linux-kernel - Re: [PATCH v4 4/4] watchdog: Add support for VIA/WonderMedia SoC watchdog functionality

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Message-ID: <ad056ba1-8fce-4735-b71e-f22c22eacc18@roeck-us.net>
Date: Wed, 21 May 2025 06:33:10 -0700
From: Guenter Roeck <linux@...ck-us.net>
To: Alexey Charkov <alchark@...il.com>
Cc: Krzysztof Kozlowski <krzk@...nel.org>,
 Daniel Lezcano <daniel.lezcano@...aro.org>,
 Thomas Gleixner <tglx@...utronix.de>, Rob Herring <robh@...nel.org>,
 Conor Dooley <conor+dt@...nel.org>, Krzysztof Kozlowski
 <krzk+dt@...nel.org>, Wim Van Sebroeck <wim@...ux-watchdog.org>,
 linux-arm-kernel@...ts.infradead.org, linux-kernel@...r.kernel.org,
 devicetree@...r.kernel.org, linux-watchdog@...r.kernel.org
Subject: Re: [PATCH v4 4/4] watchdog: Add support for VIA/WonderMedia SoC
 watchdog functionality

On 5/20/25 23:20, Alexey Charkov wrote:
> On Wed, May 21, 2025 at 3:15 AM Guenter Roeck <linux@...ck-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@...il.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@...il.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