OMAP Bootloader Overview
There are several stages of bootloaders that perform different levels of initialization on an OMAP platform, in order to eventually load and run the filesystem. This figure shows the booting sequence of the ROM code, x-loader, u-boot, and kernel, with each stage performing enough configuration in order to load and execute the next.
The Bootloader Project covers specifically the x-loader and the u-boot: a code overview, and how to obtain and build the code.
Why are there two bootloaders? The first bootloader, x-loader, is a stripped-down version of the u-boot, designed to run in OMAP's small on-chip SRAM. It initializes the main off-chip memory of the system and other necessary device drivers, and then loads the larger bootloader for Linux, u-boot.
 OMAP Boot Sequence
Detailed documentation on the OMAP boot sequence is available in the OMAP TRM, Initialization chapter. The TRMs for multiple OMAP platforms (OMAP4430, OMAP4460, OMAP34xx, etc) are available here: http://www.ti.com/general/docs/wtbu/wtbudocumentcenter.tsp?templateId=6123&navigationId=12037
 SYSBOOT Pins
The internal ROM Code can attempt to boot from several different peripheral and memory devices, including, but not limited to: Serial (UART3), SD Card, eMMC, NAND, and USB. The order in which these devices are searched for a valid first-stage booting image (x-loader) is determine by a set of GPIO configuration pins referred to as SYSBOOT. The TRM includes a table that shows the booting device list that each combination of the SYSBOOT pins refers to.
The SYSBOOT value can be read from physical address 0x480022f0, either using JTAG, or if you have linux running, use devmem2:
# devmem2 0x480022f0 b /dev/mem opened. Memory mapped at address 0x40020000. Value at address 0x480022F0 (0x400202f0): 0x2F
For OMAP34xx, there is also a tool to show the boot order: omap34xx-boot-order
 First Stage Boot
For example, the SYSBOOT pins could be set such that booting device list consists of 1) serial (UART3), 2) SD card (MMC1), and 3) NAND flash. In this case, the ROM code would first look for a valid x-loader over the serial port, then in the SD card, then in the NAND flash. Whenever it finds a valid x-loader, it proceeds with execution of that binary.
For serial boot, a simple ID is written out of the serial port. If the host responds correctly within a short window of time, the ROM will read from the serial port and transfer the data to the internal SRAM. Control is passed to the start of SDRAM if no errors are detected. UART3 is the only uart for which the ROM will attempt to load from.
SD Card Boot
If MMC is included in the booting device list, the ROM looks for an SD Card on the first MMC controller. If a card is found, the ROM then looks for the first FAT32 partition within the partition table. Once the partition is found, the root directory is scanned for a special signed file called "MLO" (which is the x-loader binary with a header containing the memory location to load the file to and the size of the file). Assuming all is well with the file, it is transfered into the internal SRAM and control is passed to it. Both MMC1 and MMC2 can be used for booting.
NAND / eMMC Boot
If NAND is included in the booting device list, the ROM attempts to load the first sector of NAND. If the sector is bad, corrupt, or blank, the ROM will try the next sector (up to 4) before exiting. Once a good sector is found, the ROM transfers the contents to SRAM and transfers control to it. (The same steps are performed for eMMC if eMMC is included in the booting device list instead of NAND.)
 Second Stage Boot
It is the job of the x-loader to transfer the 2nd stage loader into main memory, which we call the u-boot. Typically both the x-loader and u-boot come from the same storage medium. For example, typically if the x-loader is transferred via USB, the u-boot will also be transferred via USB, and if the x-loader is transferred via SD card, the u-boot will also be transferred via SD card. However, this is not required. For example, you could flash the serial x-loader into the NAND. The ROM code will load the x-loader from NAND and transfer control to the x-loader, which will wait for the u-boot to be downloaded from the serial port.
In the case of loading both the u-boot over the serial port, the x-loader waits for the host to initiate a kermit connection to reliably transfer large files into main memory. Once the file transfer is completed successfully, control is transfered.
SD Card Boot
In the case of loading the u-boot via the SD card, the SD card x-loader looks for a FAT32 partition on the first MMC controller and scans the top level directory for a file named "u-boot.bin". It then transfers the file into main memory and transfers control to it.
NAND / eMMC Boot
In the case of a u-boot stored in NAND, the x-loader expects the u-boot to be located at the 5th sector (offset 0x00800000). It transfers the image from NAND into main memory and transfers control to it. In the case of a u-boot stored in eMMC, the x-loader expects the u-boot to be located at the offset 0x200.
 x-loader overview
The x-loader is a small first stage bootloader derived from the u-boot base code. It is loaded into the internal static RAM by the OMAP ROM code. Due to the small size of the internal static RAM, the x-loader is stripped down to the essentials. The x-loader configures the pin muxing, clocks, DDR, and serial console, so that it can access and load the second stage bootloader (u-boot) into the DDR. This figure shows the code flow in the x-loader, beginning in start.S.
Please find the continue context.....