linux-kernel - Re: [PATCH v2 1/3] devicetree: bindings: Document qcom,msm-id and qcom,board-id

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Message-ID: <CAJAp7OiVwvPjRbrBxw7Dndvk4Apwz_bx=CzSqeqd3JzYhmB7Kw@mail.gmail.com>
Date:	Wed, 11 Mar 2015 08:33:04 -0700
From:	Bjorn Andersson <bjorn@...o.se>
To:	Kumar Gala <galak@...eaurora.org>
Cc:	Arnd Bergmann <arnd@...db.de>, Olof Johansson <olof@...om.net>,
	Kevin Hilman <khilman@...nel.org>,
	linux-arm-msm <linux-arm-msm@...r.kernel.org>,
	"linux-arm-kernel@...ts.infradead.org" 
	<linux-arm-kernel@...ts.infradead.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	"arm@...nel.org" <arm@...nel.org>,
	"devicetree@...r.kernel.org" <devicetree@...r.kernel.org>,
	Heiko Stübner <heiko@...ech.de>
Subject: Re: [PATCH v2 1/3] devicetree: bindings: Document qcom,msm-id and qcom,board-id

On Tue, Mar 10, 2015 at 12:57 PM, Kumar Gala <galak@...eaurora.org> wrote:
>
> On Mar 10, 2015, at 2:52 PM, Arnd Bergmann <arnd@...db.de> wrote:
>
>> On Tuesday 10 March 2015 13:10:08 Kumar Gala wrote:
>>>>>>>>>>>> The top level qcom,msm-id and qcom,board-id are utilized by bootloaders
>>>>>>>>>>>>> on Qualcomm MSM platforms to determine which device tree should be
>>>>>>>>>>>>> utilized and passed to the kernel.
>>>>>>>>>>>>>
>>>>>>>>>>>>> Cc: <devicetree@...r.kernel.org>
>>>>>>>>>>>>> Signed-off-by: Kumar Gala <galak@...eaurora.org>
>>>>>>>>>>>>
>>>>>>>>>>>> Is this the special magic that allows qcom bootloaders to take a kernel
>>>>>>>>>>>> plus multiple DTBs and figure out which DTB to pass?
>>>>>>>>>>>>
>>>>>>>>>>>> Kevin
>>>>>>>>>>>
>>>>>>>>>>> yes
>>>>>>>>>>
>>>>>>>>>> That's a bummer.
>>>>>>>>>>
>>>>>>>>>> Luckily, the solution for upstream is still quite simple: Provide only
>>>>>>>>>> one devicetree, and it'll be used, right?
>>>>>>>>>
>>>>>>>>> We can provide only one, we still need the IDs in the DT.
>>>>>>>>
>>>>>>>> How are the DTS provided? Concatenated with the kernel, or in a
>>>>>>>> wrapped data format? Or in a separate partition from the kernel?
>>>>>>>
>>>>>>> Its a wrapped data format that is than concatenated with the kernel if I remember correctly.
>>>>>>
>>>>>> Then you should be able to create a tool that can write this concatenated
>>>>>> format and insert these properties from a table that matches the boot
>>>>>> loader, right?
>>>>>>
>>>>>>     Arnd
>>>>>
>>>>> Are you suggesting the tool insert the properties in the DT?  I’m not sure I understand what the point of doing that would be.
>>>>
>>>> To insert platform-local properties that mean nothing outside of the
>>>> firmware packaging of the device trees, which is the case here?
>>>>
>>>> I think the idea of having the installer script inserting them is
>>>> quite reasonable in this case.
>>>>
>>>>
>>>> -Olof
>>>
>>> These values are static, so not sure what the point of having the installer script do that?
>>>
>>
>> It combines two hacks to work around a nonstandard boot loader.
>> Once the bootloader is fixed, you can stop using that script.
>>
>>       Arnd
>
> I feel as if I’m missing something here.  If we just have the properties in the .dts files I don’t need to change anything ever, no script, no worries about working with old or new boot loaders, etc.
>

But are these values actually used by the boot loader?

As far as I could see in 8974 the dtbTool provided by Qualcomm
extracts the values from the list of dtbs and use them to create the
table-of-content in the QCDT blob. The boot loader then looks in this
TOC to pick the right dtb to use.

Regards,
Bjorn
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