Re: [PATCH v2] ARM: tegra: add Acer Chromebook 13 device tree

[Andrew Bresticker <abrestic-F7+t8E8rja9g9hUCZPvPmw@public.gmane.org>]

On Mon, Aug 18, 2014 at 4:43 PM, Stephen Warren <swarren-3lzwWm7+Weoh9ZMKESR00Q@public.gmane.org> wrote:
> On 08/18/2014 05:24 PM, Andrew Bresticker wrote:
>>
>> On Mon, Aug 18, 2014 at 9:10 AM, Stephen Warren <swarren@wwwdotorg.org>
>> wrote:
>>>
>>> On 08/16/2014 09:20 AM, Andreas Färber wrote:
>>>>
>>>>
>>>> Hi,
>>>>
>>>> Am 13.08.2014 21:14, schrieb Dylan Reid:
>>>>>
>>>>>
>>>>> The Acer Chromebook 13, codenamed Big, contains an NVIDIA tegra124
>>>>> processor and is similar to the Venice2 reference platform.
>>>>>
>>>>> The keyboard, USB 2, audio, HDMI, sdcard, and emmc have been tested
>>>>> and work on the 1266x768 models.  The HD models haven't yet been
>>>>> tested.
>>>>>
>>>>> WiFi does not work yet, it needs at least some PMIC changes to enable
>>>>> the 32k clock.
>>>>>
>>>>> The elan trackpad is not yet functional but hopefully will be soon as
>>>>> there are patches under review.
>>>>>
>>>>> There is also an issue on reboot because the TPM isn't reset.  It will
>>>>> cause the stock firmware to enter recovery mode.  This can be worked
>>>>> around by an EC-reset, press the refresh and power keys at the same
>>>>> time.
>>>
>>>
>>>
>>>>> diff --git a/arch/arm/boot/dts/tegra124-nyan-big.dts
>>>>> b/arch/arm/boot/dts/tegra124-nyan-big.dts
>>>>> new file mode 100644
>>>>> index 0000000..79f1852
>>>>> --- /dev/null
>>>>> +++ b/arch/arm/boot/dts/tegra124-nyan-big.dts
>>>>> @@ -0,0 +1,1136 @@
>>>>> +/dts-v1/;
>>>>> +
>>>>> +#include <dt-bindings/input/input.h>
>>>>> +#include "tegra124.dtsi"
>>>>> +
>>>>> +/ {
>>>>> +       model = "Acer Chromebook 13";
>>>>> +       compatible = "google,nyan-big", "nvidia,tegra124";
>>>>
>>>>
>>>>
>>>> In light of v1 and the above commit message referring to this as Google
>>>> Big, shouldn't this be "google,big", "nvidia,tegra124" and optionally
>>>> "google,nyan" as secondary string, independent of the new file name?
>>>
>>>
>>>
>>> Despite this board having been derived from Nyan, it isn't Nyan, so I
>>> don't
>>> think Nyan should be part of any compatible value, nor a separate
>>> compatible
>>> value.
>>
>>
>> "google,nyan-big" is the compatible string that the bootloader on
>> these devices looks for.  It's also the convention we are now using
>> for our ARM devices, as Olof has already mentioned.
>
>
> I don't understand "that the bootloader looks for"; why is the bootloader
> doing anything w.r.t the compatible value in the DT that's passed to the
> kernel.

The ChromeOS bootloader ("depthcharge") boots FIT images and selects
the appropriate device-tree from the image based on the compatible
string.  On Big boards, it looks for "google,nyan-big".  This only
becomes an issue if there are multiple device-trees in the FIT image.
If there's only a single configuration (or you chain-load U-Boot),
then the bootloader will boot with that configuration.  If you build a
kernel image in the ChromiumOS environment, however, you'll end up
with multiple device-trees (all those built by the kernel config at
least) in the FIT image - this is what allows us to boot the same
binary on different boards.