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
Date: Wed, 5 May 2021 18:00:57 -0500 [thread overview]
Message-ID: <20210505230057.GA3001067@robh.at.kernel.org> (raw)
In-Reply-To: <20210428222953.235280-2-sebastian.reichel@collabora.com>
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>
next prev parent reply other threads:[~2021-05-05 23:01 UTC|newest]
Thread overview: 13+ messages / expand[flat|nested] mbox.gz Atom feed top
2021-04-28 22:29 [PATCHv2 0/5] Support for GE B1x5v2 and B1x5Pv2 Sebastian Reichel
2021-04-28 22:29 ` [PATCHv2 1/5] rtc: m41t80: add support for fixed clock Sebastian Reichel
2021-04-29 1:11 ` Saravana Kannan
2021-05-05 23:00 ` Rob Herring [this message]
2021-04-28 22:29 ` [PATCHv2 2/5] drm/imx: Add 8 pixel alignment fix Sebastian Reichel
2021-05-10 14:47 ` Philipp Zabel
2021-04-28 22:29 ` [PATCHv2 3/5] dt-bindings: vendor-prefixes: add congatec Sebastian Reichel
2021-05-05 23:01 ` Rob Herring
2021-04-28 22:29 ` [PATCHv2 4/5] dt-bindings: arm: fsl: add GE B1x5pv2 boards Sebastian Reichel
2021-05-05 23:02 ` Rob Herring
2021-04-28 22:29 ` [PATCHv2 5/5] ARM: dts: imx6: Add GE B1x5v2 Sebastian Reichel
2021-05-22 14:04 ` Shawn Guo
2021-05-24 22:29 ` (subset) [PATCHv2 0/5] Support for GE B1x5v2 and B1x5Pv2 Alexandre Belloni
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