GSOC 2015: Porting MicroMonitor to the Beaglebone Black

Thu Mar 26 20:45:10 UTC 2015

I should clarify that the terms SPL and MLO seem to be used
interchangeably.  So I guess it's pretty much a matter of figuring out
where the internal ROM-based bootloader exactly transfers the "second
stage" bootloader from (i.e. location in memory) and to what location
in the internal SRAM to do the basic initializations.  Once I can find
the sources of the SPL implementation this might be used for reference
as the functionality is originally integrated with U-boot and probably
can't be used (note: not a license expert, please correct me if I am
wrong) since it is licensed under GPLv2.

>From what I read very briefly, the AM335x internal SRAM seems to have
limited memory so "second stage" bootloader might only have basic
functionality to do basic initialization (perhaps like UART) and then
locating and transferring the real bootloader.  I'll be verifying
this.

At the time of this message, I have a little over 22 hours to finish
my proposal before the deadline.  So far it has been adjusted to
reflect that the effort is to port MicroMonitor to the Beaglebone
Black in the most general sense.  I am still lacking a bit of
specifics.  Any advice on what can constitute as something that I
should do for this project as well as any ways I can perhaps improve
my proposal?

Here is the link to my proposal:
https://docs.google.com/document/d/1pGQ60e5A4JcXl9_gkBEutIgb6AhiQ00MPJN6J1rSKT8/edit#

Thank you!

On Thu, Mar 26, 2015 at 1:08 PM, Jarielle Catbagan
<jcatbagan93 at gmail.com> wrote:
> I'll definitely check out the AM335x TRM.
>
> Referring to this link
> (http://beagleboard.org/project/U-Boot+%28V1%29/), it briefly
> elaborates that the U-boot on the Beaglebone consists of two phases.
> First phase consists of the SPL which is transferred from eMMC or uSD
> (depending on boot switch) and must be in a specific format and
> location before being transferred to the internal SRAM of the AM335x.
> This SPL is really a file that is part of the U-boot secondary program
> loader functionality, which for now I am assuming is built along side
> when U-boot is built.  This SPL performs initialization which includes
> initializing the off-chip memory, which in the case of the Beaglebone
> Black would be the DDR3 RAM.  Once external RAM is initialized, main
> U-boot bootloader image is transferred to there.
>
> I also found this thread in the U-boot mailing list:
> http://lists.denx.de/pipermail/u-boot/2013-December/168433.html
>
> Currently in the process of reading through it but for the most part I
> understand it to be focusing on where the sources of the SPL
> implementation can be found.
>
>
> On Thu, Mar 26, 2015 at 12:09 PM, Ed Sutter
> <ed.sutter at alcatel-lucent.com> wrote:
>> Refer to chapter 26 of this document (AM335x TRM):
>> http://www.ti.com/lit/ug/spruh73l/spruh73l.pdf
>> for more detail on the booting process of the chip.
>>
>> Depending on how hard it is to reconfigure the SYSBOOT pins, it may be
>> simpler to initially try to boot this off the UART.  Then once that works,
>> its likely that it would easily transition over to the uSD card. I suggest
>> this only to eliminate the need for 'dd' and burning the uSD card till we
>> get it right.
>>
>> Gotta learn about the format of this initial image pulled into the device.
>>>
>>> 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. :)
>>>
>>> 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 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.
>>>>
>>>>
>>>> [1]
>>>> https://github.com/CircuitCo/BeagleBone-Black/blob/master/BBB_SRM.pdf?raw=true
>>>
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>>
>>
>>
>> --
>> Ed Sutter
>> Alcatel-Lucent Technologies -- Bell Laboratories
>> Phone: 908-582-2351
>> Email: ed.sutter at alcatel-lucent.com
>>
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