Re: [PATCH 2/3] ARM: dts: omap3-igep0020: Add HS USB Host support

[Javier Martinez Canillas <martinez.javier@gmail.com>]

On Mon, Oct 7, 2013 at 11:13 AM, Javier Martinez Canillas
<javier.martinez@collabora.co.uk> wrote:
> On 10/07/2013 11:06 AM, Roger Quadros wrote:
>>>
>>> Well that's a very good question indeed.
>>>
>>> The thing is that the IGEP0030 is a Computer-on-Module [1] that is used in
>>> conjunction with expansion boards and some of them have USB HOST support such as
>>> IGEP Paris [2] and IGEP Berlin [3].
>>>
>>> Support for this expansion boards is still not in mainline but there are plans
>>> to add them using Device Tree overlays [4] once/if this land on mainline.
>>>
>>
>> Why would your boards need Device Tree overlays? From the looks of it, the configuration
>> of SOM + base board don't seem to change at runtime.
>>
>
> Indeed, a static configuration (DTS) would be enough now that I think about it.
>

Hi Roger,

Now that I had coffee I remember why I think that even when Device
Tree overlays are not strictly required for a SOM + base board it
could be handy to use. If we use a static configuration (DTB) then the
same firmware can't be used by any IGEP COM Module user. She would
have to choose a DTB to pass to the kernel on boot.

While using DT overlays the same firmware that provides a minimal DTB
can be used regardless of the base board used (as long as there are
all the needed fragment/overlays hooks in the DTS).

After all the SOM has a NAND flash memory and a uSD/MMC slot so a
minimal DTB is needed to boot and the support for all the peripherals
present on the base board can be added by triggering a device tree
overlay load from user-space.

Or maybe I'm misunderstanding the use case for DT overlays since I had
just read about it but I don't have practical experience with it.

Thanks a lot and best regards,
Javier