linux-kernel - Re: [PATCH] drm/msm/dp: remove limitation of link rate at 5.4G to support HBR3

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Date:   Thu, 10 Nov 2022 10:43:59 +0300
From:   Dmitry Baryshkov <dmitry.baryshkov@...aro.org>
To:     Kuogee Hsieh <quic_khsieh@...cinc.com>,
        Doug Anderson <dianders@...omium.org>
Cc:     robdclark@...il.com, sean@...rly.run, swboyd@...omium.org,
        vkoul@...nel.org, daniel@...ll.ch, airlied@...ux.ie,
        agross@...nel.org, quic_abhinavk@...cinc.com,
        quic_sbillaka@...cinc.com, freedreno@...ts.freedesktop.org,
        dri-devel@...ts.freedesktop.org, linux-arm-msm@...r.kernel.org,
        linux-kernel@...r.kernel.org,
        Bjorn Andersson <andersson@...nel.org>
Subject: Re: [PATCH] drm/msm/dp: remove limitation of link rate at 5.4G to
 support HBR3

On 10/11/2022 02:47, Kuogee Hsieh wrote:
> 
> On 11/2/2022 11:04 AM, Dmitry Baryshkov wrote:
>> On 02/11/2022 20:28, Doug Anderson wrote:
>>> Hi,
>>>
>>> On Wed, Nov 2, 2022 at 10:23 AM Dmitry Baryshkov
>>> <dmitry.baryshkov@...aro.org> wrote:
>>>>
>>>>> 1. Someone figures out how to model this with the bridge chain and
>>>>> then we only allow HBR3 if we detect we've got a TCPC that supports
>>>>> it. This seems like the cleanest / best but feels like a long pole.
>>>>> Not only have we been trying to get the TCPC-modeled-as-a-bridge stuff
>>>>> landed for a long time but even when we do it we still don't have a
>>>>> solution for how to communicate the number of lanes and other stuff
>>>>> between the TCPC and the DP controller so we have to enrich the bridge
>>>>> interface.
>>>>
>>>> I think we'd need some OOB interface. For example for DSI interfaces we
>>>> have mipi_dsi_device struct to communicate such OOB data.
>>>>
>>>> Also take a note regarding data-lanes from my previous email.
>>>
>>> Right, we can somehow communicate the max link rate through the bridge
>>> chain to the DP controller in an OOB manner that would work.
>>
>> I'd note that our dp_panel has some notion of such OOB data. So do AUX 
>> drivers including the panel-edp. My suggestion would be to consider 
>> both of them while modelling the OOB data.
>>
>>>
>>>
>>>>> 2. We add in a DT property to the display controller node that says
>>>>> the max link rate for use on this board. This feels like a hack, but
>>>>> maybe it's not too bad. Certainly it would be incredibly simple to
>>>>> implement. Actually... ...one could argue that even if we later model
>>>>> the TCPC as a bridge that this property would still be valid / useful!
>>>>> You could certainly imagine that the SoC supports HBR3 and the TCPC
>>>>> supports HBR3 but that the board routing between the SoC and the TCPC
>>>>> is bad and only supports HBR2. In this case the only way out is
>>>>> essentially a "board constraint" AKA a DT property in the DP
>>>>> controller.
>>>>
>>>> We have been discussing similar topics with Abhinav. Krzysztof 
>>>> suggested
>>>> using link-frequencies property to provide max and min values.
> 
> questions,
> 
> 1)is Krzysztof suggested had been implemented?

I can not parse this question, please excuse me.

Yes, Krzysztof suggested this being implemented as a link property, see 
media/video-interfaces.txt.

Moreover your implementation goes against both the existing definition 
(array with the list of frequencies) and Krzysztof's suggested extension 
(min and max). Listing just a single frequency goes against both these 
suggestions. In case of DP we have a fixed set of frequencies. Thus I'd 
suggest listing all supported frequencies instead.

> 2) where is link property i can add link-frequencies?

link node. Create outbound graph node, add link-frequencies there. Also 
as you are touching this part, please move the data-lanes property too.

> 
> 
>>>
>>> This sounds good to me and seems worth doing even if we eventually do 
>>> #1.
>>
>> And the bonus point is that it can be done easily.
>>

-- 
With best wishes
Dmitry