[PATCH 1/2] watchdog: add gpio watchdog driver

[Rasmus Villemoes]

On 11/05/2021 07.55, Stefan Roese wrote:
> On 10.05.21 17:47, Rasmus Villemoes wrote:
>> A rather common kind of external watchdog circuit is one that is kept
>> alive by toggling a gpio. Add a driver for handling such a watchdog.
>>
>> The compatible string is probably a little odd as it has nothing to do
>> with linux per se - however, I chose that to make .dts snippets
>> reusable between device trees used with U-Boot and linux, and this is
>> the (only) compatible string that linux' corresponding driver and DT
>> binding accepts. I have asked whether one should/could add "wdt-gpio"
>> to that binding, but the answer was no:
>>
>>   
>> https://lore.kernel.org/lkml/CAL_JsqKEGaFpiFV_oAtE+S_bnHkg4qry+bhx2EDs=NSbVf_giA@mail.gmail.com/
>>
>>
>> If someone feels strongly about this, I can certainly remove the
>> "linux," part from the string - it probably wouldn't the only place where
>> one can't reuse a DT snippet as-is between linux and U-Boot.
>>
>> Signed-off-by: Rasmus Villemoes <rasmus.villemoes at prevas.dk>
>> ---
>>   .../watchdog/gpio-wdt.txt                     | 15 ++++++
>>   drivers/watchdog/Kconfig                      |  7 +++
>>   drivers/watchdog/Makefile                     |  1 +
>>   drivers/watchdog/gpio_wdt.c                   | 51 +++++++++++++++++++
>>   4 files changed, 74 insertions(+)
>>   create mode 100644 doc/device-tree-bindings/watchdog/gpio-wdt.txt
>>   create mode 100644 drivers/watchdog/gpio_wdt.c
>>
>> diff --git a/doc/device-tree-bindings/watchdog/gpio-wdt.txt
>> b/doc/device-tree-bindings/watchdog/gpio-wdt.txt
>> new file mode 100644
>> index 0000000000..2283b7ba6e
>> --- /dev/null
>> +++ b/doc/device-tree-bindings/watchdog/gpio-wdt.txt
>> @@ -0,0 +1,15 @@
>> +GPIO watchdog timer
>> +
>> +Describes a simple watchdog timer which is reset by toggling a gpio.
>> +
>> +Required properties:
>> +
>> +- compatible: must be "linux,wdt-gpio"
>> +- gpios: gpio to toggle when wdt driver reset method is called
>> +
>> +Example:
>> +
>> +    gpio-wdt {
>> +        gpios = <&gpio0 1 0>;
>> +        compatible = "linux,wdt-gpio";
>> +    };
>> diff --git a/drivers/watchdog/Kconfig b/drivers/watchdog/Kconfig
>> index f0ff2612a6..2cf378db29 100644
>> --- a/drivers/watchdog/Kconfig
>> +++ b/drivers/watchdog/Kconfig
>> @@ -147,6 +147,13 @@ config WDT_CORTINA
>>         This driver support all CPU ISAs supported by Cortina
>>         Access CAxxxx SoCs.
>>   +config WDT_GPIO
>> +    bool "External gpio watchdog support"
>> +    depends on WDT
>> +    depends on DM_GPIO
>> +    help
>> +       Support for external watchdog fed by toggling a gpio.
>> +
>>   config WDT_MPC8xx
>>       bool "MPC8xx watchdog timer support"
>>       depends on WDT && MPC8xx
>> diff --git a/drivers/watchdog/Makefile b/drivers/watchdog/Makefile
>> index 5c7ef593fe..f14415bb8e 100644
>> --- a/drivers/watchdog/Makefile
>> +++ b/drivers/watchdog/Makefile
>> @@ -25,6 +25,7 @@ obj-$(CONFIG_WDT_BOOKE) += booke_wdt.o
>>   obj-$(CONFIG_WDT_CORTINA) += cortina_wdt.o
>>   obj-$(CONFIG_WDT_ORION) += orion_wdt.o
>>   obj-$(CONFIG_WDT_CDNS) += cdns_wdt.o
>> +obj-$(CONFIG_WDT_GPIO) += gpio_wdt.o
>>   obj-$(CONFIG_WDT_MPC8xx) += mpc8xx_wdt.o
>>   obj-$(CONFIG_WDT_MT7620) += mt7620_wdt.o
>>   obj-$(CONFIG_WDT_MT7621) += mt7621_wdt.o
>> diff --git a/drivers/watchdog/gpio_wdt.c b/drivers/watchdog/gpio_wdt.c
>> new file mode 100644
>> index 0000000000..9dba9c254e
>> --- /dev/null
>> +++ b/drivers/watchdog/gpio_wdt.c
>> @@ -0,0 +1,51 @@
>> +// SPDX-License-Identifier: GPL-2.0+
>> +
>> +#include <common.h>
>> +#include <dm.h>
>> +#include <dm/device_compat.h>
>> +#include <wdt.h>
>> +#include <asm/gpio.h>
>> +
>> +struct gpio_wdt_priv {
>> +    struct gpio_desc gpio;
>> +    int state;
>> +};
>> +
>> +static int gpio_wdt_reset(struct udevice *dev)
>> +{
>> +    struct gpio_wdt_priv *priv = dev_get_priv(dev);
>> +
>> +    priv->state = !priv->state;
>> +    return dm_gpio_set_value(&priv->gpio, priv->state);
>> +}
>> +
>> +static int dm_probe(struct udevice *dev)
>> +{
>> +    struct gpio_wdt_priv *priv = dev_get_priv(dev);
>> +    int ret;
>> +
>> +    ret = gpio_request_by_name(dev, "gpios", 0, &priv->gpio,
>> GPIOD_IS_OUT);
>> +    if (ret < 0) {
>> +        dev_err(dev, "Request for wdt gpio failed: %d\n", ret);
>> +        return ret;
>> +    }
>> +    return gpio_wdt_reset(dev);
>> +}
>> +
>> +static const struct wdt_ops gpio_wdt_ops = {
>> +    .reset = gpio_wdt_reset,
>> +};
> 
> I'm noticing that you don't have a "start" function. How is this GPIO
> watchdog enabled? Is it "always on"? Or does it start with the first
> triggering?
> 

I have yet to encounter one of these that isn't always-running - it's
more or less the only reason one would add an external watchdog circuit
instead of relying on whatever the SOC has.

> Perhaps it makes sense to add a note about this into the driver to make
> this clear.

Will do. But I now see a much bigger problem due to some refactoring in
wdt-uclass that basically requires a ->start method or GD_FLG_WDT_READY
won't be set. Sigh.

I was actually hoping to get this small driver in first, then start
doing the promised "handle _all_ registered watchdogs in wdt_uclass'
watchdog_reset", because testing that is much easier once the sandbox
test has two distinct "devices". I guess I'll just provide a dummy start
method to work around that. Then when I equip wdt_uclass with some
uclass-priv data that will contain not only the reset_period and
last_reset metadata, but also keep track of whether the device is
running (and then wdt_start() can be a no-op in that case, while the
gpio driver can set that flag in its probe function).

Rasmus