[PATCH 2/3] doc: environment: Expand on fdt_addr, initrd_addr and loadaddr

[Heinrich Schuchardt]

On 6/20/22 16:31, Tom Rini wrote:
> - Explain why fdt_addr and initrd_addr should not be set to disable
>    relocation normally.
> - Provide some advice on the typical loadaddr default value.
>
> Signed-off-by: Tom Rini <trini at konsulko.com>
> ---
>   doc/usage/environment.rst | 15 ++++++++++++---
>   1 file changed, 12 insertions(+), 3 deletions(-)
>
> diff --git a/doc/usage/environment.rst b/doc/usage/environment.rst
> index a3f23d4e4e22..a9a4702632d2 100644
> --- a/doc/usage/environment.rst
> +++ b/doc/usage/environment.rst
> @@ -204,7 +204,9 @@ fdt_high
>       to work it must reside in writable memory, have
>       sufficient padding on the end of it for u-boot to
>       add the information it needs into it, and the memory
> -    must be accessible by the kernel.
> +    must be accessible by the kernel.  This usage is strongly discouraged
> +    however as it also stops U-Boot from ensuring the device tree starting
> +    address is properly aligned and a misaligned tree will cause OS failures.

fdt_addr_r is typically 4 byte aligned. Is there a bug in some part of
the code?

>
>   fdtcontroladdr
>       if set this is the address of the control flattened
> @@ -240,14 +242,21 @@ initrd_high
>       memory. In this case U-Boot will NOT COPY the
>       ramdisk at all. This may be useful to reduce the
>       boot time on your system, but requires that this
> -    feature is supported by your Linux kernel.
> +    feature is supported by your Linux kernel.  This usage however requires
> +    that the user ensure that there will be no overlap with other parts of the

It is not the user but the developer who define $kernel_addr_r,
$initr_addr_r, and $fdt_addr_r.

Relocating initrd does not stop the user from loading the kernel to
initrd_high. The user is always responsible for avoiding overlap. In
this regard there is nothing special about initrd_high=~0UL.

> +    iamge such as the Linux kernel BSS.  It should not be enabled by default

%s/iamge/image

> +    and only done as part of optimizing a deployment.

Relocating initrd and fdt is using boot time. On many boards it is
simply superfluous. On these boards I cannot see any reason not to
disable it by default.

Best regards

Heinrich

>
>   ipaddr
>       IP address; needed for tftpboot command
>
>   loadaddr
>       Default load address for commands like "bootp",
> -    "rarpboot", "tftpboot", "loadb" or "diskboot"
> +    "rarpboot", "tftpboot", "loadb" or "diskboot".  Note that the optimal
> +    default values here will vary between architectures.  On 32bit ARM for
> +    example, some offset from start of memory is used as the Linux kernel
> +    zImage has a self decompressor and it's best if we stay out of where that
> +    will be working.
>
>   loads_echo
>       see CONFIG_LOADS_ECHO