GSOC 2015: Porting MicroMonitor to the Beaglebone Black

[Gedare Bloom]

On Thu, Mar 26, 2015 at 2:18 PM, Joel Sherrill
<joel.sherrill at oarcorp.com> wrote:
> 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.
>
Yeah you have to get explicit permission, the RTEMS license is not
compatible to the Micromonitor one in that direction.

>>> 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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> -- Joel Sherrill
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