linux-kernel - Re: [PATCHv1 1/6] rtc: m41t80: add support for protected clock

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Date:   Tue, 23 Feb 2021 02:26:57 +0100
From:   Sebastian Reichel <sre@...nel.org>
To:     Alexandre Belloni <alexandre.belloni@...tlin.com>
Cc:     linux-rtc@...r.kernel.org, Alessandro Zummo <a.zummo@...ertech.it>,
        Philipp Zabel <p.zabel@...gutronix.de>,
        devicetree@...r.kernel.org, David Airlie <airlied@...ux.ie>,
        Shawn Guo <shawnguo@...nel.org>,
        Sascha Hauer <s.hauer@...gutronix.de>,
        linux-kernel@...r.kernel.org, dri-devel@...ts.freedesktop.org,
        Rob Herring <robh+dt@...nel.org>,
        linux-mtd@...ts.infradead.org, NXP Linux Team <linux-imx@....com>,
        Pengutronix Kernel Team <kernel@...gutronix.de>,
        Miquel Raynal <miquel.raynal@...tlin.com>,
        Daniel Vetter <daniel@...ll.ch>, kernel@...labora.com,
        Fabio Estevam <festevam@...il.com>,
        linux-arm-kernel@...ts.infradead.org
Subject: Re: [PATCHv1 1/6] rtc: m41t80: add support for protected clock

Hi,

On Mon, Feb 22, 2021 at 10:26:26PM +0100, Alexandre Belloni wrote:
> On 22/02/2021 22:20:47+0100, Alexandre Belloni wrote:
> > On 22/02/2021 18:12:42+0100, Sebastian Reichel wrote:
> > > Congatec's QMX6 system on module (SoM) uses a m41t62 as RTC. The
> > > modules SQW clock output defaults to 32768 Hz. This behaviour is
> > > used to provide the i.MX6 CKIL clock. Once the RTC driver is probed,
> > > the clock is disabled and all i.MX6 functionality depending on
> > > the 32 KHz clock has undefined behaviour. On systems using hardware
> > > watchdog it seems to likely trigger a lot earlier than configured.
> > > 
> > > The proper solution would be to describe this dependency in DT,
> > > but that will result in a deadlock. The kernel will see, that
> > > i.MX6 system clock needs the RTC clock and do probe deferral.
> > > But the i.MX6 I2C module never becomes usable without the i.MX6
> > > CKIL clock and thus the RTC's clock will not be probed. So from
> > > the kernel's perspective this is a chicken-and-egg problem.
> > > 
> > 
> > Reading the previous paragraph, I was going to suggest describing the
> > dependency and wondering whether this would cause a circular dependency.
> > I guess this will keep being an issue for clocks on an I2C or SPI bus...

Yes, it is a circular dependency on this particular system on
module. It only works because the RTC enables the clock by
default. The i.MX6 CKIL is expected to be always enabled.

> > > Technically everything is fine by not touching anything, since
> > > the RTC clock correctly enables the clock on reset (i.e. on
> > > battery backup power loss) and also the bootloader enables it
> > > in case a kernel without this support has been booted.
> > > 
> > > The 'protected-clocks' property is already in use for some clocks
> > > that may not be touched because of firmware limitations and is
> > > described in Documentation/devicetree/bindings/clock/clock-bindings.txt.
> > > 
> > > Signed-off-by: Sebastian Reichel <sebastian.reichel@...labora.com>
> > Acked-by: Alexandre Belloni <alexandre.belloni@...tlin.com>
> 
> Or maybe you expected me to apply the patch, how are the following
> patches dependent on this one?

The last patch, which introduces a new board has a runtime
dependency on this patch. Without this feature the board
goes into a reboot loop because its bootloader enables the
i.MX6 watchdog and without the CKIL its timing is messed up.

But it's a pure runtime dependency for a new board, so it should
be fine to merge this via your tree. It basically means the board
is only working once your tree and arm tree have been merged,
which seems ok from my POV.

-- Sebastian

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