Subject: Re: [PATCH v2] ARM: tegra: Restore DT ABI on Tegra124 Chromebooks

From: Thierry Reding <[email-masked]>
Date: Mon, 18 Feb 2019 16:13:57 +0100

On Mon, Feb 11, 2019 at 08:44:52PM +0000, Tristan Bastian wrote:
> Thierry Reding – Mon, 11. February 2019 12:09
> > From: Thierry Reding <[email-masked]>
> > 
> > Commit 482997699ef0 ("ARM: tegra: Fix unit_address_vs_reg DTC warnings
> > for /memory") inadventently broke device tree ABI by adding a unit-
> > address to the "/memory" node because the device tree compiler flagged
> > the missing unit-address as a warning.
> > 
> > Tegra124 Chromebooks (a.k.a. Nyan) use a bootloader that relies on the
> > full name of the memory node in device tree being exactly "/memory". It
> > can be argued whether this was a good decision or not, and some other
> > bootloaders (such as U-Boot) do accept a unit-address in the name of the
> > node, but the device tree is an ABI and we can't break existing setups
> > just because the device tree compiler considers it bad practice to omit
> > the unit-address nowadays.
> > 
> > This partially reverts the offending commit and restores device tree ABI
> > compatibility.
> > 
> > Fixes: 482997699ef0 ("ARM: tegra: Fix unit_address_vs_reg DTC warnings for
> > /memory")
> > Reported-by: Tristan Bastian <[email-masked]>
> > Signed-off-by: Thierry Reding <[email-masked]>
> > ---
> > Changes in v2:
> > - remove memory@80000000 node pulled in from tegra124.dtsi
> > - add device_type = "memory" property
> > 
> > arch/arm/boot/dts/tegra124-nyan.dtsi | 17 ++++++++++++++++-
> > 1 file changed, 16 insertions(+), 1 deletion(-)
> > 
> > diff --git a/arch/arm/boot/dts/tegra124-nyan.dtsi
> > b/arch/arm/boot/dts/tegra124-nyan.dtsi
> > index d5f11d6d987e..bc85b6a166c7 100644
> > --- a/arch/arm/boot/dts/tegra124-nyan.dtsi
> > +++ b/arch/arm/boot/dts/tegra124-nyan.dtsi
> > @@ -13,10 +13,25 @@
> > stdout-path = "serial0:115200n8";
> > };
> > 
> > - memory@80000000 {
> > + /*
> > + * Note that recent version of the device tree compiler (starting with
> > + * version 1.4.2) warn about this node containing a reg property, but
> > + * missing a unit-address. However, the bootloader on these Chromebook
> > + * devices relies on the full name of this node to be exactly /memory.
> > + * Adding the unit-address causes the bootloader to create a /memory
> > + * node and write the memory bank configuration to that node, which in
> > + * turn leads the kernel to believe that the device has 2 GiB of
> > + * memory instead of the amount detected by the bootloader.
> > + *
> > + * The name of this node is effectively ABI and must not be changed.
> > + */
> > + memory {
> > + device_type = "memory";
> > reg = <0x0 0x80000000 0x0 0x80000000>;
> > };
> > 
> > + /delete-node/ memory@80000000;
> > +
> > host1x@50000000 {
> > hdmi@54280000 {
> > status = "okay";
> > -- 
> > 2.19.1
> 
> Thanks Thierry
> 
> Tristan
> 
> Tested-by: Tristan Bastian <[email-masked]>

Hi ARM SoC maintainers,

I had almost forgotten about this. Ideally I'd like this to go into v5.0
since it fixes a regression introduced in v4.19, so we'd keep the number
of versions that it needs to get backported to fairly low.

The patchwork link is here:

	http://patchwork.ozlabs.org/patch/1039743/

I don't have any other fixes for v5.0, so I don't think sending a pull
request would help you very much, but I can prepare one if you prefer.

Thanks,
Thierry
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