[U-Boot] Can u-Boot Ran from RAM?

[Eric Nelson <eric.nelson@boundarydevices.com>]

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.