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Date:   Wed, 5 May 2021 18:00:57 -0500
From:   Rob Herring <robh@kernel.org>
To:     Sebastian Reichel <sebastian.reichel@collabora.com>
Cc:     Fabio Estevam <festevam@gmail.com>,
        linux-arm-kernel@lists.infradead.org,
        Miquel Raynal <miquel.raynal@bootlin.com>,
        Alessandro Zummo <a.zummo@towertech.it>,
        Shawn Guo <shawnguo@kernel.org>,
        dri-devel@lists.freedesktop.org, Rob Herring <robh+dt@kernel.org>,
        linux-mtd@lists.infradead.org,
        Saravana Kannan <saravanak@google.com>,
        linux-kernel@vger.kernel.org,
        Sascha Hauer <s.hauer@pengutronix.de>,
        NXP Linux Team <linux-imx@nxp.com>,
        devicetree@vger.kernel.org, linux-rtc@vger.kernel.org,
        kernel@collabora.com, Philipp Zabel <p.zabel@pengutronix.de>,
        Alexandre Belloni <alexandre.belloni@bootlin.com>,
        David Airlie <airlied@linux.ie>,
        Pengutronix Kernel Team <kernel@pengutronix.de>
Subject: Re: [PATCHv2 1/5] rtc: m41t80: add support for fixed clock
Message-ID: <20210505230057.GA3001067@robh.at.kernel.org>
References: <20210428222953.235280-1-sebastian.reichel@collabora.com>
 <20210428222953.235280-2-sebastian.reichel@collabora.com>
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On Thu, 29 Apr 2021 00:29:49 +0200, 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. For example when using
> the hardware watchdog the system will likely do arbitrary reboots.
> 
> Referencing the m41t62 directly results 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.
> 
> 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 an something (e.g. an unpatched kernel) disabled this
> incorrectly.
> 
> A workaround for this issue is describing the square wave pin
> as fixed-clock, which is registered early and basically how
> this pin is used on the i.MX6.
> 
> Suggested-by: Saravana Kannan <saravanak@google.com>
> Signed-off-by: Sebastian Reichel <sebastian.reichel@collabora.com>
> ---
>  Documentation/devicetree/bindings/rtc/rtc-m41t80.txt |  9 +++++++++
>  drivers/rtc/rtc-m41t80.c                             | 12 ++++++++++++
>  2 files changed, 21 insertions(+)
> 

Reviewed-by: Rob Herring <robh@kernel.org>