linux-kernel - Re: [PATCH 0/7] arm64: dts: qcom: x1e80100: enable rtc

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Message-ID: <cc2e9b61-ec05-4b42-bcfd-e98a55082391@oss.qualcomm.com>
Date: Thu, 23 Jan 2025 13:28:31 +0100
From: Konrad Dybcio <konrad.dybcio@....qualcomm.com>
To: Johan Hovold <johan+linaro@...nel.org>,
        Alexandre Belloni <alexandre.belloni@...tlin.com>,
        Bjorn Andersson <andersson@...nel.org>
Cc: Rob Herring <robh@...nel.org>, Krzysztof Kozlowski <krzk+dt@...nel.org>,
        Conor Dooley <conor+dt@...nel.org>,
        Konrad Dybcio <konradybcio@...nel.org>,
        Jonathan Marek <jonathan@...ek.ca>, linux-arm-msm@...r.kernel.org,
        linux-rtc@...r.kernel.org, devicetree@...r.kernel.org,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH 0/7] arm64: dts: qcom: x1e80100: enable rtc

On 20.01.2025 3:41 PM, Johan Hovold wrote:
> This series adds support for utilising the UEFI firmware RTC offset to
> the Qualcomm PMIC RTC driver and uses that to enable the RTC on all X
> Elite machines.
> 
> Included is also a patch to switch the Lenovo ThinkPad X13s over to
> using the UEFI offset.
> 
> The RTCs in many Qualcomm devices are effectively broken due to the time
> registers being read-only. Instead some other non-volatile memory can be
> used to store an offset which a driver can take into account. On Windows
> on Arm laptops, the UEFI firmware (and Windows) use a UEFI variable for
> storing such an offset.
> 
> When RTC support for the X13s was added two years ago we did not yet
> have UEFI variable support for these machines in mainline and there were
> also some concerns regarding flash wear. [1] As not all Qualcomm
> platforms have UEFI firmware anyway, we instead opted to use a PMIC
> scratch register for storing the offset. [2]
> 
> On the UEFI machines in question this is however arguable not correct
> as it means that the RTC time can differ between the UEFI firmware (and
> Windows) and Linux.
> 
> Now that the (reverse engineered) UEFI variable implementation has been
> merged and thoroughly tested, let's switch to using that to store the
> RTC offset also on Linux. The flash wear concerns can be mitigated by
> deferring writes due to clock drift until shutdown.
> 
> Note that this also avoids having to wait for months for Qualcomm to
> provide a free PMIC SDAM scratch register for X1E and future platforms,
> and specifically allows us to enable the RTC on X1E laptops today.
> 
> Rob had some concerns about adding a DT property for indicating that a
> machine uses UEFI for storing the offset and suggested that the driver
> should probe for this instead. Unfortunately, this is easier said than
> done given that UEFI variable support itself is probed for and may not
> be available until after the RTC driver probes.
> 
> Hopefully this all goes away (for future platforms) once Qualcomm fix
> their UEFI implementation so that the UEFI time (and variable) services
> can be used directly.
> 
> Johan

Thanks for working on this!

Konrad