Re: [PATCH v4 5/5] rpi: Use the U-Boot control FDT for fdt_addr

[Christopher Obbard <christopher.obbard@linaro.org>]

Hi Simon,

[resending as I used the wrong e-mail address which isn't on the list,
oops]

On Thu, 2024-12-19 at 17:34 -0700, Simon Glass wrote:
> The fdt_addr variable is used in extlinux as a fallback devicetree if
> none is provided by the boot command. Otherwise the only use in U-
> Boot
> seems to me efi_install_fdt() when the internal FDT is required.
> 
> The existing mechanism uses the devicetree provided to U-Boot, but in
> its original, unrelocated position. In my testing on an rpi_4, this
> ends
> up at 2b35ef00 which is not a convenient place in memory, if the
> ramdisk
> is large.
> 
> U-Boot already deals with this sort of problem by relocating the FDT
> to a safe address.
> 
> So use the control-FDT address instead.
> 
> Remove the existing comment, which is confusing, since the FDT is not
> actually passed unmodified to the kernel: U-Boot adds various things
> using its FDT-fixup mechanism.
> 
> Note that board_get_usable_ram_top() reduces the RAM top for boards
> with
> less RAM. This behaviour is left unchanged as there is no other
> mechanism for U-Boot to handle this.
> 
> Signed-off-by: Simon Glass <sjg@chromium.org>

Reviewed-by: Christopher Obbard <christopher.obbard@linaro.org>
Tested-by: Christopher Obbard <christopher.obbard@linaro.org> # CM4 1G

> ---
> 
> Changes in v4:
> - Expand the comment on set_fdt_addr()
> - Update the commit message to talk in terms of my testing
> 
> Changes in v2:
> - Drop patch to allow expanding the devicetree during relocation
> 
>  board/raspberrypi/rpi/rpi.c | 20 ++++++++++----------
>  1 file changed, 10 insertions(+), 10 deletions(-)
> 
> diff --git a/board/raspberrypi/rpi/rpi.c
> b/board/raspberrypi/rpi/rpi.c
> index c46fe4b2350..f74006e0968 100644
> --- a/board/raspberrypi/rpi/rpi.c
> +++ b/board/raspberrypi/rpi/rpi.c
> @@ -3,6 +3,8 @@
>   * (C) Copyright 2012-2016 Stephen Warren
>   */
>  
> +#define LOG_CATEGORY	LOGC_BOARD
> +
>  #include <config.h>
>  #include <dm.h>
>  #include <env.h>
> @@ -326,18 +328,13 @@ static void set_fdtfile(void)
>  }
>  
>  /*
> - * If the firmware provided a valid FDT at boot time, let's expose
> it in
> - * ${fdt_addr} so it may be passed unmodified to the kernel.
> + * Allow U-Boot to use its control FDT with extlinux if one is not
> provided.
> + * This will then go through the usual fixups that U-Boot does,
> before being
> + * handed off to Linux
>   */
>  static void set_fdt_addr(void)
>  {
> -	if (env_get("fdt_addr"))
> -		return;
> -
> -	if (fdt_magic(fw_dtb_pointer) != FDT_MAGIC)
> -		return;
> -
> -	env_set_hex("fdt_addr", fw_dtb_pointer);
> +	env_set_hex("fdt_addr", (ulong)gd->fdt_blob);
>  }
>  
>  /*
> @@ -571,7 +568,10 @@ int ft_board_setup(void *blob, struct bd_info
> *bd)
>  {
>  	int node;
>  
> -	update_fdt_from_fw(blob, (void *)fw_dtb_pointer);
> +	if (blob == gd->fdt_blob)
> +		log_debug("Same FDT: nothing to do\n");
> +	else
> +		update_fdt_from_fw(blob, (void *)gd->fdt_blob);
>  
>  	node = fdt_node_offset_by_compatible(blob, -1, "simple-
> framebuffer");
>  	if (node < 0)