DW Watchdog can rise an interrupt in case if IRQ request mode is enabled
and timer reaches the zero value. In this case the IRQ lane is left
pending until either the next watchdog kick event (watchdog restart) or
until the WDT_EOI register is read or the device/system reset. This
interface can be used to implement the pre-timeout functionality
optionally provided by the Linux kernel watchdog devices.

IRQ mode provides a two stages timeout interface. It means the IRQ is
raised when the counter reaches zero, while the system reset occurs only
after subsequent timeout if the timer restart is not performed. Due to
this peculiarity the pre-timeout value is actually set to the achieved
hardware timeout, while the real watchdog timeout is considered to be
twice as much of it. This applies a significant limitation on the
pre-timeout values, so current implementation supports either zero value,
which disables the pre-timeout events, or non-zero values, which imply
the pre-timeout to be at least half of the current watchdog timeout.

Note that we ask the interrupt controller to detect the rising-edge
pre-timeout interrupts to prevent the high-level-IRQs flood, since
if the pre-timeout happens, the IRQ lane will be left pending until
it's cleared by the timer restart.
Signed-off-by: Serge Semin <Sergey.Semin@baikalelectronics.ru>
Reviewed-by: Guenter Roeck <linux@roeck-us.net>
Cc: Alexey Malahov <Alexey.Malahov@baikalelectronics.ru>
Cc: Thomas Bogendoerfer <tsbogend@alpha.franken.de>
Cc: Arnd Bergmann <arnd@arndb.de>
Cc: Rob Herring <robh+dt@kernel.org>
Cc: linux-mips@vger.kernel.org
Cc: devicetree@vger.kernel.org
Link: https://lore.kernel.org/r/20200530073557.22661-7-Sergey.Semin@baikalelectronics.ruSigned-off-by: Guenter Roeck <linux@roeck-us.net>
Signed-off-by: Wim Van Sebroeck <wim@linux-watchdog.org>