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

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Message-ID: <CAF6AEGsfke=x0p1b2-uNX6DuQfRyEjVbJaxTbVLDT2YvSkGJbg@mail.gmail.com>
Date: Mon, 21 Apr 2025 07:36:28 -0700
From: Rob Clark <robdclark@...il.com>
To: Rob Herring <robh@...nel.org>
Cc: Johan Hovold <johan+linaro@...nel.org>, 
	Alexandre Belloni <alexandre.belloni@...tlin.com>, Bjorn Andersson <andersson@...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, 
	Rob Clark <robdclark@...omium.org>
Subject: Re: [PATCH 0/7] arm64: dts: qcom: x1e80100: enable rtc

On Sun, Jan 26, 2025 at 4:20 PM Rob Herring <robh@...nel.org> wrote:
>
> On Mon, Jan 20, 2025 at 03:41:45PM +0100, 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.
>
> This information would be useful in the binding commit...
>
> Seems like something I would say, but this is v1 and I have no memory of
> discussing this. I would also say probe ordering is not a DT problem,
> but sounds like an OS problem. Aren't there other things needing EFI
> variables earlyish too? Do you really want to have to update the DT to
> enable this?

I was debugging why RTC offset was not working properly for me, and
eventually realized it was exactly this problem (efivars not avail
when rtc probes).

Hacking up rtc-pm8xxx to return -EPROBE_DEFER "fixes" it

BR,
-R

> OTOH, it's one property, meh.
>
> Rob
>