linux-kernel - Re: [PATCH v2 1/3] power: reset: read priority from device tree

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Date:	Mon, 1 Dec 2014 17:41:00 +0000
From:	Mark Rutland <mark.rutland@....com>
To:	Stefan Agner <stefan@...er.ch>
Cc:	"shawn.guo@...aro.org" <shawn.guo@...aro.org>,
	"kernel@...gutronix.de" <kernel@...gutronix.de>,
	"linux@...ck-us.net" <linux@...ck-us.net>,
	"arnd@...db.de" <arnd@...db.de>, "sre@...nel.org" <sre@...nel.org>,
	"fkan@....com" <fkan@....com>,
	"grant.likely@...aro.org" <grant.likely@...aro.org>,
	"robh+dt@...nel.org" <robh+dt@...nel.org>,
	"dbaryshkov@...il.com" <dbaryshkov@...il.com>,
	"dwmw2@...radead.org" <dwmw2@...radead.org>,
	"devicetree@...r.kernel.org" <devicetree@...r.kernel.org>,
	"linux-arm-kernel@...ts.infradead.org" 
	<linux-arm-kernel@...ts.infradead.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH v2 1/3] power: reset: read priority from device tree

On Mon, Dec 01, 2014 at 05:24:37PM +0000, Stefan Agner wrote:
> On 2014-12-01 18:11, Mark Rutland wrote:
> > On Mon, Dec 01, 2014 at 05:03:07PM +0000, Stefan Agner wrote:
> >> This patch adds an optional property which allows to specify the
> >> reset source priority. This priority is used by the kernel restart
> >> handler call chain to sort out the proper reset/restart method.
> >> Depending on the power design of a board or other machine/board
> >> specific peculiarity, it is not possible to pick a generic priority.
> >>
> >> Signed-off-by: Stefan Agner <stefan@...er.ch>
> >> ---
> >>  Documentation/devicetree/bindings/power/reset/syscon-reboot.txt | 3 +++
> >>  drivers/power/reset/syscon-reboot.c                             | 5 ++++-
> >>  2 files changed, 7 insertions(+), 1 deletion(-)
> >>
> >> diff --git a/Documentation/devicetree/bindings/power/reset/syscon-reboot.txt b/Documentation/devicetree/bindings/power/reset/syscon-reboot.txt
> >> index 1190631..ee41d9c 100644
> >> --- a/Documentation/devicetree/bindings/power/reset/syscon-reboot.txt
> >> +++ b/Documentation/devicetree/bindings/power/reset/syscon-reboot.txt
> >> @@ -11,6 +11,9 @@ Required properties:
> >>  - offset: offset in the register map for the reboot register (in bytes)
> >>  - mask: the reset value written to the reboot register (32 bit access)
> >>
> >> +Optional properties:
> >> +- priority: define the priority of the reset (0-255, defaults to 128)
> >> +
> > 
> > NAK. This is a leak of Linux-internal details.
> > 
> > What is this necessary for?
> > 
> > Mark.
> 
> Hi Mark,
> 
> In my case, it is necessary to be called ahead of the watchdog, which
> has a priority of 128. This syscon-reboot driver currently has a default
> priority of 128 too. I could live with a higher default priority for the
> syscon-reboot driver, in fact I proposed that in the discussion of v1 of
> that patch:
> https://lkml.org/lkml/2014/11/28/484

Thanks for the link.

> IMHO, this priority might make sense for most cases, I guess that
> dedicated "syscon" capabilities are usually better suited as a reboot
> source than watchdog.

I would think likewise.

> If dt, then the question which arises: If there are different
> capabilities to reset/reboot a whole system, how do we reflect which is
> the best suited one in dt?

I'm not sure, but I don't think that exposing a priority variable in
this way is the best, because it implicitly relies on what the kernel
may have selected for other devices and/or FW interfaces, which may not
have been described in DT.

So if we can get away with a fixed priority for now, then I would prefer
that.

Otherwise, I would imagine that most systems have a single preferred
mechanism with some possible fallback(s), for which a single
preferred-poweroff property might suffice, and has better interaction
w.r.t. priority (in that it should _always_ be tried first). Even that's
difficult to reconcile with FW bindings though, especially EFI (which we
sometimes must use in preference for variable storage and capsule
updates).

Thanks,
Mark.
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