[PATCH v2] watchdog: npcm: fix sleeping clock call in restart handler

[zhaofeng2]

From: Zhao Feng <zhaofeng2@xxxxxxxxxxxxxxxxxxxx>

The watchdog .restart callback runs in atomic context: it is invoked
from the restart handler chain via atomic_notifier_call_chain(), under
an RCU read-side critical section, on every reboot or restart.  Since
commit af084fdccfaf ("watchdog: npcm: Enable clock if provided"),
npcm_wdt_restart() calls clk_prepare_enable(), and clk_prepare()
takes a mutex unconditionally, so kernels built with
CONFIG_DEBUG_ATOMIC_SLEEP=y report

  BUG: sleeping function called from invalid context

on every reboot.  Worse, a panic-time restart can block on
prepare_lock and never issue the reset at all.

Get the clock with devm_clk_get_optional_prepared() at probe time and
keep it prepared for the lifetime of the driver, so the restart
handler only needs clk_enable(), which is safe in atomic context.
With the clock permanently prepared, npcm_wdt_start() and
npcm_wdt_stop() are simplified to clk_enable()/clk_disable() as well.

Fixes: af084fdccfaf ("watchdog: npcm: Enable clock if provided")
Cc: stable@xxxxxxxxxxxxxxx
Signed-off-by: Zhao Feng <zhaofeng2@xxxxxxxxxxxxxxxxxxxx>
---
v2: use devm_clk_get_optional_prepared() instead of open-coding
    clk_prepare() + devm_add_action_or_reset(), and simplify
    start/stop to clk_enable()/clk_disable(), per Sashiko's review.

Issue reported by Sashiko's review of Triet Hoang's pending
error-propagation patch; maintainer confirmed the problem in
https://lore.kernel.org/linux-kernel/19258f94-ec01-48f4-9d04-22e5ecb5cac2@xxxxxxxxxxxx/

Based on linux-next, which already carries
"watchdog: npcm: fix clock prepare leak when setting timeout on a
running watchdog".

Verified against linux-next.  Not runtime-tested (no Nuvoton NPCM
hardware).

 drivers/watchdog/npcm_wdt.c | 19 ++++++++++++++-----
 1 file changed, 14 insertions(+), 5 deletions(-)

--- a/drivers/watchdog/npcm_wdt.c
+++ b/drivers/watchdog/npcm_wdt.c
@@ -180,7 +180,7 @@
 {
 	struct npcm_wdt *wdt = to_npcm_wdt(wdd);
 
-	clk_prepare_enable(wdt->clk);
+	clk_enable(wdt->clk);
 
 	__npcm_wdt_start(wdd);
 
@@ -193,7 +193,7 @@
 
 	writel(0, wdt->reg);
 
-	clk_disable_unprepare(wdt->clk);
+	clk_disable(wdt->clk);
 
 	return 0;
 }
@@ -242,8 +242,12 @@
 {
 	struct npcm_wdt *wdt = to_npcm_wdt(wdd);
 
-	/* For reset, we start the WDT clock and leave it running. */
-	clk_prepare_enable(wdt->clk);
+	/*
+	 * For reset, we start the WDT clock and leave it running.
+	 * The clock was prepared at probe time; only clk_enable() is
+	 * safe in this atomic (restart notifier) context.
+	 */
+	clk_enable(wdt->clk);
 
 	writel(NPCM_WTR | NPCM_WTRE | NPCM_WTE, wdt->reg);
 	udelay(1000);
@@ -417,7 +421,12 @@
 
 	start = res->start;
 
-	wdt->clk = devm_clk_get_optional(&pdev->dev, NULL);
+	/*
+	 * Keep the clock prepared for the lifetime of the driver:
+	 * npcm_wdt_restart() runs in atomic context and may only call
+	 * clk_enable().
+	 */
+	wdt->clk = devm_clk_get_optional_prepared(&pdev->dev, NULL);
 	if (IS_ERR(wdt->clk))
 		return PTR_ERR(wdt->clk);