linux-kernel - Re: [PATCH 2/4] efi/libstub: detect panel-id

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Message-ID: <CAKv+Gu-KhPJxxJA3+J813OPcnoAD4nHq6MhiRTJSd_5y1dPNnw@mail.gmail.com>
Date:   Tue, 2 Jul 2019 22:35:16 +0200
From:   Ard Biesheuvel <ard.biesheuvel@...aro.org>
To:     Rob Clark <robdclark@...il.com>
Cc:     dri-devel <dri-devel@...ts.freedesktop.org>,
        linux-arm-msm <linux-arm-msm@...r.kernel.org>,
        freedreno@...ts.freedesktop.org, aarch64-laptops@...ts.linaro.org,
        Rob Clark <robdclark@...omium.org>,
        Ingo Molnar <mingo@...nel.org>, Will Deacon <will@...nel.org>,
        Leif Lindholm <leif.lindholm@...aro.org>,
        Alexander Graf <agraf@...e.de>,
        Steve Capper <steve.capper@....com>,
        Lukas Wunner <lukas@...ner.de>,
        Julien Thierry <julien.thierry@....com>,
        linux-efi <linux-efi@...r.kernel.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH 2/4] efi/libstub: detect panel-id

On Tue, 2 Jul 2019 at 22:26, Ard Biesheuvel <ard.biesheuvel@...aro.org> wrote:
>
> On Sun, 30 Jun 2019 at 22:36, Rob Clark <robdclark@...il.com> wrote:
> >
> > From: Rob Clark <robdclark@...omium.org>
> >
> > On snapdragon aarch64 laptops, a 'UEFIDisplayInfo' variable is provided
> > to communicate some information about the display.  Crutially it has the
> > panel-id, so the appropriate panel driver can be selected.  Read this
> > out and stash in /chosen/panel-id so that display driver can use it to
> > pick the appropriate panel.
> >
> > Signed-off-by: Rob Clark <robdclark@...omium.org>
>
> Hi Rob,
>
> I understand why you are doing this, but this *really* belongs elsewhere.
>
> So we are dealing with a platform that violates the UEFI spec, since
> it does not bother to implement variable services at runtime (because
> MS let the vendor get away with this).
>

To clarify, the above remark applies to populating the DT from the OS
rather than from the firmware.

> First of all, to pass data between the EFI stub and the OS proper, we
> should use a configuration table rather than a DT property, since the
> former is ACPI/DT agnostic. Also, I'd like the consumer of the data to
> actually interpret it, i.e., just dump the whole opaque thing into a
> config table in the stub, and do the parsing in the OS proper.
>
> However, I am not thrilled at adding code to the stub that
> unconditionally looks for some variable with some magic name on all
> ARM/arm64 EFI systems, so this will need to live under a Kconfig
> option that depends on ARM64 (and does not default to y)
>

... but saving variables at boot time for consumption at runtime is
something that we will likely see more of in the future.