Single device tree blob between linux and u-boot

[Cédric Le Goater]

On 10/13/2016 09:37 PM, Rick Altherr wrote:
> I've been working on signed kernel images which depends on building 
> FIT images that include the kernel, initrd, and dtb.  The kernel_fitimage 
> branch of my tree (https://github.com/kc8apf/openbmc) will build an 
> unsigned FIT image by including the dtb specified in the machine config 
> (usually from the kernel).  There are also provisions in there for u-boot 
> being built in two stages so a control dtb that includes the public signing 
> keys can be combined into the final u-boot binary.  From how that is structure, 
> I suspect Maxim is right that there will be some differences between u-boot's 
> dtb and linux's dtb.  That shouldn't be an issue as the u-boot dtb gets built 
> into the u-boot image while the kernel dtb is loaded from the FIT image.

No it shouldn't be too problematic. 

let's get going and see what a first draft would look like. We would need 
a first driver using a very minimal u-boot dtb to start with and I suppose 
it will be the I2C one. Could you build a uboot tree with both so we can 
experiment ? 

We need to make sure that a new uboot boots an old image also.

Thanks,

C. 



> Rick
> 
> On Thu, Oct 13, 2016 at 11:41 AM, Maxim Sloyko <maxims at google.com <mailto:maxims at google.com>> wrote:
> 
> 
> 
>     On Thu, Oct 13, 2016 at 11:20 AM, Xo Wang <xow at google.com <mailto:xow at google.com>> wrote:
> 
>         Hi folks,
> 
>         I saw Cedric had mentioned "...you can use the linux compiled [dtb]
>         and dd it at the end of the u-boot
>         partition..."
> 
>         To provide the device tree to u-boot, should we do what Cedric said
>         and also pass the address of the DTB from U-Boot to Linux? According
>         to these slides:
> 
>         https://events.linuxfoundation.org/sites/events/files/slides/petazzoni-device-tree-dummies.pdf <https://events.linuxfoundation.org/sites/events/files/slides/petazzoni-device-tree-dummies.pdf>
> 
>         U-Boot can pass the DTB address to Linux in r2 (p. 6) with the right
>         bootm command parameters.
> 
>         This means we can have U-Boot and Linux use the same DTB binary (I
>         prefer built in Linux) rather than each source tree building one.
> 
> 
>     This introduces the dependency of U-Boot on Linux. Do we plan to keep this dependency forever? This may be fine with OpenBMC, but what about mainline U-Boot and Linux?
>     Also, are we sure that they are 100% compatible? IIRC Peter was saying that in practice you need two different device trees for U-Boot and Linux.
>      
> 
> 
>         Then we can remove the CONFIG_ARM_APPENDED_DTB option in our Linux
>         build that is described as a legacy "compatibility" mechanism. We also
>         avoid having an extra copy of the DTSs in U-Boot source and avoid
>         duplicate DTBs in our image.
> 
>         Thoughts?
> 
>         cheers
>         xo
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> 
> 
> 
>     -- 
>     *M*axim *S*loyko
> 
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