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

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Date:   Tue, 15 Nov 2022 10:43:36 -0800
From:   Kuogee Hsieh <quic_khsieh@...cinc.com>
To:     Dmitry Baryshkov <dmitry.baryshkov@...aro.org>,
        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 11/9/2022 11:43 PM, Dmitry Baryshkov wrote:
> 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.

I think this proposal is kind of strange.

According to DP spec, if a link support 5,4G, then it must support 1.6, 
2.7 and 5.4.

If it support 8.1G, then it must support 1.6 , 2.7 and 5.4.

There is no link can only support 2.7 and 5.4G without supporting 1.6G.

>
>> 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.
>>>
>