linux-kernel - Re: [PATCH] of/fdt: Ignore disabled memory nodes

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Message-ID: <CAFEAcA_DRoJmnFdyEEcSvCxtPYignZFqQFnHyWkcpyijCBSrCg@mail.gmail.com>
Date:   Tue, 17 May 2022 20:19:47 +0100
From:   Peter Maydell <peter.maydell@...aro.org>
To:     Rob Herring <robh@...nel.org>
Cc:     Andre Przywara <andre.przywara@....com>,
        Frank Rowand <frowand.list@...il.com>,
        devicetree@...r.kernel.org,
        linux-arm-kernel <linux-arm-kernel@...ts.infradead.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        Ross Burton <ross.burton@....com>,
        Ard Biesheuvel <ardb@...nel.org>,
        Catalin Marinas <catalin.marinas@....com>,
        Will Deacon <will@...nel.org>,
        Russell King <linux@...linux.org.uk>
Subject: Re: [PATCH] of/fdt: Ignore disabled memory nodes

On Tue, 17 May 2022 at 18:48, Rob Herring <robh@...nel.org> wrote:
>
> On Tue, May 17, 2022 at 11:54 AM Peter Maydell <peter.maydell@...aro.org> wrote:
> >
> > On Tue, 17 May 2022 at 16:34, Rob Herring <robh@...nel.org> wrote:
> > >
> > > On Tue, May 17, 2022 at 11:14:10AM +0100, Andre Przywara wrote:
> > > > When we boot a machine using a devicetree, the generic DT code goes
> > > > through all nodes with a 'device_type = "memory"' property, and collects
> > > > all memory banks mentioned there. However it does not check for the
> > > > status property, so any nodes which are explicitly "disabled" will still
> > > > be added as a memblock.
> > > > This ends up badly for QEMU, when booting with secure firmware on
> > > > arm/arm64 machines, because QEMU adds a node describing secure-only
> > > > memory:
> > > > ===================
> > > >       secram@...0000 {
> > >
> > > BTW, 'memory' is the correct node name.
> >
> > We already have a 'memory' node, which is for the NS
> > memory. This one's for the secure-only RAM block,
> > which is why I gave it a name that hopefully helps in
> > spotting that when a human is reading the DT.
>
> You can do: secram: memory@...0000 {
>
> Where 'secram' is only a source level label until overlays come into
> the picture.

We generate the DTB with libfdt, so source-only information
isn't something we can put in, I think. (The quoted DT fragment
in this patch's commit message is the result of decompiling
the runtime generated DT binary blob with dtc.)

> > I'm not really sure to what extent node names in device trees are
> > "this is just an identifying textual label" and to what extent
> > they are "this is really ABI and you need to follow the standard",
> > though -- nothing in practice seems to care what they are,
> > suggesting the "textual label" theory, but some bits of tooling
> > complain if you do things like forget the address value or use the
> > same address for two different nodes, suggesting the "really ABI"
> > theory.
>
> Node names are supposed to follow the class of device and there's a
> list of established names in the spec.
>
> Sometimes it's ABI and sometimes not. Much of it is just good hygiene.
> memory nodes are also special because 'device_type' is used to
> identify them, but device_type is generally deprecated for FDT as its
> meaning in OpenFirmware doesn't apply (it defines what callable
> methods exist). We could use the nodename (without unit address)
> instead, but that would fail in some cases as other names have been
> used.

This seems kind of odd to me as a design, compared to
"have the node have a property that says what it is
and let the name of the node just be, well, its name"
(especially since 'device_type' and 'compatible' look an
awful lot like "this is the property that tells you what this
node actually is".)
Are we just stuck with what we have for historical reasons ?

-- PMM