Re: RFC: Copying Device Tree File into reserved area of VMLINUX before deployment

[Frank Rowand]

Hi Ulf, Rob,

On 11/20/17 15:19, Ulf Samuelsson wrote:
> 
> 
> On 2017-11-20 05:32, Frank Rowand wrote:
>> Hi Ulf,
>>
>>
>> On 11/19/17 23:23, Frank Rowand wrote:
>>> adding devicetree list, devicetree maintainers
>>>
>>> On 11/18/17 12:59, Ulf Samuelsson wrote:
>>>> I noticed when checking out the OpenWRT support for the board that they have a method to avoid having to pass the device tree address to the kernel, and can thus boot device tree based kernels with U-boots that
>>>> does not support device trees.
>>>>
>>>> Is this something that would be considered useful for including in mainstream:
>>>>
>>>> BACKGROUND:
>>>> Trying to load a yocto kernel into a MIPS target (MT7620A based),
>>>> and the U-Boot is more than stupid.
>>>> Does not support the "run" command as an example.
>>>> They modified the U-Boot MAGIC Word to complicate things.
>>>> The U-Boot is not configured to use device tree files.
>>>> The board runs a 2.6 kernel right now.
>>>>
>>>> Several attempts by me a and others to rebuild U-Boot according to
>>>> the H/W vendors source code and build instructions results in a
>>>> bricked unit. Bricked units cannot be recovered.
>>
>> Hopefully you have brought this to the attention of the vendor. U-Boot
>> is GPL v2 (or in some ways possibly GPL v2 or later), so if you can not
>> build U-Boot that is equivalent to the binary U-Boot they shipped, the
>> vendor may want to ensure that they are shipping the proper source and
>> build instructions.
>>
> 
> I am not the one in contact with the H/W vendor.
> The U-boot is pretty old, and from comments from those
> in contact with them, the U-Boot knowledge at the H/W vendor
> is minimal at best.
> It might even be that they program an U-boot where the upgrade of the U-boot is broken...
> 
> 
>>
>>>> Not my choice of H/W, so I cannot change it.
>>>>
>>>>
>>>> ===================================================================
>>>> OPENWRT:
>>>> I noticed when checking out the OpenWRT support for the board that
>>>> they have a method to avoid having to pass the device tree address
>>>> to the kernel, and can thus boot device tree based kernels with
>>>> U-boots that does not support device trees.
>>>>
>>>> What they do is to reserve 16 kB of kernel space, and tag it with
>>>> an ASCII string "OWRTDTB:". After the kernel and dtb is built, a
>>>> utility "patch-dtb" will update the vmlinux binary, copying in the
>>>> device tree file.
>>>>
>>>> ===================================================================
>>>> It would be useful to me, and I could of course patch the
>>>> mainstream kernel, but first I would like to check if this is of
>>>> interest for mainstream.
>>
>> Not in this form. Hard coding a fixed size area in the boot image
>> to contain the FDT (aka DTB) is a non-starter.
> 
> OK, Is it the fixed size, which is a problem?

Yes, it is the fixed size which is a problem.

> Is generally combining an image with a DTB into a single file also a non-starter?

Can you jump in here Rob?  My understanding is that CONFIG_ARM_APPENDED_DTB,
which is the ARM based solution that Mark mentioned, was envisioned as a
temporary stop gap until boot loaders could add devicetree support.  I don't
know if there is a desire to limit this approach or to remove it in the
future.

I'm not sure why this feature should not be permanently supported.  I'm being
cautious, just in case I'm overlooking or missing an important issue, thus
asking for Rob's input.  I do know that this feature does not advance the
desires of people who want a single kernel (single boot image?) that runs on
many different systems, instead of a boot image that is unique to each
target platform.  But I don't see why that desire precludes also having
an option to have a target specific boot image.

-Frank


>>
>> And again, I would first approach the H/W vendor before trying to
>> come up with a work around like this.
>>
>>
>>>> I envisage the support would look something like:
>>>>
>>>> ============
>>>> Kconfig.
>>>> config MIPS
>>>> ÂÂÂÂÂselectÂÂÂ HAVE_IMAGE_DTB
>>>>
>>>> configÂÂÂ HAVE_IMAGE_DTB
>>>> ÂÂÂÂÂbool
>>>>
>>>> if HAVE_IMAGE_DTB
>>>> configÂÂÂÂ IMAGE_DTB
>>>> ÂÂÂÂÂboolÂÂÂ "Allocated space for DTB within image
>>>>
>>>> configÂÂÂ DTB_SIZE
>>>> ÂÂÂÂÂintÂÂÂ "DTB space (kB)
>>>>
>>>> configÂÂÂ DTB_TAG
>>>> ÂÂÂÂÂstringÂÂÂ "DTB space tag"
>>>> ÂÂÂÂÂdefaultÂÂÂ "OWRTDTB:"
>>>> endif
>>>>
>>>> ============
>>>> Some Makefile
>>>> obj-$(CONFIG_INCLUDE_DTB) += image_dtb.o
>>>>
>>>> ============
>>>> image_dtb.S:
>>>> ÂÂÂÂÂ.text
>>>> ÂÂÂÂÂ.alignÂÂÂ 5
>>>> ÂÂÂÂÂ.asciiÂÂÂ CONFIG_DTB_TAG
>>>> ÂÂÂÂÂEXPORT(__image_dtb)
>>>> ÂÂÂÂÂ.fillÂÂÂ DTB_SIZE * 1024
>>>>
>>>> ===================
>>>> arch/mips/xxx/of.c:
>>>>
>>>> #ifÂÂÂ defined(CONFIG_IMAGE_DTB)
>>>> ÂÂÂÂÂif (<conditions to boot from dtb_space>)
>>>> ÂÂÂÂÂÂÂÂ __dt_setup_arch(__dtb_start);
>>>> ÂÂÂÂÂelse
>>>> ÂÂÂÂÂÂÂÂ __dt_setup_arch(&__image_dtb);
>>>> #else
>>>> ÂÂÂÂÂ__dt_setup_arch(__dtb_start);
>>>> #endif
>>>>
>>>> I imagine that if the support is enabled for a target, it should
>>>> be possible to override it with a CMDLINE argument
>>>> ÂÂÂÂ ÂÂÂÂ They do something similar for the CMDLINE; copying it into the vmlinux, to allow a smaller boot
>