[Gta04-owner] New SD card setup

Dr. H. Nikolaus Schaller hns at goldelico.com
Sat Sep 22 11:32:32 CEST 2012


Am 22.09.2012 um 11:13 schrieb Neil Jerram:

> Hi all,
> 
> I'm about to setup a new SD card and am wondering about the detail of
> the partitioning instructions.
> 
> The README at http://sourceforge.net/projects/qtmoko/files/GTA04/ says:
> 
>> * You need specially partitioned SD card. The partitioning is described
>>  here: http://code.google.com/p/beagleboard/wiki/LinuxBootDiskFormat 
> 
> and that beagleboard link includes a section called "Set the Geometry of
> the SD Card".
> 
> My question is whether that section really matters, and if so why.
> IIRC, I followed it when setting up my last SD card, but then I found -
> when printing the partition record again - that all those changes hadn't
> stuck.  I.e. the numbers of heads, cylinders etc. had reverted back to
> what they were initially.
> 
> Can someone explain whether and why those instructions are needed?

This is because the Boot ROM within the OMAP3 can find and load the file
"MLO" from the first FAT partition. Since the Boot ROM does not know anything
about the FAT file system format it reads from a specific sector. Formatting
the SD card in the given way makes sure that if you mount the SD card and
write the first file, it will be stored at the correct location so that it *can* be found
by the Boot ROM.

You may not notice bad formatting unless you press the AUX button during
boot because without pressing the button, the Boot ROM will fetch X-Loader
and U-Boot from NAND and then U-Boot checks if there is a SD card and loads
the kernel from there. And, if it does not find the MLO on the SD card, it
will load it from NAND even if you did press the AUX button.

So using this way of booting from SD card does not reveal an issue with
the # of heads/cylinders etc. Unless you erase NAND.

To summarize: if your X-Loader and U-Boot in NAND is ok, you don't have
to format the SD card in any special way (except making at least two partitions).
And you don't even have to store a "MLO" file on it.

Hope this helps,
Nikolaus





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