[RFC] spl_fit.c: SPL Falcon Mode return to U-Boot

[Elena Popa]

When Falcon Mode is enabled, spl_start_uboot() function must be defined. If this function returns 0 then SPL should start the kernel, if it returns 1 then U-Boot must be started. 

When spl_start_uboot() returns 1, then U-Boot must be loaded, and as far as I can tell spl_image->os should be set to IH_OS_U_BOOT before:

	if (os_takes_devicetree(spl_image->os)) {
		ret = spl_fit_append_fdt(spl_image, info, sector, &ctx);

in common/spl/spl_fit.c/ int spl_load_simple_fit(...)

this is not the case currently, and the loading fails. If set manually spl_image->os = IH_OS_U_BOOT, it works fine.

The only place I could see where it could be set is a few lines above in:

if (!spl_fit_image_get_os(ctx.fit, node, &spl_image->os))
	debug("Image OS is %s\n", genimg_get_os_name(spl_image->os));
else if (!IS_ENABLED(CONFIG_SPL_OS_BOOT))
	spl_image->os = IH_OS_U_BOOT;

but the second "if" does not execute, obviously (since we are in falcon, CONFIG_SPL_OS_BOOT is enabled). I see a few options, but I am not sure how best to fix this:

1) add something like:
else if (!IS_ENABLED(CONFIG_SPL_OS_BOOT) || spl_start_uboot())
	spl_image->os = IH_OS_U_BOOT;
but this would mean calling spl_start_uboot() twice, requiring it to return the same value. It works in my case.

2) set spl_image->os to IH_OS_U_BOOT when the spl_start_uboot() is called the first time, in:
common/spl/spl_mmc.c/spl_mmc_do_fs_boot(..)    - in several places:
#ifdef CONFIG_SPL_FS_FAT
	if (!spl_start_uboot()) {
		err = spl_load_image_fat_os(spl_image, bootdev, mmc_get_blk_desc(mmc),
...

#ifdef CONFIG_SPL_FS_EXT4
	if (!spl_start_uboot()) {
		err = spl_load_image_ext_os(spl_image, bootdev, mmc_get_blk_desc(mmc),

...

Any insight on how to proceed? I've implemented spl_start_uboot() to decide on whether to boot the kernel or the U-Boot based on key presses on the console.

Thank you,
Elena.