linux-kernel - Re: [PATCH v2 9/9] ARM: dts: chameleonv3: Add video device nodes

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Date: Mon, 26 Feb 2024 18:30:16 +0100
From: Krzysztof Kozlowski <krzysztof.kozlowski@...aro.org>
To: Paweł Anikiel <panikiel@...gle.com>
Cc: airlied@...il.com, akpm@...ux-foundation.org, conor+dt@...nel.org,
 daniel@...ll.ch, dinguyen@...nel.org, hverkuil-cisco@...all.nl,
 krzysztof.kozlowski+dt@...aro.org, maarten.lankhorst@...ux.intel.com,
 mchehab@...nel.org, mripard@...nel.org, robh+dt@...nel.org,
 tzimmermann@...e.de, devicetree@...r.kernel.org,
 dri-devel@...ts.freedesktop.org, linux-kernel@...r.kernel.org,
 linux-media@...r.kernel.org, chromeos-krk-upstreaming@...gle.com,
 ribalda@...omium.org
Subject: Re: [PATCH v2 9/9] ARM: dts: chameleonv3: Add video device nodes

On 26/02/2024 13:27, Paweł Anikiel wrote:
> On Mon, Feb 26, 2024 at 1:07 PM Krzysztof Kozlowski
> <krzysztof.kozlowski@...aro.org> wrote:
>>
>> On 26/02/2024 12:09, Paweł Anikiel wrote:
>>> On Mon, Feb 26, 2024 at 10:15 AM Krzysztof Kozlowski
>>> <krzysztof.kozlowski@...aro.org> wrote:
>>>>
>>>> On 21/02/2024 17:02, Paweł Anikiel wrote:
>>>>> Add device nodes for the video system present on the Chameleon v3.
>>>>> It consists of six framebuffers and two Intel Displayport receivers.
>>>>>
>>>>> Signed-off-by: Paweł Anikiel <panikiel@...gle.com>
>>>>> ---
>>>>
>>>> ...
>>>>
>>>>> +             dprx_sst: dp-receiver@...64000 {
>>>>> +                     compatible = "intel,dprx-20.0.1";
>>>>> +                     reg = <0xc0064000 0x800>;
>>>>> +                     interrupt-parent = <&dprx_sst_irq>;
>>>>> +                     interrupts = <0 IRQ_TYPE_EDGE_RISING>;
>>>>> +                     intel,max-link-rate = <0x1e>;
>>>>
>>>> Rate is not in hex! Rate is in Hz, at least usually...
>>>>
>>>> Fix your bindings...
>>>
>>> This is the DisplayPort link rate, for which the allowed values are
>>> 8.1 Gbps, 5.4 Gbps, 2.7 Gbps, or 1.62 Gbps. The standard way to encode
>>> them (used in the DisplayPort DPCD registers and this device's
>>
>> Then it is in bps or some other units:
>>
>> https://github.com/devicetree-org/dt-schema/blob/main/dtschema/schemas/property-units.yaml
>>
>>> configuration) is by multiples of 0.27Gbps. This value (AFAIK) is
>>> usually represented in hex, so 8.1Gbps would be 0x1e.
>>
>> No, the value is represented in logical units. Frequency in Hz. Rate in
>> bps/kbps/etc. Voltage in volts.
> 
> Okay, thanks for the info. So if I understand correctly, the max link
> rate should be represented in bps in the devicetree, and then be

or kbps

> converted to the per 0.27Gbps value by the driver?

If driver needs some register-based value, then yes.

> 
> One problem is that the values here are too large to be represented in
> bps (since the datatype is uint32). Can the property be in Mbps
> instead?

Can be. You can submit a patch to dtschema (patch to DT spec list or
github pull request) adding '-mbps' as well.

Best regards,
Krzysztof