watchdog: add gpio watchdog driver

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 corresponding linux driver apparently has support for some
watchdog circuits which can be disabled by tri-stating the gpio, but I
have never actually encountered such a chip in the wild; the whole
point of adding an external watchdog is usually that it is not in any
way under software control. For forward-compatibility, and to make DT
describe the hardware, the current driver only supports devices that
have the always-running property. I went a little back and forth on
whether I should fail ->probe or only ->start, and ended up deciding
->start was the right place.

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.

Reviewed-by: Simon Glass <sjg@chromium.org>
Reviewed-by: Stefan Roese <sr@denx.de>
Signed-off-by: Rasmus Villemoes <rasmus.villemoes@prevas.dk>

1 files changed, 68 insertions, 0 deletions

diff --git a/drivers/watchdog/gpio_wdt.c b/drivers/watchdog/gpio_wdt.c
new file mode 100644
index 0000000000..982a66b3f9
--- /dev/null
+++ b/drivers/watchdog/gpio_wdt.c
@@ -0,0 +1,68 @@
+// SPDX-License-Identifier: GPL-2.0+
+
+#include <dm.h>
+#include <dm/device_compat.h>
+#include <wdt.h>
+#include <asm/gpio.h>
+
+struct gpio_wdt_priv {
+	struct gpio_desc gpio;
+	bool always_running;
+	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 gpio_wdt_start(struct udevice *dev, u64 timeout, ulong flags)
+{
+	struct gpio_wdt_priv *priv = dev_get_priv(dev);
+
+	if (priv->always_running)
+		return 0;
+
+	return -ENOSYS;
+}
+
+static int dm_probe(struct udevice *dev)
+{
+	struct gpio_wdt_priv *priv = dev_get_priv(dev);
+	int ret;
+
+	priv->always_running = dev_read_bool(dev, "always-running");
+	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;
+	}
+
+	if (priv->always_running)
+		ret = gpio_wdt_reset(dev);
+
+	return ret;
+}
+
+static const struct wdt_ops gpio_wdt_ops = {
+	.start = gpio_wdt_start,
+	.reset = gpio_wdt_reset,
+};
+
+static const struct udevice_id gpio_wdt_ids[] = {
+	{ .compatible = "linux,wdt-gpio" },
+	{}
+};
+
+U_BOOT_DRIVER(wdt_gpio) = {
+	.name = "wdt_gpio",
+	.id = UCLASS_WDT,
+	.of_match = gpio_wdt_ids,
+	.ops = &gpio_wdt_ops,
+	.probe	= dm_probe,
+	.priv_auto = sizeof(struct gpio_wdt_priv),
+};