Re: [PATCHv5 0/4] watchdog: Extend kernel API and add early_timeout_sec feature

["Yang, Wenyou" <wenyou.yang@atmel.com>]

On 2015/4/10 21:06, Timo Kokkonen wrote:
> The watchdog kernel API is quite limited. It has support for providing
> generic device handling, but it doesn't really know anything about the
> watchdog hardware or its constraints. The watchdog drivers come with a
> lot of diversity and their own set of quirks and constraints. Some of
> their limitations are not nice for the user space, so the drivers work
> around them with all sorts of ad hoc implementations.
>
> One common pattern is to use kernel timers or work queues to allow
> longer timeout parameters than the actual hardware supports. To solve
> this problem, this patch set extends the kernel watchdog API with a few
> parameters that let the core know more about the watchdog HW and take
> care about the timeout extending.
>
> The patch set also implements "early_timeout_sec" feature that is very
> common on many production systems where early kernel or user space
> crashes must lead to a device reset. Traditional watchdog handling
> does not allow this as the watchdog is stopped (fully or emulating
> stopped state with kernel timers) before user space opens it for the
> first time.
>
> The changes are designed to be taken in use one driver at time. If the
> driver does not set the new parameters and call
> watchdog_init_params(), the watchdog behavior is exactly the same as
> before.
>
> In principle this new API makes it possible for the user space to see
> every watchdog hardware to behave the same, at least in terms of
> watchdog timeouts. Once the API is in, it should be easier to move
> even more common behavior out of the driver code to the watchdog core
> and make the drivers simpler.
>
> I'm not anticipating this patch set to go in as is, this is merely to
> help bringing up the discussion as there is a working patch set to look
> at. There are propably some corner cases that I didn't get right
> yet. Also, we could even remove the maximum timeout completely as the
> HW no longer limits the watchdog timeouts. This propably needs some
> thinking still and your commenting.
>
> Please review and give feedback.
Tested on the SAMA5D4EK.

Tested-by Wenyou Yang <wenyou.yang@atmel.com>
>
> Patch revision history:
>
> -v5: Re-think the approach to be fully generic. The early_timeout_sec
>    handling is no longer in the driver but in the watchdog core. As a
>    result the core needed to gain knowledge about the watchdog
>    hardware. Appropriate handling is added in the core. The side effect
>    for this is that drivers using the new extensions can be simplified
>    a lot and different kinds of watchdog hardware can be made to
>    behave the same for the user space.
>
> -v4: Binding documentation is now separated completely from the driver
>    patch. The documentation no longer makes any assumptions about how
>    the actual implementation is made, it just describes the actual
>    behavior the driver should implement in order to satisfy the
>    requirement.
>
> - v3: Rename the property to "early-timeout-sec" and use it as a
>    timeout value that stops the timer in the atmel driver after the
>    timeout expires. A watchdog.txt is also introduced for documenting
>    the common watchdog properties, including now this one and
>    "timeout-sec" property.
>
> - v2: Rename the property to "enable-early-reset" as the behavior
>    itself is not atmel specific. This way other drivers are free to
>    implement same behavior with the same property name.
>
> - v1: Propose property name "atmle,no-early-timer" for disabling the
>    timer that keeps the atmel watchdog running until user space opens
>    the device.
>
>
> Timo Kokkonen (4):
>    watchdog: Extend kernel API to know about HW limitations
>    watchdog: Allow watchdog to reset device at early boot
>    devicetree: Document generic watchdog properties
>    watchdog: at91sam9_wdt: Convert to use new watchdog core extensions
>
>   .../devicetree/bindings/watchdog/watchdog.txt      |  20 ++++
>   drivers/watchdog/at91sam9_wdt.c                    |  58 +++--------
>   drivers/watchdog/watchdog_core.c                   | 108 ++++++++++++++++++++-
>   drivers/watchdog/watchdog_dev.c                    |  12 +++
>   include/linux/watchdog.h                           |  24 +++++
>   5 files changed, 173 insertions(+), 49 deletions(-)
>   create mode 100644 Documentation/devicetree/bindings/watchdog/watchdog.txt
>