Controllers with several interface options - one or more drivers?

[Daniel Vetter]

On Fri, Jul 26, 2019 at 02:25:10PM +0200, Sam Ravnborg wrote:
> Hi Josef, Daniel et al.
> 
> The driver that triggered this reply is a driver that adds parallel
> support to ili9341 in a dedicated panel driver.
> The issue here is that we already have a tiny driver that supports the
> ili9341 controller - but with a slightly different configuration.
> 
> The ili9341 supports several interfaces - from the datasheet:
>     "ILI9341 supports parallel 8-/9-/16-/18-bit data bus
>      MCU interface, 6-/16-/18-bit data bus RGB interface and
>      3-/4-line serial peripheral interface (SPI)"
> 
> Noralf - in another mail explained:
> "
> The MIPI Alliance has lots of standards some wrt. display controller
> interfaces:
> - MIPI DBI - Display Bus Interface (used for commands and optionally pixels)
> - MIPI DPI - Display Pixel Interface (also called RGB interface or
> DOTCLK interface)
> - MIPI DSI - Display Serial Interface (commands and pixels)
> 
> The ili9341 supports both MIPI DBI and DPI.
> "
> 
> MIPI DPI - is a good fit for a drm_panel driver.
> MIPI DBI - requires a full display controller driver.
> 
> There are many other examples of driver SoC that in the same way
> can be seen only as a panel or as a full display controller driver.
> 
> The open question here is if we should try to support both cases in the
> same driver / file. Or shall we implment two different drivers.
> One for the panel use-case. And one for the display controller usecase?
> 
> Not sure - so asking for feedback.

I'm not sure. Currently we do have DSI and dumb RGB panels all in
drm/panel. I don't think we have DBI panels in there yet, but then
drm/tiny is the only one supporting these.

I guess we could look into move some of the DBI panel drivers into panel
drivers, but that needs a bit more glue all around. I'm honestly not sure
how the current DSI drivers in drm_panel work exactly, especially for
command mode.

Or maybe we need a new interface for command mode.

Wrt sharing code between drivers for the same chip, but different
interfaces: I wouldn't worry too much about that. Maybe try to have a
shared header file at least for registers. Long term we could end up with
one driver module which exposes different flavours of the same chip, so
multiple drm_panel drivers, or maybe we'll get something more specific for
dsi/dbi.
-Daniel
-- 
Daniel Vetter
Software Engineer, Intel Corporation
http://blog.ffwll.ch