +630,18 @@ static void sc16is7xx_handle_tx(struct uart_port *port)
 	if (likely(to_send)) {
 		/* Limit to size of TX FIFO */
 		txlen = sc16is7xx_port_read(port, SC16IS7XX_TXLVL_REG);
+		if (txlen > SC16IS7XX_FIFO_SIZE) {
+			dev_err_ratelimited(port->dev, "chip reports %d free bytes in TX "
+					"fifo, but it only has %d", txlen, SC16IS7XX_FIFO_SIZE);
+			txlen = 0;
+		}
 		to_send = (to_send > txlen) ? txlen : to_send;
 		/* prevent buffer overrun if reported txlen is flawed */
 		to_send = (to_send > sizeof(s->buf)) ? sizeof(s->buf) : to_send;
 
+		/* don't send zero-length data */
+		if (unlikely(!to_send))
+			return;
 		/* Add data to send */
 		port->icount.tx += to_send;
 
-- 
2.1.4

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