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

From: Dmitry Baryshkov <dmitry.baryshkov@linaro.org>
To: Kuogee Hsieh <quic_khsieh@quicinc.com>,
	Doug Anderson <dianders@chromium.org>
Cc: robdclark@gmail.com, sean@poorly.run, swboyd@chromium.org,
	vkoul@kernel.org, daniel@ffwll.ch, airlied@linux.ie,
	agross@kernel.org, quic_abhinavk@quicinc.com,
	quic_sbillaka@quicinc.com, freedreno@lists.freedesktop.org,
	dri-devel@lists.freedesktop.org, linux-arm-msm@vger.kernel.org,
	linux-kernel@vger.kernel.org,
	Bjorn Andersson <andersson@kernel.org>
Subject: Re: [PATCH] drm/msm/dp: remove limitation of link rate at 5.4G to support HBR3
Date: Wed, 16 Nov 2022 16:40:06 +0300	[thread overview]
Message-ID: <e85245a8-480a-0b1f-0e10-6a6d64ae76ec@linaro.org> (raw)
In-Reply-To: <dcddd992-60e9-b802-b7a3-bab877545761@quicinc.com>

On 15/11/2022 21:43, Kuogee Hsieh wrote:
> 
> 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@linaro.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.

Let me quote the docs.

   link-frequencies:
     $ref: /schemas/types.yaml#/definitions/uint64-array
     description:
       Allowed data bus frequencies. For MIPI CSI-2, for instance, this 
is the
       actual frequency of the bus, not bits per clock per lane value. 
An array
       of 64-bit unsigned integers.

Note. 'allowed data bus frequencies'. So by listing only the max 
frequency you'd break this description.

-- 
With best wishes
Dmitry