linux-kernel - Re: [PATCH 2/2] watchdog: Add Realtek Otto watchdog timer

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Date:   Fri, 15 Oct 2021 14:07:45 +0100
From:   Sander Vanheule <sander@...nheule.net>
To:     Guenter Roeck <linux@...ck-us.net>
Cc:     linux-watchdog@...r.kernel.org, devicetree@...r.kernel.org,
        Wim Van Sebroeck <wim@...ux-watchdog.org>,
        Rob Herring <robh+dt@...nel.org>, linux-kernel@...r.kernel.org
Subject: Re: [PATCH 2/2] watchdog: Add Realtek Otto watchdog timer

On Thu, 2021-10-14 at 09:56 -0700, Guenter Roeck wrote:
> On 10/14/21 3:26 AM, Sander Vanheule wrote:
> > On Wed, 2021-10-13 at 14:03 -0700, Guenter Roeck wrote:
> > > On 10/13/21 12:46 PM, Sander Vanheule wrote:
> > > > On Wed, 2021-10-13 at 11:48 -0700, Guenter Roeck wrote:
> > > > > On Wed, Oct 13, 2021 at 03:29:00PM +0200, Sander Vanheule wrote:
> > > > [...]
> > > > 
> > > > > > 
> > > > > > diff --git a/drivers/watchdog/realtek_otto_wdt.c
> > > > > > b/drivers/watchdog/realtek_otto_wdt.c
> > > > > > new file mode 100644
> > > > > > index 000000000000..64c9cba6b0b1
> > > > > > --- /dev/null
> > > > > > +++ b/drivers/watchdog/realtek_otto_wdt.c
> > > > > > @@ -0,0 +1,411 @@
> > > > > > +// SPDX-License-Identifier: GPL-2.0-only
> > > > > > +
> > > > > > +/*
> > > > > > + * Realtek Otto MIPS platform watchdog
> > > > > > + *
> > > > > > + * Watchdog timer that will reset the system after timeout, using the
> > > > > > selected
> > > > > > + * reset mode.
> > > > > > + *
> > > > > > + * Counter scaling and timeouts:
> > > > > > + * - Base prescale of (2 << 25), providing tick duration T_0: 168ms @
> > > > > > 200MHz
> > > > > > + * - PRESCALE: logarithmic prescaler adding a factor of {1, 2, 4, 8}
> > > > > > + * - Phase 1: Times out after (PHASE1 + 1) × PRESCALE × T_0
> > > > > > + *   Generates an interrupt, WDT cannot be stopped after phase 1
> > > > > > + * - Phase 2: starts after phase 1, times out after (PHASE2 + 1) ×
> > > > > > PRESCALE × T_0
> > > > > > + *   Resets the system according to RST_MODE
> > > > > 
> > > > > Why is there a phase2 interrupt if phase2 resets the chip ?
> > > > > 
> > > > 
> > > > The SoC's reset controller has an interrupt line for phase2, even though
> > > > then it then the
> > > > WDT also resets the system. I don't have any documentation about this
> > > > peripheral; just
> > > > some vendor code and there the phase2 interrupt isn't enabled. I mainly
> > > > added it here for
> > > > completeness.
> > > > 
> > > 
> > > It seems pointless to mandate an interrupt just for completeness.
> > 
> > Okay, then I will just drop it here. As I understand, the bindings should be as
> > complete as possible, so I think the phase2 interrupt definition should remain
> > there?
> > 
> 
> I still don't see the point of it if there is no known use case. At the very
> least it will need to be optional, but even then I would expect a description
> of the use case.
> 
> FWIW, technically I suspect that there is a means for the watchdog to generate
> a second interrupt instead of resetting the hardware (otherwise the second
> interrupt would not really make sense), but without hardware and without
> datasheet it is impossible to confirm that.

I haven't found any WDT reset enable/disable flag for the RTL838x and RTL839x series,
but I noticed the RTL930x series does have a potentially interesting field in their
reset controller.

WD_RST_EN: https://svanheule.net/realtek/longan/register/rst_glb_ctrl_0

Sadly we don't have any proper datasheets. All we have is the register layouts from
the source archives, which we've been trying to document ourselves. I've ordered some
hardware with an RTL9302 SoC, so I should be able to provide an update next week.

Best,
Sander