On 01/30/2012 09:07 PM, Bud Miljkovic wrote:
Hi there,
While getting acquainted with possible u-Boot development issues, I read
FAQ "14.2.1. Can U-Boot be configured such that it can be started in
RAM?" and was puzzled to learn that u-Boot cannot run from RAM.
Considering a custom platform, using i.MX536, I understand that the
i.MX53x processor has its own ROM-based code that performs boot time
essential devices initialisation, etc. In the case when NAND flash is
the program-image medium at the boot stage, first, the ROM-based code
checks for Discovered Bad Blocks Table (DBBT) presence and searches for
valid Firmware Configuration Block (FCB) on external NAND Flash.
If FCB is found that points to the NAND Flash page(s) that contain the
first 4K of initial firmware to be loaded from NAND Flash. Then, it
loads the 4K of data, pointed by FCB, into the NFC RAM buffer. These
data contain a valid Image Vector Table (IVT). Then, the ROM-based code
processes IVT, executes the Device Configuration Data (DCD) sequences to
initialize boot-related integrated peripherals (typically, these are
IOMUX, SDRAM controller and boot memory controller), then copies the
application code, also contained in IVT, to target memory (typically,
SDRAM) and jumps to it. Typically, this application code is the custom
primary bootloader that completes loading the application code (e.g.
main OS bootloader) and passes the control to it.
If FCB is not found the ROM-based code starts executing a download
protocol over UART or USB port in an attempt to get a valid IVT from a
host. If the IVT is eventually obtained it is processed as before.
So the question is can this "custom primary bootloader" role, in fact,
be played by u-Boot. Reading your text of FAQ 14.2.1., it seems to me
that it cannot. On the other hand I know of some Freescale i.MX53x demo
boards (e.g. ARD, LOCO) that use u-Boot.
I wonder what I'm missing here.
I think that FAQ is a bit misleading and really refers to using another boot
loader to load U-Boot.
On all i.MX processors I've had the pleasure of using, U-Boot runs only
from RAM. I think you probably knew this because very few i.MX designs
even have other memory (i.e. NOR flash) that allows XIP.
For instance, the LOCO certainly isn't running __from__ SD card.
Take a look at the link maps for builds of these platforms to clarify.
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