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

[Marek Vasut]

> 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.

The FAQ really means "is it possible to load u-boot from u-boot" ... which is 
possible in some cases, but officially unsupported.

> 
> 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.

Yep, MX53 is well supported by mainline u-boot.

M

> 
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