[PATCH v2 6/8] watchdog: starfive-wdt: Avoid unclocked MMIO read in starfive_wdt_get_timeleft()

[Guenter Roeck <linux@roeck-us.net>]

When the watchdog is inactive, runtime PM suspends the device and gates
off apb_clk and core_clk via starfive_wdt_runtime_suspend(). However,
reading the sysfs timeleft attribute via timeleft_show() or issuing
ioctl(WDIOC_GETTIMELEFT) calls starfive_wdt_get_timeleft() unconditionally.

Because starfive_wdt_get_timeleft() reads the counter and interrupt status
registers without checking whether the hardware watchdog is running,
querying timeleft while inactive performs unclocked MMIO reads and triggers
a bus fault.

Return 0 immediately in starfive_wdt_get_timeleft() when
watchdog_hw_running() is false. When watchdog_hw_running() is true, the
driver holds an active runtime PM reference keeping the clocks enabled.

Fixes: db728ea9c7be ("drivers: watchdog: Add StarFive Watchdog driver")
Cc: Xingyu Wu <xingyu.wu@starfivetech.com>
Cc: Ziv Xu <ziv.xu@starfivetech.com>
Signed-off-by: Guenter Roeck <linux@roeck-us.net>
---
v2: New patch

 drivers/watchdog/starfive-wdt.c | 3 +++
 1 file changed, 3 insertions(+)

diff --git a/drivers/watchdog/starfive-wdt.c b/drivers/watchdog/starfive-wdt.c
index 4142e1502038..60a97ffa940a 100644
--- a/drivers/watchdog/starfive-wdt.c
+++ b/drivers/watchdog/starfive-wdt.c
@@ -312,6 +312,9 @@ static unsigned int starfive_wdt_get_timeleft(struct watchdog_device *wdd)
 	struct starfive_wdt *wdt = watchdog_get_drvdata(wdd);
 	u32 count;
 
+	if (!watchdog_hw_running(wdd))
+		return 0;
+
 	/*
 	 * If the watchdog takes twice timeout and set half count value,
 	 * timeleft value should add the count value before first timeout.
-- 
2.45.2