Re: [RFC 18/24] dt-bindings: rtc: qcom-pm8xxx: add uefi-variable offset

From: Ard Biesheuvel <ardb@kernel.org>
To: Johan Hovold <johan@kernel.org>
Cc: Rob Herring <robh@kernel.org>,
	Johan Hovold <johan+linaro@kernel.org>,
	Alexandre Belloni <alexandre.belloni@bootlin.com>,
	Bjorn Andersson <andersson@kernel.org>,
	Andy Gross <agross@kernel.org>,
	Konrad Dybcio <konrad.dybcio@linaro.org>,
	Alessandro Zummo <a.zummo@towertech.it>,
	Krzysztof Kozlowski <krzysztof.kozlowski+dt@linaro.org>,
	Maximilian Luz <luzmaximilian@gmail.com>,
	linux-arm-msm@vger.kernel.org, linux-rtc@vger.kernel.org,
	devicetree@vger.kernel.org, linux-kernel@vger.kernel.org
Subject: Re: [RFC 18/24] dt-bindings: rtc: qcom-pm8xxx: add uefi-variable offset
Date: Thu, 9 Feb 2023 17:59:10 +0100	[thread overview]
Message-ID: <CAMj1kXGwVO_WtfMSkbYj_Uf2t7-=G2b1vt-pggOGwMqFJKEQNQ@mail.gmail.com> (raw)
In-Reply-To: <Y9qO0yQ7oLux2L9n@hovoldconsulting.com>

On Wed, 1 Feb 2023 at 17:09, Johan Hovold <johan@kernel.org> wrote:
>
> [ +CC: Ard ]
>
> On Mon, Jan 30, 2023 at 12:49:44PM -0600, Rob Herring wrote:
> > On Thu, Jan 26, 2023 at 03:20:51PM +0100, Johan Hovold wrote:
> > > On many Qualcomm platforms the PMIC RTC control and time registers are
> > > read-only so that the RTC time can not be updated. Instead an offset
> > > needs be stored in some machine-specific non-volatile memory, which a
> > > driver can take into account.
> > >
> > > Add a 'qcom,uefi-rtc-info' boolean flag which indicates that the RTC
> > > offset is stored in a Qualcomm specific UEFI variable so that the RTC
> > > time can be updated on such platforms.
> > >
> > > The UEFI variable is
> > >
> > >     882f8c2b-9646-435f-8de5-f208ff80c1bd-RTCInfo
> > >
> > > and holds a 12-byte structure where the first four bytes is a GPS time
> > > offset in little-endian byte order.
> >
> > Can't you just try to read the UEFI variable and use it if that
> > succeeds?
>
> Generally, yes. The problem here is that this UEFI variable is not used
> on all devices using these PMICs and I need a way to determine whether
> to wait for the UEFI variables to become available or not (e.g. when
> efivars support is built as module, yes, that's a thing now...).
>

Could we read this variable at boot and pass it to the kernel in a
different way? That way, we only have to defer the ability to set the
RTC, right?

> > I don't like this in DT because what if lots of devices start storing
> > lots of things in vendor specific UEFI variables. It doesn't scale.
>
> I hope we won't see that even if we already have some devices for x86
> platforms storing MAC addresses and such in UEFI variables. They
> currently access the UEFI firmware directly (i.e. not using the efivars
> abstraction) and simply assume UEFI is always there.
>
> With the Google SMI efivars implementation or the new Qualcomm SMC-based
> one, we need a way to determine whether to wait for efivars to become
> registered. For drivers where efivars is always needed we can just probe
> defer, but in this case we should not wait unless the DT indicates that
> the RTC offset is stored in UEFI on this particular machine.
>
> Just as the nvmem-cell property indicates that the offset is stored in
> some abstract nvmem, it seems reasonable to describe the offset being
> stored in UEFI when that is the case (even if it is indeed generally
> possible to probe for the latter).
>
> An alternative might be to describe the efivars fw dependency in DT too
> (e.g. for device links), but I believe you have already expressed some
> concerns over that:
>
>         https://lore.kernel.org/lkml/20230130210530.GA3339716-robh@kernel.org/
>
> Johan