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.
Signed-off-by: Florian Achleitner <achleitner.florian@xxxxxxxxxxx>
---
drivers/tty/serial/sc16is7xx.c | 12 ++++++++++++
1 file changed, 12 insertions(+)
diff --git a/drivers/tty/serial/sc16is7xx.c b/drivers/tty/serial/sc16is7xx.c
index 107a099..b00fa25 100644
--- a/drivers/tty/serial/sc16is7xx.c
+++ b/drivers/tty/serial/sc16is7xx.c
@@ -389,6 +389,13 @@ static void sc16is7xx_fifo_write(struct uart_port *port, u8 to_send)
const u8 line = sc16is7xx_line(port);
u8 addr = (SC16IS7XX_THR_REG << SC16IS7XX_REG_SHIFT) | line;
+ /*
+ * Don't send zero-length data, at least on SPI it confuses the chip reading
+ * wrong TXLVL data.
+ */
+ if (unlikely(!to_send))
+ return;
+
regcache_cache_bypass(s->regmap, true);
regmap_raw_write(s->regmap, addr, s->buf, to_send);
regcache_cache_bypass(s->regmap, false);
@@ -630,6 +637,11 @@ 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 > SC16IS7XX_FIFO_SIZE) ? SC16IS7XX_FIFO_SIZE : to_send;
--
2.5.0
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