Re: [PATCH v2 7/8] dt-bindings: display: allwinner: Split H616 DE33 layer reg space

["Jernej Škrabec" <jernej.skrabec@gmail.com>]

Dne ponedeljek, 25. maj 2026 ob 14:10:38 Srednjeevropski poletni čas je Krzysztof Kozlowski napisal(a):
> On 24/05/2026 23:33, Chen-Yu Tsai wrote:
> > Hi,
> > 
> > (resent from new email)
> > 
> > On Thu, May 14, 2026 at 2:04 PM Krzysztof Kozlowski <krzk@kernel.org> wrote:
> >>
> >> On Sat, May 09, 2026 at 09:00:14PM +0200, Jernej Skrabec wrote:
> >>> From: Jernej Skrabec <jernej.skrabec@gmail.com>
> >>>
> >>> As it turns out, current H616 DE33 binding was written based on
> >>> incomplete understanding of DE33 design. Namely, planes are shared
> >>> resource and not tied to specific mixer, which was the case for previous
> >>> generations of Display Engine (DE3 and earlier).
> >>>
> >>> This means that current DE33 binding doesn't properly reflect HW and
> >>> using it would mean that second mixer (used for second display output)
> >>> can't be supported.
> >>>
> >>> Remove layer register space, which will be represented with additional
> >>> node, and replace it with phandle, which will point to that new, shared
> >>> node. That way, all mixers can share same layers.
> >>>
> >>> There is no user of this binding yet, so changes can be made safely,
> >>> without breaking any backward compatibility.
> >>
> >> There is user. git grep gives me:
> >> drivers/gpu/drm/sun4i/sun8i_mixer.c
> >>
> >> which means this is a released ABI. As I understood, the old code was
> > 
> > We held off on merging the DT changes so that we could rework this.
> > I can't find the actual request though. It was probably over IRC.
> > 
> >> working fine but just did not support all use cases. Why this cannot be
> >> kept backwards compatible?
> > 
> > AFAIK the "planes" block is shared between two display mixers. As the
> > commit message explains, this prevents using the second mixer, since
> > only one of them can claim and map the register space. And on the H700
> > (which is the same die as the H616 discussed here but with more exposed
> > interfaces), there could actually be a use case for the second mixer.
> 
> It explains why you want to make the changes but not why you cannot keep
> it backwards compatible.

I guess it can be backward compatible, but I don't think it makes sense.
Yes, original driver implemented original DT bindings, but there is no node
which uses that binding. If there is no user of that, why would driver
need to support it nevertheless? Supporting only actually used DT binding
allows for better code architecture, as there is no need to support second,
unused path. It also simplifies testing, since developer doesn't need to
test both paths if code is changed in that area.

Best regards,
Jernej

Best regards,
Jernej