linux-kernel - Re: Missing ARM TWD Watchdog driver?

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Date:   Mon, 17 Jan 2022 17:02:25 +0000
From:   Mark Rutland <mark.rutland@....com>
To:     Guenter Roeck <linux@...ck-us.net>
Cc:     Boris Lysov <arzamas-16@...l.ee>, linux-kernel@...r.kernel.org,
        devicetree@...r.kernel.org, linux-watchdog@...r.kernel.org,
        linux-arm-kernel@...ts.infradead.org
Subject: Re: Missing ARM TWD Watchdog driver?

On Mon, Jan 17, 2022 at 08:34:57AM -0800, Guenter Roeck wrote:
> On 1/17/22 8:01 AM, Boris Lysov wrote:
> > Hello everyone,
> > 
> > I stumbled upon the ARM TWD Watchdog driver which seems to be missing (or I'm
> > just blind).
> > 
> > As per commit a33f5c380c4bd3fa5278d690421b72052456d9fe ("Merge tag
> > 'xfs-5.17-merge-3' of git://git.kernel.org/pub/scm/fs/xfs/xfs-linux"), multiple
> > device trees declare usage of arm-twd:
> > 
> > arch/arm/boot/dts/arm-realview-eb-mp.dtsi
> > arch/arm/boot/dts/arm-realview-pb11mp.dts
> > arch/arm/boot/dts/arm-realview-pbx-a9.dts
> > arch/arm/boot/dts/bcm5301x.dtsi
> > arch/arm/boot/dts/bcm63138.dtsi
> > arch/arm/boot/dts/bcm-hr2.dtsi
> > arch/arm/boot/dts/bcm-nsp.dtsi
> > arch/arm/boot/dts/berlin2cd.dtsi
> > arch/arm/boot/dts/highbank.dts
> > arch/arm/boot/dts/mmp3.dtsi
> > arch/arm/boot/dts/owl-s500.dtsi
> > arch/arm/boot/dts/spear13xx.dtsi
> > arch/arm/boot/dts/ste-dbx5x0.dtsi
> > arch/arm/boot/dts/vexpress-v2p-ca5s.dts
> > arch/arm/boot/dts/vexpress-v2p-ca9.dts
> > 
> > and it is documented in
> > Documentation/devicetree/bindings/watchdog/arm,twd-wdt.yaml
> > 
> > However I could not find the driver itself. I tried running case-insensitive
> > grep and ripgrep to no avail. Does this driver actually exist? Is it gone?
> > 
> 
> I may be missing it, but I do not see any evidence that it ever existed.

Likewise, AFAICT we added the binding but never the driver.

Looking at the git history, that schema was introduced in commit:

  50e02e9a030a9ae3 ("dt-bindings: timer: arm,twd: Convert to json-schema")

... and replaced Documentation/devicetree/bindings/timer/arm,twd.txt

Per:

  $ git log --follow Documentation/devicetree/bindings/timer/arm,twd.txt

That was introduced alongside a driver in commit:

  d8e0364364d333fe ("ARM: smp_twd: add device tree support")

... which only introduced a driver for the timer, and not the watchdog,
as the commit message notes.

Thanks.
Mark.