[U-Boot] [PATCH 1/2] ARM: rpi: update memory layout env. var. documentation

[Stephen Warren <swarren@wwwdotorg.org>]

On 02/06/2016 12:30 AM, Jonathan Liu wrote:
> Hi Stephen,
> 
> I actually read the DT loaded by RPi's binary firmware on an RPi 2 in a
> U-Boot script:
> fdt addr ${fdt_addr_r} && fdt get value bootargs /chosen bootargs
> fatload mmc 0:1 ${kernel_addr_r} uImage
> bootm ${kernel_addr_r} - ${fdt_addr_r}
> 
> Essentially this loads the kernel with the same arguments and DT that
> RPi's binary firmware would have used if it booted the kernel directly
> with device tree support. This allows for the normal patching of the
> kernel arguments and device tree to be done by the RPi binary firmware
> so that things like reading the serial number in /proc/cpuinfo works.
> 
> A trailer is added to u-boot.bin with "mkknlimg --dtok u-boot.bin
> u-boot.bin" for the FW to enable device tree support and load the
> patched device tree to 0x00000100.
> 
> So I am not sure about the comment that the DT loaded by the FW is
> typically ignored by U-Boot scripts.

This is a very unusual use-case. Typically the reason for using U-Boot
in the first place is so that U-Boot has full control over the kernel,
DT, and command-line. This way, users can configure all these aspects
the exact same way on an RPi running U-Boot as on any other system
running U-Boot. Mixing configuration between config.txt and the
scripts/config-files that U-Boot reads/executes isn't typical, since it
involves board-specific config file. As such, I believe the comment is
correct for the common case, and already admits that other cases are
possible.