Re: [Query] : DT: Adding mezzanine board

[Laurent Pinchart <laurent.pinchart@ideasonboard.com>]

Hi Prabhakar,

On Wed, Feb 19, 2020 at 12:25:37PM +0000, Lad, Prabhakar wrote:
> On Wed, Feb 19, 2020 at 12:08 PM Laurent Pinchart wrote:
> > On Wed, Feb 19, 2020 at 11:42:42AM +0000, Lad, Prabhakar wrote:
> > > Hi,
> > >
> > > I am working Renesas RZ/Gx platforms, these boards have support for
> > > connecting MIPI adapter mezzanine board [1]. To enable this support I
> > > am planning to create a dtsi which will include the board file and
> > > enable the necessary VIN/CSI and the camera endpoint.
> > >
> > > For example: to add support for G2E board I would create a file named
> > > r8a774c0-cat874-mipi-adapter.dtsi which will include the board file
> > > r8a774c0.dtsi and enable necessary VIN/CSI/Camera endpoint and when
> > > user wants to enable this support for the G2E board he could just
> > > include r8a774c0-cat874-mipi-adapter.dtsi to r8a774c0-cat874.dts
> > >
> > > Would this be a good approach or is there an alternative way which I am missing.
> >
> > DT overlays are another option. They can't be applied on a running
> > system though. It doesn't seem like the mezzanine boards can be detected
> > by the boot loader, so overlays can't be applied automatically before
> > the kernel is started. One option would still be to provide them to
> > U-Boot and apply a selected overlay based on environment variables.
> > Another option would be to apply overlays as part of the build process,
> > which shouldn't require modifying the DT sources to #include the .dtsi
> > file for the mezzanine board.
>
> Thank you for the insight, overlay is something which I wanted to
> avoid and have the dtb as part of kernel itself. How about just
> creating r8a774c0-cat874-mipi-adapter.dts which would include
> r8a774c0-cat874.dts file and enable the VIN/CSI/Camera endpoint, so
> the user would just use r8a774c0-cat874-mipi-adapter.dtb file when the
> mipi adapter is connected ?

That would restrict usage of the mezzanine board to a particular base
board, while in the general case there could be different types of base
boards compatible with a set of mezzanines.

The reason why I mentioned overlay isn't really to apply them
dynamically, but for the set of tools around them. Bootloaders can apply
overlays, and we can also apply overlays to a DTB as part of a build
process, without patching the source. Is there a particular reason why
you would prefer avoiding them ?

> > > [1] https://github.com/Kevin-WSCU/96Boards-Camera/blob/master/AISTARVISION%20MIPI%20Adapter%20V2.1/AISTARVISION%20MIPI%20Adapter%20V2.1.pdf

-- 
Regards,

Laurent Pinchart