Re: [RFC][PATCH 2/3] drm/modes: Make width-mm/height-mm mandatory in of_get_drm_panel_display_mode()

[Rob Herring <robh@kernel.org>]

On Sat, Apr 02, 2022 at 07:55:59PM +0200, Marek Vasut wrote:
> On 4/2/22 19:08, Noralf Trønnes wrote:
> > 
> > 
> > Den 02.04.2022 18.39, skrev Marek Vasut:
> > > On 4/2/22 09:45, Noralf Trønnes wrote:
> > > > 
> > > > 
> > > > Den 02.04.2022 06.28, skrev Marek Vasut:
> > > > > On 4/2/22 05:19, Laurent Pinchart wrote:
> > > > > > On Fri, Apr 01, 2022 at 10:36:24PM +0200, Marek Vasut wrote:
> > > > > > > On 4/1/22 20:46, Laurent Pinchart wrote:
> > > > > > > > On Fri, Apr 01, 2022 at 06:37:54PM +0200, Marek Vasut wrote:
> > > > > > > > > Make the width-mm/height-mm panel properties mandatory in
> > > > > > > > > of_get_drm_panel_display_mode(), print error message and
> > > > > > > > > return -ve in case these DT properties are not present.
> > > > > > > > > This is needed to correctly report panel dimensions.
> > > > > > > > 
> > > > > > > > Can we guarantee this won't cause a regression ?
> > > > > > > 
> > > > > > > For the upstream DTs, I think we can.
> > > > > > > For downstream DTs, we cannot know.
> > > > > > 
> > > > > > Are there users of this function whose DT bindings don't require the
> > > > > > width-mm and height-mm properties ?
> > > > > 
> > > > > There is literally one user of this function upstream:
> > > > > drivers/gpu/drm/tiny/panel-mipi-dbi.c
> > > > 
> > > > Yes, the function was added for that driver since it was so generic in
> > > > nature. What about adding an argument to of_get_drm_panel_display_mode()
> > > > that tells if the properties are mandatory or not?
> > > 
> > > Sure, we can do that, but maybe the question here is even bigger than
> > > this series.
> > > 
> > > Should every panel set mandatory width_mm/height_mm so e.g. the user
> > > space can infer DPI from it and set up scaling accordingly, or should
> > > width_mm/height_mm be optional ?
> > > 
> > > I think width_mm/height_mm should be mandatory for all panels.
> > > 
> > > Thoughts ?
> > 
> > If this had come up during the review of the driver I would have no
> > problem making it mandatory. It makes sense for DPI. Maybe it's possible
> > to get around the ABI break by getting in a change through -fixes before
> > 5.18 is released? I'm fine with that.
> 
> Well that's awesome, the dbi-spi.yaml didn't land in any kernel release yet,
> so we still have a chance to fix it ? Rob ?

Yes, it can be fixed. And the binding, not the kernel, is the place to 
enforce it being mandatory IMO.

Rob