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

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Date:   Sat, 6 Mar 2021 11:56:45 -0800
From:   Rob Herring <robh@...nel.org>
To:     Sebastian Reichel <sre@...nel.org>
Cc:     Alexandre Belloni <alexandre.belloni@...tlin.com>,
        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,
        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

On Tue, Feb 23, 2021 at 02:26:57AM +0100, Sebastian Reichel wrote:
> 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.

I think you should describe the circular clocking and then provide a way 
to break the dependency. It's a somewhat common issue.

Rob