[U-Boot] [PATCH v2] common: fit: Allow U-Boot images to be booted

[Mario Six]

On Wed, Jul 20, 2016 at 2:59 PM, Robert P. J. Day <rpjday at crashcourse.ca> wrote:
> On Wed, 20 Jul 2016, Mario Six wrote:
>
>> In certain circumstances it comes in handy to be able to boot into a second
>> U-Boot. But as of now it is not possible to boot a U-Boot binary that is inside
>> a FIT image, which is problematic for projects that e.g. need to guarantee a
>> unbroken chain of trust from SOC all the way into the OS, since the FIT signing
>> mechanism cannot be used.
>>
>> This patch adds the capability to load such FIT images.
>>
>> An example .its snippet (utilizing signature verification) might look
>> like the following:
>>
>> images {
>>       firmware at 1 {
>>               description = "2nd stage U-Boot image";
>>               data = /incbin/("u-boot-dtb.img.gz");
>>               type = "firmware";
>>               arch = "arm";
>>               os = "u-boot";
>>               compression = "gzip";
>>               load = <0x8FFFC0>;
>>               entry = <0x900000>;
>>               signature at 1 {
>>                       algo = "sha256,rsa4096";
>>                       key-name-hint = "key";
>>               };
>>       };
>> };
>
>   i'm sure i'm about to embarrass myself, but the above represents
> loading a second version of u-boot into RAM, no? but i thought u-boot
> didn't support being run out of RAM (except in special circumstances).
> so what am i misunderstanding here?
>
> rday
>

Well, if by "special circumstances" you mean "load the U-Boot image exactly
where CONFIG_SYS_TEXT_BASE expects it to be loaded," then these are special
circumstances; the U-Boot in the example snippet has CONFIG_SYS_TEXT_BASE of
0x900000, we load it to 0x8FFFC0 (offset for the 0x40 byte header), and the
bootm routine simply jumps to the entry point.

Accordingly, the second stage U-Boot you want to run must not have the save
CONFIG_SYS_TEXT_BASE as the U-Boot you want to run it out of (otherwise you
would overwrite the existing image in memory); something like

#ifdef CONFIG_SECONDUBOOT
#define    CONFIG_SYS_TEXT_BASE    0x00800000
#else
#define    CONFIG_SYS_TEXT_BASE    0x00900000
#endif

is needed if you want to build both with the same header file.

Hope that makes things a bit clearer.

Best regards,

Mario