VIA/WonderMedia SoCs can use their system timer's first channel as a
watchdog device which will reset the system if the clocksource counter
matches the value given in its match register 0 and if the watchdog
function is enabled.

Since the watchdog function is tightly coupled to the timer itself, it
is implemented as an auxiliary device of the timer device

Signed-off-by: Alexey Charkov <alchark@gmail.com>
---
 MAINTAINERS                   |  1 +
 drivers/watchdog/Kconfig      | 15 ++++++++
 drivers/watchdog/Makefile     |  1 +
 drivers/watchdog/vt8500-wdt.c | 80 +++++++++++++++++++++++++++++++++++++++++++
 4 files changed, 97 insertions(+)

diff --git a/MAINTAINERS b/MAINTAINERS
index 5362095240627f613638197fda275db6edc16cf7..97d1842625dbdf7fdca3556260662dab469ed091 100644
--- a/MAINTAINERS
+++ b/MAINTAINERS
@@ -3447,6 +3447,7 @@ F:	drivers/tty/serial/vt8500_serial.c
 F:	drivers/video/fbdev/vt8500lcdfb.*
 F:	drivers/video/fbdev/wm8505fb*
 F:	drivers/video/fbdev/wmt_ge_rops.*
+F:	drivers/watchdog/vt8500-wdt.c
 F:	include/linux/vt8500-timer.h
 
 ARM/ZYNQ ARCHITECTURE
diff --git a/drivers/watchdog/Kconfig b/drivers/watchdog/Kconfig
index 0d8d37f712e8cfb4bf8156853baa13c23a57d6d9..8049ae630301a98123f97f6e3ee868bd3bf1534a 100644
--- a/drivers/watchdog/Kconfig
+++ b/drivers/watchdog/Kconfig
@@ -2003,6 +2003,21 @@ config PIC32_DMT
 	  To compile this driver as a loadable module, choose M here.
 	  The module will be called pic32-dmt.
 
+config VT8500_WATCHDOG
+	tristate "VIA/WonderMedia VT8500 watchdog support"
+	depends on ARCH_VT8500 || COMPILE_TEST
+	select WATCHDOG_CORE
+	select AUXILIARY_BUS
+	help
+	  VIA/WonderMedia SoCs can use their system timer as a hardware
+	  watchdog, as long as the first timer channel is free from other
+	  uses and respective function is enabled in its registers. To
+	  make use of it, say Y here and ensure that the device tree
+	  lists at least two interrupts for the VT8500 timer device.
+
+	  To compile this driver as a module, choose M here.
+	  The module will be called vt8500-wdt.
+
 # PARISC Architecture
 
 # POWERPC Architecture
diff --git a/drivers/watchdog/Makefile b/drivers/watchdog/Makefile
index c9482904bf870a085c7fce2a439ac5089b6e6fee..3072786bf226c357102be3734fe6e701f753d45b 100644
--- a/drivers/watchdog/Makefile
+++ b/drivers/watchdog/Makefile
@@ -101,6 +101,7 @@ obj-$(CONFIG_MSC313E_WATCHDOG) += msc313e_wdt.o
 obj-$(CONFIG_APPLE_WATCHDOG) += apple_wdt.o
 obj-$(CONFIG_SUNPLUS_WATCHDOG) += sunplus_wdt.o
 obj-$(CONFIG_MARVELL_GTI_WDT) += marvell_gti_wdt.o
+obj-$(CONFIG_VT8500_WATCHDOG) += vt8500-wdt.o
 
 # X86 (i386 + ia64 + x86_64) Architecture
 obj-$(CONFIG_ACQUIRE_WDT) += acquirewdt.o
diff --git a/drivers/watchdog/vt8500-wdt.c b/drivers/watchdog/vt8500-wdt.c
new file mode 100644
index 0000000000000000000000000000000000000000..54fe5ad7695de36f6b3c1d28e955f00bef00e9db
--- /dev/null
+++ b/drivers/watchdog/vt8500-wdt.c
@@ -0,0 +1,80 @@
+// SPDX-License-Identifier: GPL-2.0
+/* Copyright (C) 2025 Alexey Charkov <alchark@gmail.com */
+
+#include <linux/auxiliary_bus.h>
+#include <linux/container_of.h>
+#include <linux/io.h>
+#include <linux/limits.h>
+#include <linux/module.h>
+#include <linux/types.h>
+#include <linux/watchdog.h>
+#include <linux/vt8500-timer.h>
+
+static int vt8500_watchdog_start(struct watchdog_device *wdd)
+{
+	struct vt8500_wdt_info *info = watchdog_get_drvdata(wdd);
+	u64 deadline = info->timer_next(wdd->timeout * VT8500_TIMER_HZ);
+
+	writel((u32)deadline, info->wdt_match);
+	writel(TIMER_WD_EN, info->wdt_en);
+	return 0;
+}
+
+static int vt8500_watchdog_stop(struct watchdog_device *wdd)
+{
+	struct vt8500_wdt_info *info = watchdog_get_drvdata(wdd);
+
+	writel(0, info->wdt_en);
+	return 0;
+}
+
+static const struct watchdog_ops vt8500_watchdog_ops = {
+	.start			= vt8500_watchdog_start,
+	.stop			= vt8500_watchdog_stop,
+};
+
+static const struct watchdog_info vt8500_watchdog_info = {
+	.identity		= "VIA VT8500 watchdog",
+	.options		= WDIOF_MAGICCLOSE |
+				  WDIOF_KEEPALIVEPING |
+				  WDIOF_SETTIMEOUT,
+};
+
+static int vt8500_wdt_probe(struct auxiliary_device *auxdev,
+			    const struct auxiliary_device_id *id)
+{
+	struct vt8500_wdt_info *info;
+	struct watchdog_device *wdd;
+
+	wdd = devm_kzalloc(&auxdev->dev, sizeof(*wdd), GFP_KERNEL);
+	if (!wdd)
+		return -ENOMEM;
+
+	wdd->info = &vt8500_watchdog_info;
+	wdd->ops = &vt8500_watchdog_ops;
+	wdd->max_hw_heartbeat_ms = U32_MAX / (VT8500_TIMER_HZ / 1000);
+	wdd->parent = &auxdev->dev;
+
+	info = container_of(auxdev, struct vt8500_wdt_info, auxdev);
+	watchdog_set_drvdata(wdd, info);
+
+	return devm_watchdog_register_device(&auxdev->dev, wdd);
+}
+
+static const struct auxiliary_device_id vt8500_wdt_ids[] = {
+	{ .name = "timer_vt8500.vt8500-wdt" },
+	{},
+};
+
+MODULE_DEVICE_TABLE(auxiliary, my_auxiliary_id_table);
+
+static struct auxiliary_driver vt8500_wdt_driver = {
+	.name =	"vt8500-wdt",
+	.probe = vt8500_wdt_probe,
+	.id_table = vt8500_wdt_ids,
+};
+module_auxiliary_driver(vt8500_wdt_driver);
+
+MODULE_AUTHOR("Alexey Charkov <alchark@gmail.com>");
+MODULE_DESCRIPTION("Driver for the VIA VT8500 watchdog timer");
+MODULE_LICENSE("GPL");

-- 
2.49.0

On 5/20/25 13:01, Alexey Charkov wrote:
> VIA/WonderMedia SoCs can use their system timer's first channel as a
> watchdog device which will reset the system if the clocksource counter
> matches the value given in its match register 0 and if the watchdog
> function is enabled.
> 
> Since the watchdog function is tightly coupled to the timer itself, it
> is implemented as an auxiliary device of the timer device
> 
> Signed-off-by: Alexey Charkov <alchark@gmail.com>
> ---
>   MAINTAINERS                   |  1 +
>   drivers/watchdog/Kconfig      | 15 ++++++++
>   drivers/watchdog/Makefile     |  1 +
>   drivers/watchdog/vt8500-wdt.c | 80 +++++++++++++++++++++++++++++++++++++++++++
>   4 files changed, 97 insertions(+)
> 
> diff --git a/MAINTAINERS b/MAINTAINERS
> index 5362095240627f613638197fda275db6edc16cf7..97d1842625dbdf7fdca3556260662dab469ed091 100644
> --- a/MAINTAINERS
> +++ b/MAINTAINERS
> @@ -3447,6 +3447,7 @@ F:	drivers/tty/serial/vt8500_serial.c
>   F:	drivers/video/fbdev/vt8500lcdfb.*
>   F:	drivers/video/fbdev/wm8505fb*
>   F:	drivers/video/fbdev/wmt_ge_rops.*
> +F:	drivers/watchdog/vt8500-wdt.c
>   F:	include/linux/vt8500-timer.h
>   
>   ARM/ZYNQ ARCHITECTURE
> diff --git a/drivers/watchdog/Kconfig b/drivers/watchdog/Kconfig
> index 0d8d37f712e8cfb4bf8156853baa13c23a57d6d9..8049ae630301a98123f97f6e3ee868bd3bf1534a 100644
> --- a/drivers/watchdog/Kconfig
> +++ b/drivers/watchdog/Kconfig
> @@ -2003,6 +2003,21 @@ config PIC32_DMT
>   	  To compile this driver as a loadable module, choose M here.
>   	  The module will be called pic32-dmt.
>   
> +config VT8500_WATCHDOG
> +	tristate "VIA/WonderMedia VT8500 watchdog support"
> +	depends on ARCH_VT8500 || COMPILE_TEST
> +	select WATCHDOG_CORE
> +	select AUXILIARY_BUS
> +	help
> +	  VIA/WonderMedia SoCs can use their system timer as a hardware
> +	  watchdog, as long as the first timer channel is free from other
> +	  uses and respective function is enabled in its registers. To
> +	  make use of it, say Y here and ensure that the device tree
> +	  lists at least two interrupts for the VT8500 timer device.
> +
> +	  To compile this driver as a module, choose M here.
> +	  The module will be called vt8500-wdt.
> +
>   # PARISC Architecture
>   
>   # POWERPC Architecture
> diff --git a/drivers/watchdog/Makefile b/drivers/watchdog/Makefile
> index c9482904bf870a085c7fce2a439ac5089b6e6fee..3072786bf226c357102be3734fe6e701f753d45b 100644
> --- a/drivers/watchdog/Makefile
> +++ b/drivers/watchdog/Makefile
> @@ -101,6 +101,7 @@ obj-$(CONFIG_MSC313E_WATCHDOG) += msc313e_wdt.o
>   obj-$(CONFIG_APPLE_WATCHDOG) += apple_wdt.o
>   obj-$(CONFIG_SUNPLUS_WATCHDOG) += sunplus_wdt.o
>   obj-$(CONFIG_MARVELL_GTI_WDT) += marvell_gti_wdt.o
> +obj-$(CONFIG_VT8500_WATCHDOG) += vt8500-wdt.o
>   
>   # X86 (i386 + ia64 + x86_64) Architecture
>   obj-$(CONFIG_ACQUIRE_WDT) += acquirewdt.o
> diff --git a/drivers/watchdog/vt8500-wdt.c b/drivers/watchdog/vt8500-wdt.c
> new file mode 100644
> index 0000000000000000000000000000000000000000..54fe5ad7695de36f6b3c1d28e955f00bef00e9db
> --- /dev/null
> +++ b/drivers/watchdog/vt8500-wdt.c
> @@ -0,0 +1,80 @@
> +// SPDX-License-Identifier: GPL-2.0
> +/* Copyright (C) 2025 Alexey Charkov <alchark@gmail.com */
> +
> +#include <linux/auxiliary_bus.h>
> +#include <linux/container_of.h>
> +#include <linux/io.h>
> +#include <linux/limits.h>
> +#include <linux/module.h>
> +#include <linux/types.h>
> +#include <linux/watchdog.h>
> +#include <linux/vt8500-timer.h>
> +
> +static int vt8500_watchdog_start(struct watchdog_device *wdd)
> +{
> +	struct vt8500_wdt_info *info = watchdog_get_drvdata(wdd);
> +	u64 deadline = info->timer_next(wdd->timeout * VT8500_TIMER_HZ);
> +

wdd->timeout is the soft timeout, which may be larger
than the maximum hardware timeout. That means you'll need
to use something like "min(wdd->timeout, U32_MAX / VT8500_TIMER_HZ)
* VT8500_TIMER_HZ" as parameter to the timer_next call.

> +	writel((u32)deadline, info->wdt_match);

Can deadline overflow ?

> +	writel(TIMER_WD_EN, info->wdt_en);
> +	return 0;
> +}
> +
> +static int vt8500_watchdog_stop(struct watchdog_device *wdd)
> +{
> +	struct vt8500_wdt_info *info = watchdog_get_drvdata(wdd);
> +
> +	writel(0, info->wdt_en);
> +	return 0;
> +}
> +
> +static const struct watchdog_ops vt8500_watchdog_ops = {
> +	.start			= vt8500_watchdog_start,
> +	.stop			= vt8500_watchdog_stop,
> +};
> +
> +static const struct watchdog_info vt8500_watchdog_info = {
> +	.identity		= "VIA VT8500 watchdog",
> +	.options		= WDIOF_MAGICCLOSE |
> +				  WDIOF_KEEPALIVEPING |
> +				  WDIOF_SETTIMEOUT,
> +};
> +
> +static int vt8500_wdt_probe(struct auxiliary_device *auxdev,
> +			    const struct auxiliary_device_id *id)
> +{
> +	struct vt8500_wdt_info *info;
> +	struct watchdog_device *wdd;
> +
> +	wdd = devm_kzalloc(&auxdev->dev, sizeof(*wdd), GFP_KERNEL);
> +	if (!wdd)
> +		return -ENOMEM;
> +
> +	wdd->info = &vt8500_watchdog_info;
> +	wdd->ops = &vt8500_watchdog_ops;
> +	wdd->max_hw_heartbeat_ms = U32_MAX / (VT8500_TIMER_HZ / 1000);
> +	wdd->parent = &auxdev->dev;
> +
> +	info = container_of(auxdev, struct vt8500_wdt_info, auxdev);
> +	watchdog_set_drvdata(wdd, info);
> +
> +	return devm_watchdog_register_device(&auxdev->dev, wdd);
> +}
> +
> +static const struct auxiliary_device_id vt8500_wdt_ids[] = {
> +	{ .name = "timer_vt8500.vt8500-wdt" },
> +	{},
> +};
> +
> +MODULE_DEVICE_TABLE(auxiliary, my_auxiliary_id_table);
> +
> +static struct auxiliary_driver vt8500_wdt_driver = {
> +	.name =	"vt8500-wdt",
> +	.probe = vt8500_wdt_probe,
> +	.id_table = vt8500_wdt_ids,
> +};
> +module_auxiliary_driver(vt8500_wdt_driver);
> +
> +MODULE_AUTHOR("Alexey Charkov <alchark@gmail.com>");
> +MODULE_DESCRIPTION("Driver for the VIA VT8500 watchdog timer");
> +MODULE_LICENSE("GPL");
>

On Wed, May 21, 2025 at 3:15 AM Guenter Roeck <linux@roeck-us.net> wrote:
>
> On 5/20/25 13:01, Alexey Charkov wrote:
> > VIA/WonderMedia SoCs can use their system timer's first channel as a
> > watchdog device which will reset the system if the clocksource counter
> > matches the value given in its match register 0 and if the watchdog
> > function is enabled.
> >
> > Since the watchdog function is tightly coupled to the timer itself, it
> > is implemented as an auxiliary device of the timer device
> >
> > Signed-off-by: Alexey Charkov <alchark@gmail.com>
> > ---
> >   MAINTAINERS                   |  1 +
> >   drivers/watchdog/Kconfig      | 15 ++++++++
> >   drivers/watchdog/Makefile     |  1 +
> >   drivers/watchdog/vt8500-wdt.c | 80 +++++++++++++++++++++++++++++++++++++++++++
> >   4 files changed, 97 insertions(+)
> >
> > diff --git a/MAINTAINERS b/MAINTAINERS
> > index 5362095240627f613638197fda275db6edc16cf7..97d1842625dbdf7fdca3556260662dab469ed091 100644
> > --- a/MAINTAINERS
> > +++ b/MAINTAINERS
> > @@ -3447,6 +3447,7 @@ F:      drivers/tty/serial/vt8500_serial.c
> >   F:  drivers/video/fbdev/vt8500lcdfb.*
> >   F:  drivers/video/fbdev/wm8505fb*
> >   F:  drivers/video/fbdev/wmt_ge_rops.*
> > +F:   drivers/watchdog/vt8500-wdt.c
> >   F:  include/linux/vt8500-timer.h
> >
> >   ARM/ZYNQ ARCHITECTURE
> > diff --git a/drivers/watchdog/Kconfig b/drivers/watchdog/Kconfig
> > index 0d8d37f712e8cfb4bf8156853baa13c23a57d6d9..8049ae630301a98123f97f6e3ee868bd3bf1534a 100644
> > --- a/drivers/watchdog/Kconfig
> > +++ b/drivers/watchdog/Kconfig
> > @@ -2003,6 +2003,21 @@ config PIC32_DMT
> >         To compile this driver as a loadable module, choose M here.
> >         The module will be called pic32-dmt.
> >
> > +config VT8500_WATCHDOG
> > +     tristate "VIA/WonderMedia VT8500 watchdog support"
> > +     depends on ARCH_VT8500 || COMPILE_TEST
> > +     select WATCHDOG_CORE
> > +     select AUXILIARY_BUS
> > +     help
> > +       VIA/WonderMedia SoCs can use their system timer as a hardware
> > +       watchdog, as long as the first timer channel is free from other
> > +       uses and respective function is enabled in its registers. To
> > +       make use of it, say Y here and ensure that the device tree
> > +       lists at least two interrupts for the VT8500 timer device.
> > +
> > +       To compile this driver as a module, choose M here.
> > +       The module will be called vt8500-wdt.
> > +
> >   # PARISC Architecture
> >
> >   # POWERPC Architecture
> > diff --git a/drivers/watchdog/Makefile b/drivers/watchdog/Makefile
> > index c9482904bf870a085c7fce2a439ac5089b6e6fee..3072786bf226c357102be3734fe6e701f753d45b 100644
> > --- a/drivers/watchdog/Makefile
> > +++ b/drivers/watchdog/Makefile
> > @@ -101,6 +101,7 @@ obj-$(CONFIG_MSC313E_WATCHDOG) += msc313e_wdt.o
> >   obj-$(CONFIG_APPLE_WATCHDOG) += apple_wdt.o
> >   obj-$(CONFIG_SUNPLUS_WATCHDOG) += sunplus_wdt.o
> >   obj-$(CONFIG_MARVELL_GTI_WDT) += marvell_gti_wdt.o
> > +obj-$(CONFIG_VT8500_WATCHDOG) += vt8500-wdt.o
> >
> >   # X86 (i386 + ia64 + x86_64) Architecture
> >   obj-$(CONFIG_ACQUIRE_WDT) += acquirewdt.o
> > diff --git a/drivers/watchdog/vt8500-wdt.c b/drivers/watchdog/vt8500-wdt.c
> > new file mode 100644
> > index 0000000000000000000000000000000000000000..54fe5ad7695de36f6b3c1d28e955f00bef00e9db
> > --- /dev/null
> > +++ b/drivers/watchdog/vt8500-wdt.c
> > @@ -0,0 +1,80 @@
> > +// SPDX-License-Identifier: GPL-2.0
> > +/* Copyright (C) 2025 Alexey Charkov <alchark@gmail.com */
> > +
> > +#include <linux/auxiliary_bus.h>
> > +#include <linux/container_of.h>
> > +#include <linux/io.h>
> > +#include <linux/limits.h>
> > +#include <linux/module.h>
> > +#include <linux/types.h>
> > +#include <linux/watchdog.h>
> > +#include <linux/vt8500-timer.h>
> > +
> > +static int vt8500_watchdog_start(struct watchdog_device *wdd)
> > +{
> > +     struct vt8500_wdt_info *info = watchdog_get_drvdata(wdd);
> > +     u64 deadline = info->timer_next(wdd->timeout * VT8500_TIMER_HZ);
> > +
>
> wdd->timeout is the soft timeout, which may be larger
> than the maximum hardware timeout. That means you'll need
> to use something like "min(wdd->timeout, U32_MAX / VT8500_TIMER_HZ)
> * VT8500_TIMER_HZ" as parameter to the timer_next call.

Indeed. For some reason I thought the core splits up large user
requested timeout values into at most max_hw_heartbeat_ms long chunks
and feeds those to the driver via the timeout field, but now I see it
doesn't.

Do I get it right that the core worker will try and do its last ping
of the hardware at exactly max_hw_heartbeat_ms before the user
specified deadline?

> > +     writel((u32)deadline, info->wdt_match);
>
> Can deadline overflow ?

Yes. The underlying hardware counter is a u32 value incrementing at
VT8500_TIMER_HZ and continuing past wraparound. This register sets the
next match value: once the hardware counter reaches the value of
(u32)deadline (which might be after the counter wrapping around iff
deadline > U32_MAX) the system resets. Perhaps it warrants a comment
in code.

Thanks for your review, Guenter!

Best regards,
Alexey

On 5/20/25 23:20, Alexey Charkov wrote:
> On Wed, May 21, 2025 at 3:15 AM Guenter Roeck <linux@roeck-us.net> wrote:
>>
>> On 5/20/25 13:01, Alexey Charkov wrote:
>>> VIA/WonderMedia SoCs can use their system timer's first channel as a
>>> watchdog device which will reset the system if the clocksource counter
>>> matches the value given in its match register 0 and if the watchdog
>>> function is enabled.
>>>
>>> Since the watchdog function is tightly coupled to the timer itself, it
>>> is implemented as an auxiliary device of the timer device
>>>
>>> Signed-off-by: Alexey Charkov <alchark@gmail.com>
>>> ---
>>>    MAINTAINERS                   |  1 +
>>>    drivers/watchdog/Kconfig      | 15 ++++++++
>>>    drivers/watchdog/Makefile     |  1 +
>>>    drivers/watchdog/vt8500-wdt.c | 80 +++++++++++++++++++++++++++++++++++++++++++
>>>    4 files changed, 97 insertions(+)
>>>
>>> diff --git a/MAINTAINERS b/MAINTAINERS
>>> index 5362095240627f613638197fda275db6edc16cf7..97d1842625dbdf7fdca3556260662dab469ed091 100644
>>> --- a/MAINTAINERS
>>> +++ b/MAINTAINERS
>>> @@ -3447,6 +3447,7 @@ F:      drivers/tty/serial/vt8500_serial.c
>>>    F:  drivers/video/fbdev/vt8500lcdfb.*
>>>    F:  drivers/video/fbdev/wm8505fb*
>>>    F:  drivers/video/fbdev/wmt_ge_rops.*
>>> +F:   drivers/watchdog/vt8500-wdt.c
>>>    F:  include/linux/vt8500-timer.h
>>>
>>>    ARM/ZYNQ ARCHITECTURE
>>> diff --git a/drivers/watchdog/Kconfig b/drivers/watchdog/Kconfig
>>> index 0d8d37f712e8cfb4bf8156853baa13c23a57d6d9..8049ae630301a98123f97f6e3ee868bd3bf1534a 100644
>>> --- a/drivers/watchdog/Kconfig
>>> +++ b/drivers/watchdog/Kconfig
>>> @@ -2003,6 +2003,21 @@ config PIC32_DMT
>>>          To compile this driver as a loadable module, choose M here.
>>>          The module will be called pic32-dmt.
>>>
>>> +config VT8500_WATCHDOG
>>> +     tristate "VIA/WonderMedia VT8500 watchdog support"
>>> +     depends on ARCH_VT8500 || COMPILE_TEST
>>> +     select WATCHDOG_CORE
>>> +     select AUXILIARY_BUS
>>> +     help
>>> +       VIA/WonderMedia SoCs can use their system timer as a hardware
>>> +       watchdog, as long as the first timer channel is free from other
>>> +       uses and respective function is enabled in its registers. To
>>> +       make use of it, say Y here and ensure that the device tree
>>> +       lists at least two interrupts for the VT8500 timer device.
>>> +
>>> +       To compile this driver as a module, choose M here.
>>> +       The module will be called vt8500-wdt.
>>> +
>>>    # PARISC Architecture
>>>
>>>    # POWERPC Architecture
>>> diff --git a/drivers/watchdog/Makefile b/drivers/watchdog/Makefile
>>> index c9482904bf870a085c7fce2a439ac5089b6e6fee..3072786bf226c357102be3734fe6e701f753d45b 100644
>>> --- a/drivers/watchdog/Makefile
>>> +++ b/drivers/watchdog/Makefile
>>> @@ -101,6 +101,7 @@ obj-$(CONFIG_MSC313E_WATCHDOG) += msc313e_wdt.o
>>>    obj-$(CONFIG_APPLE_WATCHDOG) += apple_wdt.o
>>>    obj-$(CONFIG_SUNPLUS_WATCHDOG) += sunplus_wdt.o
>>>    obj-$(CONFIG_MARVELL_GTI_WDT) += marvell_gti_wdt.o
>>> +obj-$(CONFIG_VT8500_WATCHDOG) += vt8500-wdt.o
>>>
>>>    # X86 (i386 + ia64 + x86_64) Architecture
>>>    obj-$(CONFIG_ACQUIRE_WDT) += acquirewdt.o
>>> diff --git a/drivers/watchdog/vt8500-wdt.c b/drivers/watchdog/vt8500-wdt.c
>>> new file mode 100644
>>> index 0000000000000000000000000000000000000000..54fe5ad7695de36f6b3c1d28e955f00bef00e9db
>>> --- /dev/null
>>> +++ b/drivers/watchdog/vt8500-wdt.c
>>> @@ -0,0 +1,80 @@
>>> +// SPDX-License-Identifier: GPL-2.0
>>> +/* Copyright (C) 2025 Alexey Charkov <alchark@gmail.com */
>>> +
>>> +#include <linux/auxiliary_bus.h>
>>> +#include <linux/container_of.h>
>>> +#include <linux/io.h>
>>> +#include <linux/limits.h>
>>> +#include <linux/module.h>
>>> +#include <linux/types.h>
>>> +#include <linux/watchdog.h>
>>> +#include <linux/vt8500-timer.h>
>>> +
>>> +static int vt8500_watchdog_start(struct watchdog_device *wdd)
>>> +{
>>> +     struct vt8500_wdt_info *info = watchdog_get_drvdata(wdd);
>>> +     u64 deadline = info->timer_next(wdd->timeout * VT8500_TIMER_HZ);
>>> +
>>
>> wdd->timeout is the soft timeout, which may be larger
>> than the maximum hardware timeout. That means you'll need
>> to use something like "min(wdd->timeout, U32_MAX / VT8500_TIMER_HZ)
>> * VT8500_TIMER_HZ" as parameter to the timer_next call.
> 
> Indeed. For some reason I thought the core splits up large user
> requested timeout values into at most max_hw_heartbeat_ms long chunks
> and feeds those to the driver via the timeout field, but now I see it
> doesn't.
> 
> Do I get it right that the core worker will try and do its last ping
> of the hardware at exactly max_hw_heartbeat_ms before the user
> specified deadline?
> 

Where do you see that ? In the watchdog core:

         hw_heartbeat_ms = min_not_zero(timeout_ms, wdd->max_hw_heartbeat_ms);
         keepalive_interval = ms_to_ktime(hw_heartbeat_ms / 2);

>>> +     writel((u32)deadline, info->wdt_match);
>>
>> Can deadline overflow ?
> 
> Yes. The underlying hardware counter is a u32 value incrementing at
> VT8500_TIMER_HZ and continuing past wraparound. This register sets the
> next match value: once the hardware counter reaches the value of
> (u32)deadline (which might be after the counter wrapping around iff
> deadline > U32_MAX) the system resets. Perhaps it warrants a comment
> in code.
> 

Ok. Yes, a comment would help.

Thanks,
Guenter

> Thanks for your review, Guenter!
> 
> Best regards,
> Alexey

On Wed, May 21, 2025 at 5:33 PM Guenter Roeck <linux@roeck-us.net> wrote:
>
> On 5/20/25 23:20, Alexey Charkov wrote:
> > On Wed, May 21, 2025 at 3:15 AM Guenter Roeck <linux@roeck-us.net> wrote:
> >>
> >> On 5/20/25 13:01, Alexey Charkov wrote:
> >>> VIA/WonderMedia SoCs can use their system timer's first channel as a
> >>> watchdog device which will reset the system if the clocksource counter
> >>> matches the value given in its match register 0 and if the watchdog
> >>> function is enabled.
> >>>
> >>> Since the watchdog function is tightly coupled to the timer itself, it
> >>> is implemented as an auxiliary device of the timer device
> >>>
> >>> Signed-off-by: Alexey Charkov <alchark@gmail.com>
> >>> ---
> >>>    MAINTAINERS                   |  1 +
> >>>    drivers/watchdog/Kconfig      | 15 ++++++++
> >>>    drivers/watchdog/Makefile     |  1 +
> >>>    drivers/watchdog/vt8500-wdt.c | 80 +++++++++++++++++++++++++++++++++++++++++++
> >>>    4 files changed, 97 insertions(+)
> >>>
> >>> diff --git a/MAINTAINERS b/MAINTAINERS
> >>> index 5362095240627f613638197fda275db6edc16cf7..97d1842625dbdf7fdca3556260662dab469ed091 100644
> >>> --- a/MAINTAINERS
> >>> +++ b/MAINTAINERS
> >>> @@ -3447,6 +3447,7 @@ F:      drivers/tty/serial/vt8500_serial.c
> >>>    F:  drivers/video/fbdev/vt8500lcdfb.*
> >>>    F:  drivers/video/fbdev/wm8505fb*
> >>>    F:  drivers/video/fbdev/wmt_ge_rops.*
> >>> +F:   drivers/watchdog/vt8500-wdt.c
> >>>    F:  include/linux/vt8500-timer.h
> >>>
> >>>    ARM/ZYNQ ARCHITECTURE
> >>> diff --git a/drivers/watchdog/Kconfig b/drivers/watchdog/Kconfig
> >>> index 0d8d37f712e8cfb4bf8156853baa13c23a57d6d9..8049ae630301a98123f97f6e3ee868bd3bf1534a 100644
> >>> --- a/drivers/watchdog/Kconfig
> >>> +++ b/drivers/watchdog/Kconfig
> >>> @@ -2003,6 +2003,21 @@ config PIC32_DMT
> >>>          To compile this driver as a loadable module, choose M here.
> >>>          The module will be called pic32-dmt.
> >>>
> >>> +config VT8500_WATCHDOG
> >>> +     tristate "VIA/WonderMedia VT8500 watchdog support"
> >>> +     depends on ARCH_VT8500 || COMPILE_TEST
> >>> +     select WATCHDOG_CORE
> >>> +     select AUXILIARY_BUS
> >>> +     help
> >>> +       VIA/WonderMedia SoCs can use their system timer as a hardware
> >>> +       watchdog, as long as the first timer channel is free from other
> >>> +       uses and respective function is enabled in its registers. To
> >>> +       make use of it, say Y here and ensure that the device tree
> >>> +       lists at least two interrupts for the VT8500 timer device.
> >>> +
> >>> +       To compile this driver as a module, choose M here.
> >>> +       The module will be called vt8500-wdt.
> >>> +
> >>>    # PARISC Architecture
> >>>
> >>>    # POWERPC Architecture
> >>> diff --git a/drivers/watchdog/Makefile b/drivers/watchdog/Makefile
> >>> index c9482904bf870a085c7fce2a439ac5089b6e6fee..3072786bf226c357102be3734fe6e701f753d45b 100644
> >>> --- a/drivers/watchdog/Makefile
> >>> +++ b/drivers/watchdog/Makefile
> >>> @@ -101,6 +101,7 @@ obj-$(CONFIG_MSC313E_WATCHDOG) += msc313e_wdt.o
> >>>    obj-$(CONFIG_APPLE_WATCHDOG) += apple_wdt.o
> >>>    obj-$(CONFIG_SUNPLUS_WATCHDOG) += sunplus_wdt.o
> >>>    obj-$(CONFIG_MARVELL_GTI_WDT) += marvell_gti_wdt.o
> >>> +obj-$(CONFIG_VT8500_WATCHDOG) += vt8500-wdt.o
> >>>
> >>>    # X86 (i386 + ia64 + x86_64) Architecture
> >>>    obj-$(CONFIG_ACQUIRE_WDT) += acquirewdt.o
> >>> diff --git a/drivers/watchdog/vt8500-wdt.c b/drivers/watchdog/vt8500-wdt.c
> >>> new file mode 100644
> >>> index 0000000000000000000000000000000000000000..54fe5ad7695de36f6b3c1d28e955f00bef00e9db
> >>> --- /dev/null
> >>> +++ b/drivers/watchdog/vt8500-wdt.c
> >>> @@ -0,0 +1,80 @@
> >>> +// SPDX-License-Identifier: GPL-2.0
> >>> +/* Copyright (C) 2025 Alexey Charkov <alchark@gmail.com */
> >>> +
> >>> +#include <linux/auxiliary_bus.h>
> >>> +#include <linux/container_of.h>
> >>> +#include <linux/io.h>
> >>> +#include <linux/limits.h>
> >>> +#include <linux/module.h>
> >>> +#include <linux/types.h>
> >>> +#include <linux/watchdog.h>
> >>> +#include <linux/vt8500-timer.h>
> >>> +
> >>> +static int vt8500_watchdog_start(struct watchdog_device *wdd)
> >>> +{
> >>> +     struct vt8500_wdt_info *info = watchdog_get_drvdata(wdd);
> >>> +     u64 deadline = info->timer_next(wdd->timeout * VT8500_TIMER_HZ);
> >>> +
> >>
> >> wdd->timeout is the soft timeout, which may be larger
> >> than the maximum hardware timeout. That means you'll need
> >> to use something like "min(wdd->timeout, U32_MAX / VT8500_TIMER_HZ)
> >> * VT8500_TIMER_HZ" as parameter to the timer_next call.
> >
> > Indeed. For some reason I thought the core splits up large user
> > requested timeout values into at most max_hw_heartbeat_ms long chunks
> > and feeds those to the driver via the timeout field, but now I see it
> > doesn't.
> >
> > Do I get it right that the core worker will try and do its last ping
> > of the hardware at exactly max_hw_heartbeat_ms before the user
> > specified deadline?
> >
>
> Where do you see that ? In the watchdog core:
>
>          hw_heartbeat_ms = min_not_zero(timeout_ms, wdd->max_hw_heartbeat_ms);
>          keepalive_interval = ms_to_ktime(hw_heartbeat_ms / 2);

This comment [1] which follows the lines you've pasted: "To ensure
that the watchdog times out wdd->timeout seconds after the most recent
ping from userspace, the last worker ping has to come in
hw_heartbeat_ms before this timeout."

I'm not exactly sure I'm following the code path there 100%, so wanted
to double check. Meanwhile, I've just sent a new version [2] adding
the min function as you pointed out.

[1] https://elixir.bootlin.com/linux/v6.14.6/source/drivers/watchdog/watchdog_dev.c#L120
[2] https://lore.kernel.org/all/20250521-vt8500-timer-updates-v5-0-7e4bd11df72e@gmail.com/

Best regards,
Alexey

On 5/21/25 07:15, Alexey Charkov wrote:
...
>>> Do I get it right that the core worker will try and do its last ping
>>> of the hardware at exactly max_hw_heartbeat_ms before the user
>>> specified deadline?
>>>
>>
>> Where do you see that ? In the watchdog core:
>>
>>           hw_heartbeat_ms = min_not_zero(timeout_ms, wdd->max_hw_heartbeat_ms);
>>           keepalive_interval = ms_to_ktime(hw_heartbeat_ms / 2);
> 
> This comment [1] which follows the lines you've pasted: "To ensure
> that the watchdog times out wdd->timeout seconds after the most recent
> ping from userspace, the last worker ping has to come in
> hw_heartbeat_ms before this timeout."
> 

Ah, yes. Sorry, I misunderstood your question. This is absolutely correct.
If timeout is, say, 10 seconds, and the maximum hardware timeout is 8 seconds,
the last heartbeat must be triggered by the kernel 2 seconds after the last
userspace heartbeat request to ensure that the actual timeout happens 10
seconds after the most recent heartbeat request from userspace.

Guenter