[PATCH] arm: init: add an option to use FDT from previous bootloader

[Caleb Connolly]

On 24/10/2023 19:03, Simon Glass wrote:
> Hi Caleb,
> 
> On Tue, 24 Oct 2023 at 04:32, Caleb Connolly <caleb.connolly at linaro.org> wrote:
>>
>> Add a new config option to allow u-boot to reuse the FDT provided by the
> 
> U-Boot (please fix throughout)

Will do!
> 
>> previous stage bootloader when available.
>>
>> On some boards the previous stage bootloader can populate
>> platform-specific parts of the devicetree such as the memory node, this
>> allows us to avoid hardcoding it in u-boot and instead determine it
>> dynamically at runtime.
>>
>> Signed-off-by: Caleb Connolly <caleb.connolly at linaro.org>
>> ---
>> This patch will improve generic support for Qualcomm boards by enabling
>> us to configure the memory map at runtime rather than having hardcoded
>> maps on a per-device basis. I've gone for this approach initially to try
>> and avoid introducing board specific code where possible, but I'm happy
>> to rework this into mach-snapdragon if that's preferred.
> 
> Do you think it could use bloblist instead? I did send a patch to use
> the FDT in the bloblist:

If it would require changes to the previous stage bootloader then
unfortunately not. We're mostly stuck with the behaviour that we've got
for now...

This mechanism of retrieving the DTB is also used on the apple M1 I
think, and any other board where we're booting U-Boot as a drop-in for
the kernel on arm64.
> 
> https://patchwork.ozlabs.org/project/uboot/patch/20230926141514.2101787-40-sjg@chromium.org/
> 
> Regards,
> Simon
> 
>> ---
>> base-commit: e65b5d35c9116485366bb08138043d51220551da
>>
>> // Caleb (they/them)
>> ---
>>  arch/arm/lib/save_prev_bl_data.c |  7 +++++++
>>  boot/Kconfig                     | 10 ++++++++++
>>  include/init.h                   |  9 +++++++++
>>  lib/fdtdec.c                     |  7 ++++++-
>>  4 files changed, 32 insertions(+), 1 deletion(-)
>>
>> diff --git a/arch/arm/lib/save_prev_bl_data.c b/arch/arm/lib/save_prev_bl_data.c
>> index f7b23faf0d66..b0608502535e 100644
>> --- a/arch/arm/lib/save_prev_bl_data.c
>> +++ b/arch/arm/lib/save_prev_bl_data.c
>> @@ -45,6 +45,13 @@ bool is_addr_accessible(phys_addr_t addr)
>>         return false;
>>  }
>>
>> +phys_addr_t get_prev_bl_fdt_addr(void)
>> +{
>> +       if (!is_addr_accessible((phys_addr_t)reg0))
>> +               return 0;
>> +       return reg0;
>> +}
>> +
>>  int save_prev_bl_data(void)
>>  {
>>         struct fdt_header *fdt_blob;
>> diff --git a/boot/Kconfig b/boot/Kconfig
>> index a01e6cb8aafe..c127ba254589 100644
>> --- a/boot/Kconfig
>> +++ b/boot/Kconfig
>> @@ -1599,6 +1599,16 @@ config SAVE_PREV_BL_INITRAMFS_START_ADDR
>>           If no initramfs was provided by previous bootloader, no env variables
>>           will be created.
>>
>> +config USE_PREV_BL_FDT
>> +       depends on SAVE_PREV_BL_FDT_ADDR && !OF_BOARD
>> +       bool "Use the FDT provided by the previous stage bootloader"
>> +       help
>> +         When u-boot is chain-loaded from a previous bootloader, enable this option
>> +         to use the FDT provided by the previous bootloader instead of any built-in
>> +         to u-boot.
>> +
>> +         If no FDT was available, u-boot will fall back to its internal FDT.
>> +
>>  menu "Configuration editor"
>>
>>  config CEDIT
>> diff --git a/include/init.h b/include/init.h
>> index 4e7fe26c2004..58604cd98758 100644
>> --- a/include/init.h
>> +++ b/include/init.h
>> @@ -168,6 +168,15 @@ defined(CONFIG_SAVE_PREV_BL_FDT_ADDR)
>>   * Return: 0 if ok; -ENODATA on error
>>   */
>>  int save_prev_bl_data(void);
>> +
>> +/**
>> + * get_prev_bl_fdt_addr - When u-boot is chainloaded, get the address
>> + * of the FDT passed by the previous bootloader.
>> + *
>> + * Return: the address of the FDT passed by the previous bootloader
>> + * or 0 if not found.
>> + */
>> +phys_addr_t get_prev_bl_fdt_addr(void);
>>  #endif
>>
>>  /**
>> diff --git a/lib/fdtdec.c b/lib/fdtdec.c
>> index 7a6916764835..85425f2dc1ee 100644
>> --- a/lib/fdtdec.c
>> +++ b/lib/fdtdec.c
>> @@ -1222,7 +1222,7 @@ static int uncompress_blob(const void *src, ulong sz_src, void **dstp)
>>   */
>>  static void *fdt_find_separate(void)
>>  {
>> -       void *fdt_blob = NULL;
>> +       void *fdt_blob = NULL, *prevbl_fdt_blob;
>>
>>         if (IS_ENABLED(CONFIG_SANDBOX))
>>                 return NULL;
>> @@ -1234,6 +1234,11 @@ static void *fdt_find_separate(void)
>>         else
>>                 fdt_blob = (ulong *)__bss_end;
>>  #else
>> +#if defined(CONFIG_USE_PREV_BL_FDT)
>> +       prevbl_fdt_blob = (void *)get_prev_bl_fdt_addr();
>> +       if (prevbl_fdt_blob)
>> +               return prevbl_fdt_blob;
>> +#endif
>>         /* FDT is at end of image */
>>         fdt_blob = (ulong *)_end;
>>
>>
> 
> Regards,
> SImon

-- 
// Caleb (they/them)