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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> Reviewed-by: Saravana Kannan <saravanak@google.com> Reviewed-by: Rob Herring <robh@kernel.org> Signed-off-by: Alexandre Belloni <alexandre.belloni@bootlin.com> Link: https://lore.kernel.org/r/20210428222953.235280-2-sebastian.reichel@collabora.com
Linux kernel
============
There are several guides for kernel developers and users. These guides can
be rendered in a number of formats, like HTML and PDF. Please read
Documentation/admin-guide/README.rst first.
In order to build the documentation, use ``make htmldocs`` or
``make pdfdocs``. The formatted documentation can also be read online at:
https://www.kernel.org/doc/html/latest/
There are various text files in the Documentation/ subdirectory,
several of them using the Restructured Text markup notation.
Please read the Documentation/process/changes.rst file, as it contains the
requirements for building and running the kernel, and information about
the problems which may result by upgrading your kernel.