linux-kernel - Re: [PATCH 0/4] drm+dt+efi: support devices with multiple possible panels

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Date:   Sun, 30 Jun 2019 14:05:21 -0700
From:   Rob Clark <robdclark@...il.com>
To:     Laurent Pinchart <laurent.pinchart@...asonboard.com>
Cc:     dri-devel <dri-devel@...ts.freedesktop.org>,
        linux-arm-msm <linux-arm-msm@...r.kernel.org>,
        freedreno <freedreno@...ts.freedesktop.org>,
        aarch64-laptops@...ts.linaro.org,
        Rob Clark <robdclark@...omium.org>,
        Ard Biesheuvel <ard.biesheuvel@...aro.org>,
        Catalin Marinas <catalin.marinas@....com>,
        "open list:OPEN FIRMWARE AND FLATTENED DEVICE TREE BINDINGS" 
        <devicetree@...r.kernel.org>, Ingo Molnar <mingo@...nel.org>,
        Julien Thierry <julien.thierry@....com>,
        "open list:EXTENSIBLE FIRMWARE INTERFACE (EFI)" 
        <linux-efi@...r.kernel.org>,
        open list <linux-kernel@...r.kernel.org>,
        Lukas Wunner <lukas@...ner.de>,
        Steve Capper <steve.capper@....com>,
        Will Deacon <will@...nel.org>
Subject: Re: [PATCH 0/4] drm+dt+efi: support devices with multiple possible panels

On Sun, Jun 30, 2019 at 1:47 PM Laurent Pinchart
<laurent.pinchart@...asonboard.com> wrote:
>
> Hi Rob,
>
> Thank you for the patch.
>
> On Sun, Jun 30, 2019 at 01:36:04PM -0700, Rob Clark wrote:
> > From: Rob Clark <robdclark@...omium.org>
> >
> > Now that we can deal gracefully with bootloader (firmware) initialized
> > display on aarch64 laptops[1], the next step is to deal with the fact
> > that the same model of laptop can have one of multiple different panels.
> > (For the yoga c630 that I have, I know of at least two possible panels,
> > there might be a third.)
>
> I have to ask the obvious question: why doesn't the boot loader just
> pass a correct DT to Linux ? There's no point in passing a list of
> panels that are not there, this seems quite a big hack to me. A proper
> boot loader should construct the DT based on hardware detection.

Hi Laurent,

Actually the bootloader on these devices is passing *no* dt (they boot
ACPI, we are loading dtb from grub currently)

I think normally a device built w/ dt in mind would populate
/chosen/panel-id directly (rather than the way it is currently
populated based on reading an efi variable prior to ExitBootServices).
But that is considerably easier ask than having it re-write of_graph
bindings.  Either way, we aren't in control of the bootloader on these
devices, so it is a matter of coming up with something that works on
actual hw that we don't like rather than idealized hw that we don't
have ;-)

BR,
-R