[PATCH 2/2] serial: sifive: avoid division by zero in the clock notifier

[Sebastian Alba Vives]

From: Sebastian Alba <sebasjosue84@xxxxxxxxxxxxxx>

sifive_serial_probe() registers the clock rate-change notifier before
it initialises the two fields that notifier divides by.  ssp comes from
devm_kzalloc(), so between clk_notifier_register() and the assignments
a few lines below, both ssp->baud_rate and ssp->port.uartclk are zero.

A rate change taken in that window reaches:

  PRE_RATE_CHANGE:  udelay(DIV_ROUND_UP(12 * 1000 * 1000, ssp->baud_rate))
  POST_RATE_CHANGE: __ssp_update_div() ->
                      DIV_ROUND_UP(ssp->port.uartclk, ssp->baud_rate) - 1

On RISC-V an integer division by zero does not trap; the ISA defines the
result as all bits set.  DIV_ROUND_UP() therefore yields ULONG_MAX and
udelay() busy-waits essentially forever, hanging the CPU, rather than
producing a clean oops.  The POST_RATE_CHANGE path additionally programs
a bogus divisor into the hardware.

On the FU740 the UART clock is derived from the CPU clock through a
synchronous divide-by-two, as the notifier's own comment describes, so
any CPU rate change during probe can hit this.

Initialise uartclk and baud_rate before registering the notifier, and
bail out early if the notifier is ever called with no baud rate set.

Fixes: 45c054d0815b ("tty: serial: add driver for the SiFive UART")
Signed-off-by: Sebastian Alba <sebasjosue84@xxxxxxxxxxxxxx>
---
 drivers/tty/serial/sifive.c | 13 ++++++++-----
 1 file changed, 8 insertions(+), 5 deletions(-)

diff --git a/drivers/tty/serial/sifive.c b/drivers/tty/serial/sifive.c
index 4a445d285..cd2925b6a 100644
--- a/drivers/tty/serial/sifive.c
+++ b/drivers/tty/serial/sifive.c
@@ -604,6 +604,9 @@ static int sifive_serial_clk_notifier(struct notifier_block *nb,
 	struct clk_notifier_data *cnd = data;
 	struct sifive_serial_port *ssp = notifier_to_sifive_serial_port(nb);
 
+	if (!ssp->baud_rate)
+		return NOTIFY_OK;
+
 	if (event == PRE_RATE_CHANGE) {
 		/*
 		 * The TX watermark is always set to 1 by this driver, which
@@ -1022,6 +1025,11 @@ static int sifive_serial_probe(struct platform_device *pdev)
 	ssp->clk = clk;
 	ssp->clk_notifier.notifier_call = sifive_serial_clk_notifier;
 
+	/* Set up clock divider before the notifier can observe this port */
+	ssp->port.uartclk = clk_get_rate(ssp->clk);
+	ssp->baud_rate = SIFIVE_DEFAULT_BAUD_RATE;
+	__ssp_update_div(ssp);
+
 	r = clk_notifier_register(ssp->clk, &ssp->clk_notifier);
 	if (r) {
 		dev_err(&pdev->dev, "could not register clock notifier: %d\n",
@@ -1029,11 +1037,6 @@ static int sifive_serial_probe(struct platform_device *pdev)
 		goto probe_out1;
 	}
 
-	/* Set up clock divider */
-	ssp->port.uartclk = clk_get_rate(ssp->clk);
-	ssp->baud_rate = SIFIVE_DEFAULT_BAUD_RATE;
-	__ssp_update_div(ssp);
-
 	platform_set_drvdata(pdev, ssp);
 
 	/* Enable transmits and set the watermark level to 1 */
-- 
2.43.0