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

[Krzysztof Kozlowski <krzk@kernel.org>]

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.


Best regards,
Krzysztof