GSOC 2015: Porting MicroMonitor to the Beaglebone Black

[Joel Sherrill]

On 03/26/2015 11:35 AM, Jarielle Catbagan wrote:
> It looks like the internal ROM-based bootloader looks for a secondary
> program loader (SPL) that initializes the necessary devices to
> continue the boot process and pass control to a third-stage
> bootloader.  So now I believe it's a matter of finding whether there
> are existing code implementations of this SPL, or last-case scenario
> this would have to be implemented.  Time for more investigating. :)
>
My off the top of my head recommendation is to document the
sequence the BB goes through during initialization. Then
start with a goal of replacing U-Boot in that sequence with
Micromonitor. This keeps us in the world of at least documented
with code. However U-Boot has to be formatted, linked, etc.
is exactly what you will have to do with your Micromonitor.

U-Boot can be a reference but not a source of code.

The previous stages may be able to be replaced later but
I wouldn't worry about that now.
> On Thu, Mar 26, 2015 at 9:27 AM, Jarielle Catbagan
> <jcatbagan93 at gmail.com>  wrote:
>> To put things into context in regards to the conversation that I was
>> having with Ed, Dr. Joel, and Gedare:
>>
>> I am currently in the process of looking into porting MicroMonitor to
>> the Beaglebone Black.  As indicated by Ed, "[t]he difficulty of the
>> port will depend on how much existing CPU-initialization
>> (clocks, cache, etc..) code we can reuse."
>>
Ed and the authors of the RTEMS code would have to agree but,
in principle, anything in the Beagle BSP should be fair game for
reuse in Micromonitor.
>> Ed has also indicated to me that there might be an internal bootloader
>> stored in a ROM-based memory that might look for an image in a
>> specific format.  I will definitely be investigating more into this.
>> I did manage to briefly browse through the Beaglebone Black System
>> Reference Manual Rev C.1 [1], and I have found that the boot
>> configuration/process is briefly elaborated in section 6.7.  For
>> convenience, since it's a short section I will post it here:
>>
>> "The design supports two groups of boot options on the board. The user
>> can switch between these modes via the Boot button. The primary boot
>> source is the onboard eMMC device. By holding the Boot button, the
>> user can force the board to boot from the microSD slot. This enables
>> the eMMC to be overwritten when needed or to just boot an alternate
>> image...
>>
>> [T]the processor-external boot code is composed of two stages. After
>> the primary boot code in the processor ROM passes control, a secondary
>> stage (secondary program loader -- "SPL" or "MLO") takes over. The SPL
>> stage initializes only the required devices to continue the boot
>> process, and then control is transferred to the third stage "U-boot".
>> Based on the settings of the boot pins, the ROM knows where to go and
>> get the SPL and UBoot code. In the case of the BeagleBone Black, that
>> is either eMMC or microSD based on the position of the boot switch."
>>
>> I was kindly guided to look into programming a uSD card as it might be
>> more efficient to run MicroMonitor off of the uSD for quick testing
>> after every build.  If all goes well, either an application image will
>> be located and booted off of the same SD card or via a network boot.
>> For serial debugging I have an FTDI 3.3V USB-to-Serial cable that I
>> have been previously using to access the U-boot monitor on the
>> Beaglebone Black.
>>
This is how we do a lot of our testing. It isn't worth the effort
to put something on the board itself. Plus it places wear and
tear on a replaceable item, not on the built-in flash.
>> [1] https://github.com/CircuitCo/BeagleBone-Black/blob/master/BBB_SRM.pdf?raw=true
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