[PATCH] drm/panel: panel-simple: Support panel-dpi

[Rob Herring]

On Fri, Mar 8, 2019 at 11:12 AM Sam Ravnborg <sam at ravnborg.org> wrote:
>
> Hi Thierry.
> On Fri, Mar 08, 2019 at 02:39:24PM +0100, Thierry Reding wrote:
> > On Fri, Mar 08, 2019 at 02:01:48PM +0100, Sam Ravnborg wrote:
> > > > One thing that's not clear to me is whether or not we want to allow
> > > > video timings to be specified in DT. I used to think that we didn't,
> > > > because the video timings are implied by the specific compatible string
> > > > (which we already determined is mandatory anyway),
> > >
> > > We often have two users of the timings for a simple panel.
> > > First we have the bootloader that may present something on the
> > > panel - next step it then the kernel.
> > >
> > > Bootloaders such as U-boot and barebox supports devicetree.
> > > So with the timings specified in the devicetree there are three
> > > users that can use the timings, and it is simple to share the
> > > timing specifications.
> >
> > I think this is not true in practice. As far as I know U-Boot and Linux
> > don't share the device tree. So we wouldn't actually be sharing the
> > video timings, we'd be duplicating them. And whether we duplicate them
> > in code or DT isn't really all that different.
> U-boot copies selected DT files from the kernel to U-boot.
> barebox has a (sanitized?) set of DT files from the kernel, with
> barebox specifics added on top of it. This receives updates from the kernel
> more or less for each kernel rc.
> The origin of the DT files are the kernel, but they keep copies
> for various reasons. And their update process from the kernel differs.
>
> In other words - barebox uses the kernel DT files in practice.
> U-boot in practice have their own copy (as I see it - did not look to close)

While barebox does a more automated sync, u-boot does do ad-hoc
copying of dts files from the kernel. And while it's typically a dtb
for u-boot and a dtb for the kernel, there is effort to make that be
the same dtb.

> > > As it is now one has to patch the kernel to add a panel to panel-simple,
> > > and add timing to device tree to let barebox use it.
> > >
> > > So it would be good once and for all to have the rules specified.
> > > And the preferred solution is to have timing in the devicetree
> > > so we can use it both in the kernel and in the bootloaders.
> >
> > This is *exactly* the same argument that I've heard many times before.
> > And it is still overly simplistic. Video timings are just one part of
> > the description of the panel. In most cases you need at least also a
> > power sequence.
> There are panels that are compatibe with panel-simple and there are the
> other panels.
> My comment is solely for the panel-simple compatible panels, where
> we already know stuff like power sequence and such.
>
> There are today a lot of panels in that group and in my tree there
> is patches waiting to add another three panels.
> This is panels that "just works" with barebox with timings(*) specified
> in the DT and for the kernel requires a patch to panle-simple.
>
> There are obviously a lot of panels that have additioanl requirements,
> but then this is not "panle-simple" compatible panels and outside the
> scope of my request.

Part of the problem with panel-simple is that it works at the
beginning and not as a platform progresses. Initially perhaps there is
no power control, but then that's added later. I've seen all to often
folks claiming panels "follow the simple-panel binding" and yet it
turns out that the panel has multiple power rails.

Rob