If the bootmode pins are configured as

B0-B7: 11010010, B8-B15: 00101000

for primary boot from eMMC alternate and backup boot from the eMMC UDA FS (only supported mode) then the R5 SPL fails to boot the A53 SPL from the eMMC backup boot (UDA FS) in case the eMMC boot partitions are empty (eMMC alternate boot) as shown by the following logs:

[15:56:39.990] U-Boot SPL 2025.01-00464-g1d6ba4a32cdd (Jul 04 2025 - 15:43:52 +0530)
[15:56:40.020] SYSFW ABI: 4.0 (firmware rev 0x000b '11.0.7--v11.00.07 (Fancy Rat)')
[15:56:40.084] Changed A53 CPU frequency to 1250000000Hz (T grade) in DT
[15:56:40.101] SPL initial stack usage: 13424 bytes
[15:56:40.116] Trying to boot from MMC1
[15:56:40.148] mmc_load_image_raw_sector: mmc block read error
[15:56:40.165] Partition 1 invalid on device 0
[15:56:40.166] spl_register_fat_device: fat register err - -1
[15:56:40.168] spl_load_image_fat: error reading image tispl.bin, err - -1
[15:56:40.170] SPL: failed to boot from all boot devices
[15:56:40.181] ### ERROR ### Please RESET the board ### 

This is caused by the missing handling of the backup boot in the `spl_mmc_boot_mode` function used by the R5 SPL to determine the hardware partititon and the mode to use for loading the A53 SPL.

If the `bootindex` variable is used to determine if the ROM booted the R5 SPL from the eMMC backup boot then the function could return the FS mode. This resolves the issue as discussed here:

https://e2e.ti.com/support/processors-group/processors/f/processors-forum/1343329/sk-am62-booting-from-emmc-in-backup-boot-mode-fails/5129071#5129071

Fix in upstream and ti-u-boot-2026.01:
https://git.ti.com/cgit/ti-u-boot/ti-u-boot/commit/?h=ti-u-boot-2026.01&id=6fd45dd488f9605e2f634c7e5da4ac2925c118c8