Controllers with several interface options - one or more drivers?

[Sam Ravnborg]

Hi Daniel.

Added Noralf - somehow I missed him on the original mail.

On Fri, Jul 26, 2019 at 05:06:03PM +0200, Daniel Vetter wrote:
> Also probably should add a few more (drm_bridge) people, I think
> that's also somewhat relevant here.
> -Daniel
> 
> On Fri, Jul 26, 2019 at 4:55 PM Daniel Vetter <daniel at ffwll.ch> wrote:
> >
> > 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.
If I get around to do a driver for the ssd1306 then I will try to sewhat
makes sense then. For now we shall not stall the ili9341 driver.
> >
> > 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.
This part should be the minimum. Somthing like include/drm/mipi/?

	Sam

> > 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
> 
> 
> 
> -- 
> Daniel Vetter
> Software Engineer, Intel Corporation
> +41 (0) 79 365 57 48 - http://blog.ffwll.ch