From: Julius Werner <[email protected]>
This patch adds a watchdog driver that can be used on ARM systems
with the appropriate watchdog implemented in Secure Monitor firmware.
The driver communicates with firmware via a Secure Monitor Call.
This may be useful for platforms using TrustZone that want
the Secure Monitor firmware to have the final control over the watchdog.
This is implemented on mt8173 chromebook devices oak, elm and hana in
arm trusted firmware file plat/mediatek/mt8173/drivers/wdt/wdt.c.
Signed-off-by: Julius Werner <[email protected]>
Signed-off-by: Evan Benn <[email protected]>
---
Changes in v5:
- Make timeleft return 0 on error
- Use type punning on smc_func_id to save an alloc
- Change compatible to arm,smc-wdt
Changes in v4:
- Get smc-id from of property
- Return a1 instead of a0 in timeleft
Changes in v3:
- Add optional get_timeleft op
- change name to arm_smc_wdt
Changes in v2:
- use watchdog_stop_on_reboot
- use watchdog_stop_on_unregister
- use devm_watchdog_register_device
- remove smcwd_shutdown, smcwd_remove
- change error codes
MAINTAINERS | 1 +
arch/arm64/configs/defconfig | 1 +
drivers/watchdog/Kconfig | 13 +++
drivers/watchdog/Makefile | 1 +
drivers/watchdog/arm_smc_wdt.c | 189 +++++++++++++++++++++++++++++++++
5 files changed, 205 insertions(+)
create mode 100644 drivers/watchdog/arm_smc_wdt.c
diff --git a/MAINTAINERS b/MAINTAINERS
index 0f2b39767bfa9..2b782bbff200a 100644
--- a/MAINTAINERS
+++ b/MAINTAINERS
@@ -1462,6 +1462,7 @@ M: Julius Werner <[email protected]>
R: Evan Benn <[email protected]>
S: Maintained
F: devicetree/bindings/watchdog/arm-smc-wdt.yaml
+F: drivers/watchdog/arm_smc_wdt.c
ARM SMMU DRIVERS
M: Will Deacon <[email protected]>
diff --git a/arch/arm64/configs/defconfig b/arch/arm64/configs/defconfig
index 24e534d850454..0619df80f7575 100644
--- a/arch/arm64/configs/defconfig
+++ b/arch/arm64/configs/defconfig
@@ -513,6 +513,7 @@ CONFIG_UNIPHIER_THERMAL=y
CONFIG_WATCHDOG=y
CONFIG_ARM_SP805_WATCHDOG=y
CONFIG_ARM_SBSA_WATCHDOG=y
+CONFIG_ARM_SMC_WATCHDOG=y
CONFIG_S3C2410_WATCHDOG=y
CONFIG_DW_WATCHDOG=y
CONFIG_SUNXI_WATCHDOG=m
diff --git a/drivers/watchdog/Kconfig b/drivers/watchdog/Kconfig
index 0663c604bd642..c440b576d23bf 100644
--- a/drivers/watchdog/Kconfig
+++ b/drivers/watchdog/Kconfig
@@ -867,6 +867,19 @@ config DIGICOLOR_WATCHDOG
To compile this driver as a module, choose M here: the
module will be called digicolor_wdt.
+config ARM_SMC_WATCHDOG
+ tristate "ARM Secure Monitor Call based watchdog support"
+ depends on ARM || ARM64
+ depends on OF
+ depends on HAVE_ARM_SMCCC
+ select WATCHDOG_CORE
+ help
+ Say Y here to include support for a watchdog timer
+ implemented by the EL3 Secure Monitor on ARM platforms.
+ Requires firmware support.
+ To compile this driver as a module, choose M here: the
+ module will be called arm_smc_wdt.
+
config LPC18XX_WATCHDOG
tristate "LPC18xx/43xx Watchdog"
depends on ARCH_LPC18XX || COMPILE_TEST
diff --git a/drivers/watchdog/Makefile b/drivers/watchdog/Makefile
index 6de2e4ceef190..97bed1d3d97cb 100644
--- a/drivers/watchdog/Makefile
+++ b/drivers/watchdog/Makefile
@@ -94,6 +94,7 @@ obj-$(CONFIG_UNIPHIER_WATCHDOG) += uniphier_wdt.o
obj-$(CONFIG_RTD119X_WATCHDOG) += rtd119x_wdt.o
obj-$(CONFIG_SPRD_WATCHDOG) += sprd_wdt.o
obj-$(CONFIG_PM8916_WATCHDOG) += pm8916_wdt.o
+obj-$(CONFIG_ARM_SMC_WATCHDOG) += arm_smc_wdt.o
# X86 (i386 + ia64 + x86_64) Architecture
obj-$(CONFIG_ACQUIRE_WDT) += acquirewdt.o
diff --git a/drivers/watchdog/arm_smc_wdt.c b/drivers/watchdog/arm_smc_wdt.c
new file mode 100644
index 0000000000000..7e14909bddda2
--- /dev/null
+++ b/drivers/watchdog/arm_smc_wdt.c
@@ -0,0 +1,189 @@
+// SPDX-License-Identifier: GPL-2.0-only
+/*
+ * ARM Secure Monitor Call watchdog driver
+ *
+ * Copyright 2020 Google LLC.
+ * Julius Werner <[email protected]>
+ * Based on mtk_wdt.c
+ */
+
+#include <linux/arm-smccc.h>
+#include <linux/err.h>
+#include <linux/module.h>
+#include <linux/moduleparam.h>
+#include <linux/of.h>
+#include <linux/platform_device.h>
+#include <linux/types.h>
+#include <linux/watchdog.h>
+#include <uapi/linux/psci.h>
+
+#define DRV_NAME "arm_smc_wdt"
+#define DRV_VERSION "1.0"
+
+enum smcwd_call {
+ SMCWD_INIT = 0,
+ SMCWD_SET_TIMEOUT = 1,
+ SMCWD_ENABLE = 2,
+ SMCWD_PET = 3,
+ SMCWD_GET_TIMELEFT = 4,
+};
+
+static bool nowayout = WATCHDOG_NOWAYOUT;
+static unsigned int timeout;
+
+static int smcwd_call(struct watchdog_device *wdd, enum smcwd_call call,
+ unsigned long arg, struct arm_smccc_res *res)
+{
+ struct arm_smccc_res local_res;
+
+ if (!res)
+ res = &local_res;
+
+ arm_smccc_smc((u32)(uintptr_t)watchdog_get_drvdata(wdd), call, arg, 0,
+ 0, 0, 0, 0, res);
+
+ if (res->a0 == PSCI_RET_NOT_SUPPORTED)
+ return -ENODEV;
+ if (res->a0 == PSCI_RET_INVALID_PARAMS)
+ return -EINVAL;
+ if (res->a0 != PSCI_RET_SUCCESS)
+ return -EIO;
+ return 0;
+}
+
+static int smcwd_ping(struct watchdog_device *wdd)
+{
+ return smcwd_call(wdd, SMCWD_PET, 0, NULL);
+}
+
+static unsigned int smcwd_get_timeleft(struct watchdog_device *wdd)
+{
+ struct arm_smccc_res res;
+
+ smcwd_call(wdd, SMCWD_GET_TIMELEFT, 0, &res);
+ if (res.a0)
+ return 0;
+ return res.a1;
+}
+
+static int smcwd_set_timeout(struct watchdog_device *wdd, unsigned int timeout)
+{
+ int res;
+
+ res = smcwd_call(wdd, SMCWD_SET_TIMEOUT, timeout, NULL);
+ if (!res)
+ wdd->timeout = timeout;
+ return res;
+}
+
+static int smcwd_stop(struct watchdog_device *wdd)
+{
+ return smcwd_call(wdd, SMCWD_ENABLE, 0, NULL);
+}
+
+static int smcwd_start(struct watchdog_device *wdd)
+{
+ return smcwd_call(wdd, SMCWD_ENABLE, 1, NULL);
+}
+
+static const struct watchdog_info smcwd_info = {
+ .identity = DRV_NAME,
+ .options = WDIOF_SETTIMEOUT |
+ WDIOF_KEEPALIVEPING |
+ WDIOF_MAGICCLOSE,
+};
+
+static const struct watchdog_ops smcwd_ops = {
+ .start = smcwd_start,
+ .stop = smcwd_stop,
+ .ping = smcwd_ping,
+ .set_timeout = smcwd_set_timeout,
+};
+
+static const struct watchdog_ops smcwd_timeleft_ops = {
+ .start = smcwd_start,
+ .stop = smcwd_stop,
+ .ping = smcwd_ping,
+ .set_timeout = smcwd_set_timeout,
+ .get_timeleft = smcwd_get_timeleft,
+};
+
+static int smcwd_probe(struct platform_device *pdev)
+{
+ struct watchdog_device *wdd;
+ int err;
+ struct arm_smccc_res res;
+ u32 smc_func_id;
+
+ wdd = devm_kzalloc(&pdev->dev, sizeof(*wdd), GFP_KERNEL);
+ if (!wdd)
+ return -ENOMEM;
+ platform_set_drvdata(pdev, wdd);
+
+ err = of_property_read_u32(pdev->dev.of_node, "arm,smc-id",
+ &smc_func_id);
+ if (err < 0)
+ return err;
+ watchdog_set_drvdata(wdd, (void *)(uintptr_t)smc_func_id);
+
+ err = smcwd_call(wdd, SMCWD_INIT, 0, &res);
+ if (err < 0)
+ return err;
+
+ wdd->info = &smcwd_info;
+ /* get_timeleft is optional */
+ if (smcwd_call(wdd, SMCWD_GET_TIMELEFT, 0, NULL))
+ wdd->ops = &smcwd_ops;
+ else
+ wdd->ops = &smcwd_timeleft_ops;
+ wdd->timeout = res.a2;
+ wdd->max_timeout = res.a2;
+ wdd->min_timeout = res.a1;
+ wdd->parent = &pdev->dev;
+
+ watchdog_stop_on_reboot(wdd);
+ watchdog_stop_on_unregister(wdd);
+ watchdog_set_nowayout(wdd, nowayout);
+ watchdog_init_timeout(wdd, timeout, &pdev->dev);
+ err = smcwd_set_timeout(wdd, wdd->timeout);
+ if (err)
+ return err;
+
+ err = devm_watchdog_register_device(&pdev->dev, wdd);
+ if (err)
+ return err;
+
+ dev_info(&pdev->dev,
+ "Watchdog registered (timeout=%d sec, nowayout=%d)\n",
+ wdd->timeout, nowayout);
+
+ return 0;
+}
+
+static const struct of_device_id smcwd_dt_ids[] = {
+ { .compatible = "arm,smc-wdt" },
+ {}
+};
+MODULE_DEVICE_TABLE(of, smcwd_dt_ids);
+
+static struct platform_driver smcwd_driver = {
+ .probe = smcwd_probe,
+ .driver = {
+ .name = DRV_NAME,
+ .of_match_table = smcwd_dt_ids,
+ },
+};
+
+module_platform_driver(smcwd_driver);
+
+module_param(timeout, uint, 0);
+MODULE_PARM_DESC(timeout, "Watchdog heartbeat in seconds");
+
+module_param(nowayout, bool, 0);
+MODULE_PARM_DESC(nowayout, "Watchdog cannot be stopped once started (default="
+ __MODULE_STRING(WATCHDOG_NOWAYOUT) ")");
+
+MODULE_LICENSE("GPL");
+MODULE_AUTHOR("Julius Werner <[email protected]>");
+MODULE_DESCRIPTION("ARM Secure Monitor Call Watchdog Driver");
+MODULE_VERSION(DRV_VERSION);
--
2.26.2.303.gf8c07b1a785-goog
On 4/22/20 9:40 PM, Evan Benn wrote:
> From: Julius Werner <[email protected]>
>
> This patch adds a watchdog driver that can be used on ARM systems
> with the appropriate watchdog implemented in Secure Monitor firmware.
> The driver communicates with firmware via a Secure Monitor Call.
> This may be useful for platforms using TrustZone that want
> the Secure Monitor firmware to have the final control over the watchdog.
>
> This is implemented on mt8173 chromebook devices oak, elm and hana in
> arm trusted firmware file plat/mediatek/mt8173/drivers/wdt/wdt.c.
>
> Signed-off-by: Julius Werner <[email protected]>
> Signed-off-by: Evan Benn <[email protected]>
>
Your patch doesn't apply to the mainline kernel, not even with
"patch -p 1". What is your kernel baseline ?
Guenter
> ---
>
> Changes in v5:
> - Make timeleft return 0 on error
> - Use type punning on smc_func_id to save an alloc
> - Change compatible to arm,smc-wdt
>
> Changes in v4:
> - Get smc-id from of property
> - Return a1 instead of a0 in timeleft
>
> Changes in v3:
> - Add optional get_timeleft op
> - change name to arm_smc_wdt
>
> Changes in v2:
> - use watchdog_stop_on_reboot
> - use watchdog_stop_on_unregister
> - use devm_watchdog_register_device
> - remove smcwd_shutdown, smcwd_remove
> - change error codes
>
> MAINTAINERS | 1 +
> arch/arm64/configs/defconfig | 1 +
> drivers/watchdog/Kconfig | 13 +++
> drivers/watchdog/Makefile | 1 +
> drivers/watchdog/arm_smc_wdt.c | 189 +++++++++++++++++++++++++++++++++
> 5 files changed, 205 insertions(+)
> create mode 100644 drivers/watchdog/arm_smc_wdt.c
>
> diff --git a/MAINTAINERS b/MAINTAINERS
> index 0f2b39767bfa9..2b782bbff200a 100644
> --- a/MAINTAINERS
> +++ b/MAINTAINERS
> @@ -1462,6 +1462,7 @@ M: Julius Werner <[email protected]>
> R: Evan Benn <[email protected]>
> S: Maintained
> F: devicetree/bindings/watchdog/arm-smc-wdt.yaml
> +F: drivers/watchdog/arm_smc_wdt.c
>
> ARM SMMU DRIVERS
> M: Will Deacon <[email protected]>
> diff --git a/arch/arm64/configs/defconfig b/arch/arm64/configs/defconfig
> index 24e534d850454..0619df80f7575 100644
> --- a/arch/arm64/configs/defconfig
> +++ b/arch/arm64/configs/defconfig
> @@ -513,6 +513,7 @@ CONFIG_UNIPHIER_THERMAL=y
> CONFIG_WATCHDOG=y
> CONFIG_ARM_SP805_WATCHDOG=y
> CONFIG_ARM_SBSA_WATCHDOG=y
> +CONFIG_ARM_SMC_WATCHDOG=y
> CONFIG_S3C2410_WATCHDOG=y
> CONFIG_DW_WATCHDOG=y
> CONFIG_SUNXI_WATCHDOG=m
> diff --git a/drivers/watchdog/Kconfig b/drivers/watchdog/Kconfig
> index 0663c604bd642..c440b576d23bf 100644
> --- a/drivers/watchdog/Kconfig
> +++ b/drivers/watchdog/Kconfig
> @@ -867,6 +867,19 @@ config DIGICOLOR_WATCHDOG
> To compile this driver as a module, choose M here: the
> module will be called digicolor_wdt.
>
> +config ARM_SMC_WATCHDOG
> + tristate "ARM Secure Monitor Call based watchdog support"
> + depends on ARM || ARM64
> + depends on OF
> + depends on HAVE_ARM_SMCCC
> + select WATCHDOG_CORE
> + help
> + Say Y here to include support for a watchdog timer
> + implemented by the EL3 Secure Monitor on ARM platforms.
> + Requires firmware support.
> + To compile this driver as a module, choose M here: the
> + module will be called arm_smc_wdt.
> +
> config LPC18XX_WATCHDOG
> tristate "LPC18xx/43xx Watchdog"
> depends on ARCH_LPC18XX || COMPILE_TEST
> diff --git a/drivers/watchdog/Makefile b/drivers/watchdog/Makefile
> index 6de2e4ceef190..97bed1d3d97cb 100644
> --- a/drivers/watchdog/Makefile
> +++ b/drivers/watchdog/Makefile
> @@ -94,6 +94,7 @@ obj-$(CONFIG_UNIPHIER_WATCHDOG) += uniphier_wdt.o
> obj-$(CONFIG_RTD119X_WATCHDOG) += rtd119x_wdt.o
> obj-$(CONFIG_SPRD_WATCHDOG) += sprd_wdt.o
> obj-$(CONFIG_PM8916_WATCHDOG) += pm8916_wdt.o
> +obj-$(CONFIG_ARM_SMC_WATCHDOG) += arm_smc_wdt.o
>
> # X86 (i386 + ia64 + x86_64) Architecture
> obj-$(CONFIG_ACQUIRE_WDT) += acquirewdt.o
> diff --git a/drivers/watchdog/arm_smc_wdt.c b/drivers/watchdog/arm_smc_wdt.c
> new file mode 100644
> index 0000000000000..7e14909bddda2
> --- /dev/null
> +++ b/drivers/watchdog/arm_smc_wdt.c
> @@ -0,0 +1,189 @@
> +// SPDX-License-Identifier: GPL-2.0-only
> +/*
> + * ARM Secure Monitor Call watchdog driver
> + *
> + * Copyright 2020 Google LLC.
> + * Julius Werner <[email protected]>
> + * Based on mtk_wdt.c
> + */
> +
> +#include <linux/arm-smccc.h>
> +#include <linux/err.h>
> +#include <linux/module.h>
> +#include <linux/moduleparam.h>
> +#include <linux/of.h>
> +#include <linux/platform_device.h>
> +#include <linux/types.h>
> +#include <linux/watchdog.h>
> +#include <uapi/linux/psci.h>
> +
> +#define DRV_NAME "arm_smc_wdt"
> +#define DRV_VERSION "1.0"
> +
> +enum smcwd_call {
> + SMCWD_INIT = 0,
> + SMCWD_SET_TIMEOUT = 1,
> + SMCWD_ENABLE = 2,
> + SMCWD_PET = 3,
> + SMCWD_GET_TIMELEFT = 4,
> +};
> +
> +static bool nowayout = WATCHDOG_NOWAYOUT;
> +static unsigned int timeout;
> +
> +static int smcwd_call(struct watchdog_device *wdd, enum smcwd_call call,
> + unsigned long arg, struct arm_smccc_res *res)
> +{
> + struct arm_smccc_res local_res;
> +
> + if (!res)
> + res = &local_res;
> +
> + arm_smccc_smc((u32)(uintptr_t)watchdog_get_drvdata(wdd), call, arg, 0,
> + 0, 0, 0, 0, res);
> +
> + if (res->a0 == PSCI_RET_NOT_SUPPORTED)
> + return -ENODEV;
> + if (res->a0 == PSCI_RET_INVALID_PARAMS)
> + return -EINVAL;
> + if (res->a0 != PSCI_RET_SUCCESS)
> + return -EIO;
> + return 0;
> +}
> +
> +static int smcwd_ping(struct watchdog_device *wdd)
> +{
> + return smcwd_call(wdd, SMCWD_PET, 0, NULL);
> +}
> +
> +static unsigned int smcwd_get_timeleft(struct watchdog_device *wdd)
> +{
> + struct arm_smccc_res res;
> +
> + smcwd_call(wdd, SMCWD_GET_TIMELEFT, 0, &res);
> + if (res.a0)
> + return 0;
> + return res.a1;
> +}
> +
> +static int smcwd_set_timeout(struct watchdog_device *wdd, unsigned int timeout)
> +{
> + int res;
> +
> + res = smcwd_call(wdd, SMCWD_SET_TIMEOUT, timeout, NULL);
> + if (!res)
> + wdd->timeout = timeout;
> + return res;
> +}
> +
> +static int smcwd_stop(struct watchdog_device *wdd)
> +{
> + return smcwd_call(wdd, SMCWD_ENABLE, 0, NULL);
> +}
> +
> +static int smcwd_start(struct watchdog_device *wdd)
> +{
> + return smcwd_call(wdd, SMCWD_ENABLE, 1, NULL);
> +}
> +
> +static const struct watchdog_info smcwd_info = {
> + .identity = DRV_NAME,
> + .options = WDIOF_SETTIMEOUT |
> + WDIOF_KEEPALIVEPING |
> + WDIOF_MAGICCLOSE,
> +};
> +
> +static const struct watchdog_ops smcwd_ops = {
> + .start = smcwd_start,
> + .stop = smcwd_stop,
> + .ping = smcwd_ping,
> + .set_timeout = smcwd_set_timeout,
> +};
> +
> +static const struct watchdog_ops smcwd_timeleft_ops = {
> + .start = smcwd_start,
> + .stop = smcwd_stop,
> + .ping = smcwd_ping,
> + .set_timeout = smcwd_set_timeout,
> + .get_timeleft = smcwd_get_timeleft,
> +};
> +
> +static int smcwd_probe(struct platform_device *pdev)
> +{
> + struct watchdog_device *wdd;
> + int err;
> + struct arm_smccc_res res;
> + u32 smc_func_id;
> +
> + wdd = devm_kzalloc(&pdev->dev, sizeof(*wdd), GFP_KERNEL);
> + if (!wdd)
> + return -ENOMEM;
> + platform_set_drvdata(pdev, wdd);
> +
> + err = of_property_read_u32(pdev->dev.of_node, "arm,smc-id",
> + &smc_func_id);
> + if (err < 0)
> + return err;
> + watchdog_set_drvdata(wdd, (void *)(uintptr_t)smc_func_id);
> +
> + err = smcwd_call(wdd, SMCWD_INIT, 0, &res);
> + if (err < 0)
> + return err;
> +
> + wdd->info = &smcwd_info;
> + /* get_timeleft is optional */
> + if (smcwd_call(wdd, SMCWD_GET_TIMELEFT, 0, NULL))
> + wdd->ops = &smcwd_ops;
> + else
> + wdd->ops = &smcwd_timeleft_ops;
> + wdd->timeout = res.a2;
> + wdd->max_timeout = res.a2;
> + wdd->min_timeout = res.a1;
> + wdd->parent = &pdev->dev;
> +
> + watchdog_stop_on_reboot(wdd);
> + watchdog_stop_on_unregister(wdd);
> + watchdog_set_nowayout(wdd, nowayout);
> + watchdog_init_timeout(wdd, timeout, &pdev->dev);
> + err = smcwd_set_timeout(wdd, wdd->timeout);
> + if (err)
> + return err;
> +
> + err = devm_watchdog_register_device(&pdev->dev, wdd);
> + if (err)
> + return err;
> +
> + dev_info(&pdev->dev,
> + "Watchdog registered (timeout=%d sec, nowayout=%d)\n",
> + wdd->timeout, nowayout);
> +
> + return 0;
> +}
> +
> +static const struct of_device_id smcwd_dt_ids[] = {
> + { .compatible = "arm,smc-wdt" },
> + {}
> +};
> +MODULE_DEVICE_TABLE(of, smcwd_dt_ids);
> +
> +static struct platform_driver smcwd_driver = {
> + .probe = smcwd_probe,
> + .driver = {
> + .name = DRV_NAME,
> + .of_match_table = smcwd_dt_ids,
> + },
> +};
> +
> +module_platform_driver(smcwd_driver);
> +
> +module_param(timeout, uint, 0);
> +MODULE_PARM_DESC(timeout, "Watchdog heartbeat in seconds");
> +
> +module_param(nowayout, bool, 0);
> +MODULE_PARM_DESC(nowayout, "Watchdog cannot be stopped once started (default="
> + __MODULE_STRING(WATCHDOG_NOWAYOUT) ")");
> +
> +MODULE_LICENSE("GPL");
> +MODULE_AUTHOR("Julius Werner <[email protected]>");
> +MODULE_DESCRIPTION("ARM Secure Monitor Call Watchdog Driver");
> +MODULE_VERSION(DRV_VERSION);
>
> + err = of_property_read_u32(pdev->dev.of_node, "arm,smc-id",
> + &smc_func_id);
> + if (err < 0)
> + return err;
> + watchdog_set_drvdata(wdd, (void *)(uintptr_t)smc_func_id);
Your device tree binding says there's a default and this is optional.
I think you need to change the code so that that's actually true.
On Fri, Apr 24, 2020 at 8:17 AM Julius Werner <[email protected]> wrote:
>
> > + err = of_property_read_u32(pdev->dev.of_node, "arm,smc-id",
> > + &smc_func_id);
> > + if (err < 0)
> > + return err;
> > + watchdog_set_drvdata(wdd, (void *)(uintptr_t)smc_func_id);
>
> Your device tree binding says there's a default and this is optional.
> I think you need to change the code so that that's actually true.
I think I have misunderstood the device tree json-schema spec.
My intention was for the device tree to fill in a default value in the dtb for
arm,smc-id if it was omitted in the dts. But now I see that does not seem to
happen, I cannot really find any documentation of `default`, so I will just put
a documentation string in instead and force the default in the driver.
> I think I have misunderstood the device tree json-schema spec.
> My intention was for the device tree to fill in a default value in the dtb for
> arm,smc-id if it was omitted in the dts. But now I see that does not seem to
> happen, I cannot really find any documentation of `default`, so I will just put
> a documentation string in instead and force the default in the driver.
The bindings in Documentation/device-tree are just for informational
purposes, they do not have any direct effect on anything. If you want
there to be a default, you'll have to write the kernel code to fill it
in.