[RFC 1/3] watchdog: change watchdog_need_worker logic

From: Rasmus Villemoes
Date: Thu Jul 14 2016 - 05:26:38 EST

If the driver indicates that the watchdog is running, the framework
should feed it until userspace opens the device, regardless of whether
the driver has set max_hw_heartbeat_ms.

This patch only affects the case where wdd->max_hw_heartbeat_ms is
zero, wdd->timeout is non-zero, the watchdog is not active and the
hardware device is running (*):

- If wdd->timeout is zero, watchdog_need_worker() returns false both
before and after this patch, and watchdog_next_keepalive() is not

- If watchdog_active(wdd), the return value from watchdog_need_worker
is also the same as before (namely, hm && t > hm). Hence in that case,
watchdog_next_keepalive() is only called if hm == max_hw_heartbeat_ms
is non-zero, so the change to min_not_zero there is a no-op.

- If the watchdog is not active and the device is not running, we
return false from watchdog_need_worker just as before.

That leaves the watchdog_hw_running(wdd) && !watchdog_active(wdd) &&
wdd->timeout case. Again, it's easy to see that if
wdd->max_hw_heartbeat_ms is non-zero, we return true from
watchdog_need_worker with and without this patch, and the logic in
watchdog_next_keepalive is unchanged. Finally, if
wdd->max_hw_heartbeat_ms is 0, we used to end up in the
cancel_delayed_work branch, whereas with this patch we end up
scheduling a ping timeout_ms/2 from now.

(*) This should imply that no current kernel drivers are affected,
since the only drivers which explicitly set WDOG_HW_RUNNING are
imx2_wdt.c and dw_wdt.c, both of which also provide a non-zero value
for max_hw_heartbeat_ms. The watchdog core also sets WDOG_HW_RUNNING,
but only when the driver doesn't provide ->stop, in which case it
must, according to Documentation/watchdog/watchdog-kernel-api.txt, set

Signed-off-by: Rasmus Villemoes <rasmus.villemoes@xxxxxxxxx>
drivers/watchdog/watchdog_dev.c | 10 +++++++---
1 file changed, 7 insertions(+), 3 deletions(-)

diff --git a/drivers/watchdog/watchdog_dev.c b/drivers/watchdog/watchdog_dev.c
index 3595cff..14f8a92 100644
--- a/drivers/watchdog/watchdog_dev.c
+++ b/drivers/watchdog/watchdog_dev.c
@@ -92,9 +92,13 @@ static inline bool watchdog_need_worker(struct watchdog_device *wdd)
* thus is aware that the framework supports generating heartbeat
* requests.
* - Userspace requests a longer timeout than the hardware can handle.
+ *
+ * Alternatively, if userspace has not opened the watchdog
+ * device, we take care of feeding the watchdog if it is
+ * running.
- return hm && ((watchdog_active(wdd) && t > hm) ||
- (t && !watchdog_active(wdd) && watchdog_hw_running(wdd)));
+ return (hm && watchdog_active(wdd) && t > hm) ||
+ (t && !watchdog_active(wdd) && watchdog_hw_running(wdd));

static long watchdog_next_keepalive(struct watchdog_device *wdd)
@@ -107,7 +111,7 @@ static long watchdog_next_keepalive(struct watchdog_device *wdd)
unsigned int hw_heartbeat_ms;

virt_timeout = wd_data->last_keepalive + msecs_to_jiffies(timeout_ms);
- hw_heartbeat_ms = min(timeout_ms, wdd->max_hw_heartbeat_ms);
+ hw_heartbeat_ms = min_not_zero(timeout_ms, wdd->max_hw_heartbeat_ms);
keepalive_interval = msecs_to_jiffies(hw_heartbeat_ms / 2);

if (!watchdog_active(wdd))