Beagle: FDT support in BSP as a GSoC project?

[Christian Mauderer]

On 12/08/2019 03:33, Chris Johns wrote:
> On 12/8/19 9:22 am, Joel Sherrill wrote:
>>
>>
>> On Sun, Aug 11, 2019, 5:47 PM Chris Johns <chrisj at rtems.org
>> <mailto:chrisj at rtems.org>> wrote:
>>
>>     On 12/8/19 3:28 am, Joel Sherrill wrote:
>>     > On Sun, Aug 11, 2019, 10:59 AM Christian Mauderer <list at c-mauderer.de
>>     <mailto:list at c-mauderer.de>
>>     > <mailto:list at c-mauderer.de <mailto:list at c-mauderer.de>>> wrote:
>>     >
>>     >     Hello,
>>     >
>>     >     while mentoring Vijays GSoC project this year I noted that some drivers
>>     >     in the Beagle BSPs have quite horrible hard coded values for things like
>>     >     pinmux initialization. Maybe it would be a nice GSoC project for next
>>     >     year to replace this stuff with a fdt based initialization. I would like
>>     >     to ask for feedback before creating a ticket for it because it would
>>     >     mean a quite big change for the BSP (maybe even the name - see below).
>>     >
>>     >     Basically such a project would include the following parts:
>>     >
>>     >     - Parse the pinmux settings from FDT and create a two part driver for a
>>     >     'pinctrl-single' compatible FDT entry. One part generic, one device
>>     >     specific (similar to FreeBSD or Linux).
>>     >
>>     >     - Remove pinmux initialization from all drivers.
>>     >
>>     >     - Initialize drivers based on the FDT (instead of functions like
>>     >     bbb_register_i2c_1(...))
>>     >
>>     >     - Taking a more detailed look at the FDT what else could be initialized
>>     >     from it (maybe clocks?)
>>     >
>>     >     It could be a quite nice project for a RTEMS beginner. Due to the
>>     >     distributed initialization a lot of drivers have to be touched (at least
>>     >     i2c, spi and pwm). So a potential student would get a nice overview over
>>     >     the parts.
>>     >
>>     >     Note that this would be a big change for the BSP. Currently the BSP can
>>     >     be used without an FDT (as far as I know). Only libbsd needs one. After
>>     >     that a FDT would be mandatory. Despite that, I think that it would be an
>>     >     improvement.
>>     >
>>     >     Maybe it would be possible to merge the four beagle* BSPs that we have
>>     >     into only one "beagle" or "am33xx" BSP with that change. That would
>>     >     allow to support new Beagle variants like the Pocket Beagle without much
>>     >     effort (most likely only a change in the FDT).
>>     >
>>     >     What do you think? Should I create a ticket for it?
>>     >
>>
>>     I love it. Yes please create a ticket.

OK. I'll create one in the next days.

>>
>>     > I think this is a good idea if we can still avoid bloating apps with all
>>     > drivers. Make sure it has the right tags and shows up on the project page.
>>
>>     The beagle has a lot of RAM. Is this as important for this BSP?

Most likely that's true for a lot of other FDT based BSPs too. Most of
the time FDT is used together with Linux systems.

>>
>> Not really but we don't want bad patterns starting. 
>>>
> How does a user then configure a build to get the minimal set for them?
> 
> Without dynamically loading does mean we need a bunch of build options? Is
> building for dynamic loading something we should consider?

I think there would be two possibilities:

1. Introduce a module system similar to FreeBSD / libbsd. You
explicitely have to create a module reference if you want a driver.

2. Do it the other way round: Take care that device drivers are only
referenced via one init function. If a user wants a small config, he can
overwrite the function in his application which removes the driver.

I would prefer 2 because:

- Most likely it's simpler for the average user to just have everything
available without a special configuration.

- It's simpler to implement.

- A user that really needs the last few bytes on a system like Beagle is
unlikely.

- If someone really needs the bytes, most likely he knew that when
creating the draft of the system. I would expect that this is a more
experienced user who either knows of the tricks or asks on the mailing list.

Best regards

Christian

> 
> Chris

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