[PATCH] ARM: Distro boot: document the need for fdtfile variable to be set

[Stephen Warren]

On 9/3/20 2:14 PM, Dennis Gilmore wrote:
> On Thu, Sep 3, 2020 at 2:15 PM Stephen Warren <swarren at wwwdotorg.org> wrote:
>>
>> On 9/3/20 10:40 AM, Dennis Gilmore wrote:
>>> When testing builds provided in https://github.com/openwrt/openwrt/pull/3360
>>> I discovered that fdtfile was not set and as a result the firmware was not
>>> functional. So I am documenting what is needed.
>>>
>>> Signed-off-by: Dennis Gilmore <dennis at ausil.us>
>>>
>>> Cc: Atish Patra <atish.patra at wdc.com>
>>> Cc: Lukas Auer <lukas.auer at aisec.fraunhofer.de>
>>> Cc: Tom Rini <trini at konsulko.com>
>>> Cc: Masahiro Yamada <yamada.masahiro at socionext.com>
>>> Cc: Vagrant Cascadian <vagrant at debian.org>
>>> Cc: Stephen Warren <swarren at nvidia.com>
>>> Cc: Karsten Merker <merker at debian.org>
>>> ---
>>>  doc/README.distro | 8 ++++++++
>>>  1 file changed, 8 insertions(+)
>>>
>>> diff --git a/doc/README.distro b/doc/README.distro
>>> index 5076bebd18..3eb70aeb14 100644
>>> --- a/doc/README.distro
>>> +++ b/doc/README.distro
>>> @@ -224,6 +224,14 @@ fdt_addr_r:
>>>
>>>    A size of 1MB for the FDT/DTB seems reasonable.
>>>
>>> +fdtfile:
>>> +
>>> +  Mandatory. the name of the DTB file for the specific board for instance
>>> +  the espressobin v5 board the value is "marvell/armada-3720-espressobin.dtb"
>>> +  while on a clearfog pro it is "armada-388-clearfog-pro.dtb" in the case of
>>> +  a board providing its firmware based DTB this value can be used to override
>>> +  the DTB with a different DTB.
>>
>> IIRC this variable isn't mandatory; if the DT filename follows expected
>> ${soc}-${board}.dtb naming, then U-Boot has a default value that will
>> work without the user or U-Boot author having to manually set this variable.
>>
>> So it's certainly mandatory that U-Boot know this value at runtime, but
>> perhaps the text should be expanded to indicate that sometimes U-Boot
>> can provide the value itself, but sometimes the variable needs to be set?
> 
> in include/config_distro_bootcmd.h we have the following
> /*
>  * On 32bit ARM systems there is a reasonable number of systems that follow
>  * the $soc-$board$boardver.dtb name scheme for their device trees. Use that
>  * scheme if we don't have an explicit fdtfile variable.
>  */
> #define BOOTENV_EFI_SET_FDTFILE_FALLBACK                                  \
>         "if test -z \"${fdtfile}\" -a -n \"${soc}\"; then "               \
>           "setenv efi_fdtfile ${soc}-${board}${boardver}.dtb; "           \
>         "fi; "
> #else
> #define BOOTENV_EFI_SET_FDTFILE_FALLBACK
> #endif
> 
> that schema is not true on AArch64 and the method only works with efi
> booting. I will update the text to list out the conditions you could
> get away with not setting the variable

I was thinking more of cmd/pxe_utils.c:label_boot(), which covers the
extlinux.conf rather then EFI case. This certainly applies to both 32-
and 64-bit Tegra systems at least.

Specifically:
>                        f1 = env_get("fdtfile");
>                         if (f1) {
>                                 f2 = "";
>                                 f3 = "";
>                                 f4 = "";
>                         } else {
>                                 /*
>                                  * For complex cases where this code doesn't
>                                  * generate the correct filename, the board
>                                  * code should set $fdtfile during early boot,
>                                  * or the boot scripts should set $fdtfile
>                                  * before invoking "pxe" or "sysboot".
>                                  */
>                                 f1 = env_get("soc");
>                                 f2 = "-";
>                                 f3 = env_get("board");
>                                 f4 = ".dtb";
>                         }