[PATCH] drm/imx: parallel-display: Adjust bus_flags and bus_format handling

[Boris Brezillon]

On Mon, 9 Mar 2020 21:23:06 +0200
Laurent Pinchart <laurent.pinchart at ideasonboard.com> wrote:

> On Mon, Mar 09, 2020 at 11:50:59AM +0100, Philipp Zabel wrote:
> > On Thu, 2019-11-14 at 14:17 +0100, Marek Vasut wrote:  
> > > The bus_flags and bus_format handling logic does not seem to cover
> > > all potential usecases. Specifically, this seems to fail with an
> > > "edt,etm0700g0edh6" display attached to an 24bit display interface,
> > > with interface-pix-fmt = "rgb24" set in DT.  
> > 
> > interface-pix-fmt is a legacy property that was never intended to be
> > used as an override for the panel bus format. The bus flags were
> > supposed to be set from the display-timings node, back when there was no
> > of-graph connected panel at all.
> > 
> > That being said, there isn't really a proper alternative that allows to
> > override the bus format requested by the panel driver in the device tree
> > to account for weird wiring. We could reuse the bus-width endpoint
> > property documented in [1], but that wouldn't completely specify how the
> > RGB components are to be mapped onto the parallel bus.
> > 
> > [1] Documentation/devicetree/bindings/media/video-interfaces.txt  
> 
> Things are funny sometimes, I've run into the exact same problem with a
> different display controller today.
> 
> Shouldn't we use the data-shift property from [1] to specify this ?
> Combined with Boris' bus format negotiation for bridges, I think we
> would have all the components in place to solve this problem properly.

I wonder if we shouldn't take more complex pin mappings into account
now and go directly for a data-mapping property describing those
mappings using a string. This way we'd have a single property that
would work for both fully parallel buses (DPI/RGB) and serial (or
partially parallel) ones (LVDS).