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Message-ID: <20250721230640.2244915-1-aplattner@nvidia.com>
Date: Mon, 21 Jul 2025 16:06:39 -0700
From: Aaron Plattner <aplattner@...dia.com>
To: Guenter Roeck <linux@...ck-us.net>, Wim Van Sebroeck
<wim@...ux-watchdog.org>
CC: Aaron Plattner <aplattner@...dia.com>, <linux-watchdog@...r.kernel.org>,
<linux-kernel@...r.kernel.org>, Timur Tabi <ttabi@...dia.com>
Subject: [PATCH v2] watchdog: sbsa: Adjust keepalive timeout to avoid MediaTek WS0 race condition
The MediaTek implementation of the sbsa_gwdt watchdog has a race
condition where a write to SBSA_GWDT_WRR is ignored if it occurs while
the hardware is processing a timeout refresh that asserts WS0.
Detect this based on the hardware implementer and adjust
wdd->min_hw_heartbeat_ms to avoid the race by forcing the keepalive ping
to be one second later.
Signed-off-by: Aaron Plattner <aplattner@...dia.com>
Acked-by: Timur Tabi <ttabi@...dia.com>
---
Thanks for the suggestion of adjusting wdd->min_hw_heartbeat_ms, Guenter. I
confirmed that doing this causes the keepalive to fire at the later time and
this avoids the race:
[ 137.500462] sbsa-gwdt sbsa-gwdt.0: version: 1, impl: 0x426, need_ws0_race_workaround: 1
[ 137.500473] sbsa-gwdt sbsa-gwdt.0: Set min_hw_heartbeat_ms to 6000
[ 137.500944] sbsa-gwdt sbsa-gwdt.0: Initialized with 10s timeout @ 1000000000 Hz, action=0. [enabled]
[ 143.501475] sbsa-gwdt sbsa-gwdt.0: ping
[ 149.502044] sbsa-gwdt sbsa-gwdt.0: ping
[ 155.502351] sbsa-gwdt sbsa-gwdt.0: ping
I also confirmed that this no longer exposes the adjusted keepalive time to
userspace:
# wdctl
Device: /dev/watchdog0
Identity: SBSA Generic Watchdog [version 0]
Timeout: 10 seconds
(in v1 this would have reported 8 seconds)
-- Aaron
drivers/watchdog/sbsa_gwdt.c | 50 +++++++++++++++++++++++++++++++++---
1 file changed, 47 insertions(+), 3 deletions(-)
diff --git a/drivers/watchdog/sbsa_gwdt.c b/drivers/watchdog/sbsa_gwdt.c
index 5f23913ce3b49..6ce1bfb390641 100644
--- a/drivers/watchdog/sbsa_gwdt.c
+++ b/drivers/watchdog/sbsa_gwdt.c
@@ -75,11 +75,17 @@
#define SBSA_GWDT_VERSION_MASK 0xF
#define SBSA_GWDT_VERSION_SHIFT 16
+#define SBSA_GWDT_IMPL_MASK 0x7FF
+#define SBSA_GWDT_IMPL_SHIFT 0
+#define SBSA_GWDT_IMPL_MEDIATEK 0x426
+
/**
* struct sbsa_gwdt - Internal representation of the SBSA GWDT
* @wdd: kernel watchdog_device structure
* @clk: store the System Counter clock frequency, in Hz.
* @version: store the architecture version
+ * @need_ws0_race_workaround:
+ * indicate whether to adjust wdd->timeout to avoid a race with WS0
* @refresh_base: Virtual address of the watchdog refresh frame
* @control_base: Virtual address of the watchdog control frame
*/
@@ -87,6 +93,7 @@ struct sbsa_gwdt {
struct watchdog_device wdd;
u32 clk;
int version;
+ bool need_ws0_race_workaround;
void __iomem *refresh_base;
void __iomem *control_base;
};
@@ -161,6 +168,31 @@ static int sbsa_gwdt_set_timeout(struct watchdog_device *wdd,
*/
sbsa_gwdt_reg_write(((u64)gwdt->clk / 2) * timeout, gwdt);
+ /*
+ * Some watchdog hardware has a race condition where it will ignore
+ * sbsa_gwdt_keepalive() if it is called at the exact moment that a
+ * timeout occurs and WS0 is being asserted. Unfortunately, the default
+ * behavior of the watchdog core is very likely to trigger this race
+ * when action=0 because it programs WOR to be half of the desired
+ * timeout, and watchdog_next_keepalive() chooses the exact same time to
+ * send keepalive pings.
+ *
+ * This triggers a race where sbsa_gwdt_keepalive() can be called right
+ * as WS0 is being asserted, and affected hardware will ignore that
+ * write and continue to assert WS0. After another (timeout / 2)
+ * seconds, the same race happens again. If the driver wins then the
+ * explicit refresh will reset WS0 to false but if the hardware wins,
+ * then WS1 is asserted and the system resets.
+ *
+ * Avoid the problem by scheduling keepalive heartbeats one second later
+ * than the WOR timeout.
+ *
+ * This workaround might not be needed in a future revision of the
+ * hardware.
+ */
+ if (gwdt->need_ws0_race_workaround)
+ wdd->min_hw_heartbeat_ms = timeout * 500 + 1000;
+
return 0;
}
@@ -202,12 +234,15 @@ static int sbsa_gwdt_keepalive(struct watchdog_device *wdd)
static void sbsa_gwdt_get_version(struct watchdog_device *wdd)
{
struct sbsa_gwdt *gwdt = watchdog_get_drvdata(wdd);
- int ver;
+ int iidr, ver, impl;
- ver = readl(gwdt->control_base + SBSA_GWDT_W_IIDR);
- ver = (ver >> SBSA_GWDT_VERSION_SHIFT) & SBSA_GWDT_VERSION_MASK;
+ iidr = readl(gwdt->control_base + SBSA_GWDT_W_IIDR);
+ ver = (iidr >> SBSA_GWDT_VERSION_SHIFT) & SBSA_GWDT_VERSION_MASK;
+ impl = (iidr >> SBSA_GWDT_IMPL_SHIFT) & SBSA_GWDT_IMPL_MASK;
gwdt->version = ver;
+ gwdt->need_ws0_race_workaround =
+ !action && (impl == SBSA_GWDT_IMPL_MEDIATEK);
}
static int sbsa_gwdt_start(struct watchdog_device *wdd)
@@ -299,6 +334,15 @@ static int sbsa_gwdt_probe(struct platform_device *pdev)
else
wdd->max_hw_heartbeat_ms = GENMASK_ULL(47, 0) / gwdt->clk * 1000;
+ if (gwdt->need_ws0_race_workaround) {
+ /*
+ * A timeout of 3 seconds means that WOR will be set to 1.5
+ * seconds and the heartbeat will be scheduled every 2.5
+ * seconds.
+ */
+ wdd->min_timeout = 3;
+ }
+
status = readl(cf_base + SBSA_GWDT_WCS);
if (status & SBSA_GWDT_WCS_WS1) {
dev_warn(dev, "System reset by WDT.\n");
--
2.50.1
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