linux-kernel - Re: [PATCH v2 1/5] dt-bindings: display: simple-framebuffer: Add interconnects property

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Message-ID: <85521ded-734d-48e8-8f76-c57739102ded@kernel.org>
Date: Mon, 30 Jun 2025 10:24:06 +0200
From: Krzysztof Kozlowski <krzk@...nel.org>
To: Hans de Goede <hdegoede@...hat.com>, Luca Weiss <luca.weiss@...rphone.com>
Cc: Maarten Lankhorst <maarten.lankhorst@...ux.intel.com>,
 Maxime Ripard <mripard@...nel.org>, Thomas Zimmermann <tzimmermann@...e.de>,
 David Airlie <airlied@...il.com>, Simona Vetter <simona@...ll.ch>,
 Rob Herring <robh@...nel.org>, Krzysztof Kozlowski <krzk+dt@...nel.org>,
 Conor Dooley <conor+dt@...nel.org>,
 Javier Martinez Canillas <javierm@...hat.com>, Helge Deller <deller@....de>,
 linux-fbdev@...r.kernel.org, dri-devel@...ts.freedesktop.org,
 devicetree@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH v2 1/5] dt-bindings: display: simple-framebuffer: Add
 interconnects property

On 29/06/2025 14:07, Hans de Goede wrote:
> Hi Krzysztof,
> 
> On 28-Jun-25 1:49 PM, Krzysztof Kozlowski wrote:
>> On 27/06/2025 11:48, Luca Weiss wrote:
>>> Hi Krzysztof,
>>>
>>> On Fri Jun 27, 2025 at 10:08 AM CEST, Krzysztof Kozlowski wrote:
>>>> On Mon, Jun 23, 2025 at 08:44:45AM +0200, Luca Weiss wrote:
>>>>> Document the interconnects property which is a list of interconnect
>>>>> paths that is used by the framebuffer and therefore needs to be kept
>>>>> alive when the framebuffer is being used.
>>>>>
>>>>> Acked-by: Thomas Zimmermann <tzimmermann@...e.de>
>>>>> Signed-off-by: Luca Weiss <luca.weiss@...rphone.com>
>>>>> ---
>>>>>  Documentation/devicetree/bindings/display/simple-framebuffer.yaml | 3 +++
>>>>>  1 file changed, 3 insertions(+)
>>>>>
>>>>> diff --git a/Documentation/devicetree/bindings/display/simple-framebuffer.yaml b/Documentation/devicetree/bindings/display/simple-framebuffer.yaml
>>>>> index 296500f9da05e296dbbeec50ba5186b6b30aaffc..f0fa0ef23d91043dfb2b220c654b80e2e80850cd 100644
>>>>> --- a/Documentation/devicetree/bindings/display/simple-framebuffer.yaml
>>>>> +++ b/Documentation/devicetree/bindings/display/simple-framebuffer.yaml
>>>>> @@ -79,6 +79,9 @@ properties:
>>>>>    power-domains:
>>>>>      description: List of power domains used by the framebuffer.
>>>>>  
>>>>> +  interconnects:
>>>>> +    description: List of interconnect paths used by the framebuffer.
>>>>> +
>>>>
>>>> maxItems: 1, or this is not a simple FB anymore. Anything which needs
>>>> some sort of resources in unknown way is not simple anymore. You need
>>>> device specific bindings.
>>>
>>> The bindings support an arbitrary number of clocks, regulators,
>>> power-domains. Why should I artificially limit the interconnects to only
>>> one?
>>
>> And IMO they should not. Bindings are not supposed to be generic.
> 
> The simplefb binding is a binding to allow keeping the firmware, e.g.
> uboot setup framebuffer alive to e.g. show a boot splash until
> the native display-engine drive loads. Needing display-engine
> specific bindings totally contradicts the whole goal of 

No, it does not. DT is well designed for that through expressing
compatibility. I did not say you cannot have generic fallback for simple
use case.

But this (and previous patchset) grows this into generic binding ONLY
and that is not correct.


> 
> It is generic by nature and I really do not see how clocks and
> regulators are any different then interconnects here.

Yeah, they are also wrong. I already commented on this.

> 
> Note that we had a huge discussion about adding clock
> and regulators to simplefb many years ago with pretty
> much the same arguments against doing so. In the end it was
> decided to add regulator and clocks support to the simplefb
> bindings and non of the feared problems with e.g. ordening
> of turning things on happened.
> 
> A big part of this is that the claiming of clks / regulators /
> interconnects by the simplefb driver is there to keep things on,
> so it happens before the kernel starts tuning off unused resources
> IOW everything is already up and running and this really is about
> avoiding turning things off.

No one asks to drop them from the driver. I only want specific front
compatible which will list and constrain the properties. It is not
contradictory to your statements, U-boot support, driver support. I
really do not see ANY argument why this cannot follow standard DT rules.

Best regards,
Krzysztof