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The Lenovo X13s firmware does not implement the UEFI time runtime services so the RTC in the PM8280K PMIC needs to be accessed directly. To complicate things further, the RTC control and time registers are read-only on this platform so an offset must be stored in some other machine-specific non-volatile memory which an RTC driver can take into account when reading or updating the time. The UEFI firmware (and Windows) use a UEFI variable for this: 882f8c2b-9646-435f-8de5-f208ff80c1bd-RTCInfo but the offset can only be accessed via the Qualcomm UEFI Secure Application residing in the TEE as the firmware does not implement the variable runtime services either. While it is possible to access this UEFI variable from Linux on the X13s, this requires using a fairly complex and reverse-engineered firmware interface. As the only benefit of doing so is to make sure that the UEFI (Windows) and Linux time never gets out of sync, it seems preferable to use the PMIC scratch registers for storing an offset instead. This also avoids flash wear in case of RTC drift, etc. So instead of using the UEFI RTC offset, reserve four bytes in one of the PMIC SDAM scratch-register blocks to hold the RTC offset. Signed-off-by: Johan Hovold <johan+linaro@kernel.org> Signed-off-by: Bjorn Andersson <andersson@kernel.org> Link: https://lore.kernel.org/r/20230202155448.6715-23-johan+linaro@kernel.org
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.