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

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Message-ID: <8a3ad930-bfb1-4531-9d34-fdf7d437f352@redhat.com>
Date: Sun, 29 Jun 2025 14:07:58 +0200
From: Hans de Goede <hdegoede@...hat.com>
To: Krzysztof Kozlowski <krzk@...nel.org>,
 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

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 

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

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.

Regards,

Hans