Date:   Mon, 20 Nov 2017 21:19:26 +0100
From:   Ulf Samuelsson <linux-kernel@...gii.com>
To:     Frank Rowand <frowand.list@...il.com>,
        LKML <linux-kernel@...r.kernel.org>,
        "devicetree@...r.kernel.org" <devicetree@...r.kernel.org>,
        Rob Herring <robh+dt@...nel.org>
Subject: Re: RFC: Copying Device Tree File into reserved area of VMLINUX
 before deployment



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


> 
> 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
>>>      select    HAVE_IMAGE_DTB
>>>
>>> config    HAVE_IMAGE_DTB
>>>      bool
>>>
>>> if HAVE_IMAGE_DTB
>>> config     IMAGE_DTB
>>>      bool    "Allocated space for DTB within image
>>>
>>> config    DTB_SIZE
>>>      int    "DTB space (kB)
>>>
>>> config    DTB_TAG
>>>      string    "DTB space tag"
>>>      default    "OWRTDTB:"
>>> endif
>>>
>>> ============
>>> Some Makefile
>>> obj-$(CONFIG_INCLUDE_DTB) += image_dtb.o
>>>
>>> ============
>>> image_dtb.S:
>>>      .text
>>>      .align    5
>>>      .ascii    CONFIG_DTB_TAG
>>>      EXPORT(__image_dtb)
>>>      .fill    DTB_SIZE * 1024
>>>
>>> ===================
>>> arch/mips/xxx/of.c:
>>>
>>> #if    defined(CONFIG_IMAGE_DTB)
>>>      if (<conditions to boot from dtb_space>)
>>>          __dt_setup_arch(__dtb_start);
>>>      else
>>>          __dt_setup_arch(&__image_dtb);
>>> #else
>>>      __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
>>>      
>>>      
>>> They do something similar for the CMDLINE; copying it into the vmlinux, to allow a smaller boot

-- 
Best Regards
Ulf Samuelsson

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