GSOC 2015: Porting MicroMonitor to the Beaglebone Black

[Ed Sutter]

On 3/26/2015 2:22 PM, Gedare Bloom wrote:
> 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.
Keep in mind we're referring to Apache license for Micromonitor now, so 
as far as I
know (keep in mind I am by no means a license guru), RTEMS and 
Micromonitor can
play together.  Right?
>
>>>> 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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>>
>>
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-- 
Ed Sutter
Alcatel-Lucent Technologies -- Bell Laboratories
Phone: 908-582-2351
Email: ed.sutter at alcatel-lucent.com