Beagle: FDT support in BSP as a GSoC project?

[Chris Johns]

On 12/8/19 3:51 pm, Christian Mauderer wrote:
> 
> 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.

I agree.

> - 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.
> 

This is reasonable.

Thanks
Chris