Re: [PATCH v4 2/4] drm: mxsfb: Add optional DPI output bus-width configuration

[Francesco Dolcini <francesco@dolcini.it>]

Hello Luca,

On Wed, Sep 16, 2026 at 10:42:13AM +0200, Luca Ceresoli wrote:
> > LCDIF programs LCD_DATABUS_WIDTH from the selected media bus format. The
> > format reported by the downstream panel or bridge describes the display
> > input, but it does not describe how the LCDIF data pins are physically
> > wired on the board.
> >
> > These can differ. For example, a 16-bit LCDIF bus can be connected to a
> > 24-bit display by wiring the available color bits to the corresponding
> > display inputs. In that case, using the display's 24-bit format to
> > configure LCDIF selects the wrong data-bus mode and changes the assignment
> > of color bits on the LCD_DATA pins.
> >
> > Read the optional bus-width endpoint property from the LCDIF output port
> > and use it to select the media bus format used to configure LCDIF. This
> > allows the LCDIF bus mode to describe the physical interface
> > independently of the downstream display format.
> >
> > When the optional property is absent, or it has an invalid value, continue
> > using the format reported by the downstream display device, preserving
> > the existing behavior. The code is not validating the DT for
> > correctness and just fall back to the previous behavior in case of
> > errors.
> 
> Why? Generally speaking errors are better spotted immediately, not ignored
> silently.

My understanding is that the C driver code should not validate the DT
and that this is the general recommendation when parsing properties from
the DT.

The DT checker is going to spot errors in the DT even earlier and in a
static way.

So the decision is to just fall-back to the previous driver behavior
when this property was not supported at all.

Francesco