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

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Date: Tue, 27 Feb 2024 12:26:30 +0100
From: Paweł Anikiel <panikiel@...gle.com>
To: Krzysztof Kozlowski <krzysztof.kozlowski@...aro.org>
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 Mon, Feb 26, 2024 at 6:30 PM Krzysztof Kozlowski
<krzysztof.kozlowski@...aro.org> wrote:
>
> 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

The one that's already present in dtschema is kBps (kilobytes per
second) which isn't right for this case IMO.

>
> > 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.

I sent a PR with both kbps and mbps.