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

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Date:   Wed, 2 Nov 2022 20:15:06 +0300
From:   Dmitry Baryshkov <dmitry.baryshkov@...aro.org>
To:     Doug Anderson <dianders@...omium.org>
Cc:     Kuogee Hsieh <quic_khsieh@...cinc.com>, 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 01/11/2022 17:37, Doug Anderson wrote:
> Hi,
> 
> On Mon, Oct 31, 2022 at 5:15 PM Dmitry Baryshkov
> <dmitry.baryshkov@...aro.org> wrote:
>>
>> On 01/11/2022 03:08, Doug Anderson wrote:
>>> Hi,
>>>
>>> On Mon, Oct 31, 2022 at 2:11 PM Kuogee Hsieh <quic_khsieh@...cinc.com> wrote:
>>>>
>>>> Hi Dmitry,
>>>>
>>>>
>>>> Link rate is advertised by sink, but adjusted (reduced the link rate)
>>>> by host during link training.
>>>>
>>>> Therefore should be fine if host did not support HBR3 rate.
>>>>
>>>> It will reduce to lower link rate during link training procedures.
>>>>
>>>> kuogee
>>>>
>>>> On 10/31/2022 11:46 AM, Dmitry Baryshkov wrote:
>>>>> On 31/10/2022 20:27, Kuogee Hsieh wrote:
>>>>>> An HBR3-capable device shall also support TPS4. Since TPS4 feature
>>>>>> had been implemented already, it is not necessary to limit link
>>>>>> rate at HBR2 (5.4G). This patch remove this limitation to support
>>>>>> HBR3 (8.1G) link rate.
>>>>>
>>>>> The DP driver supports several platforms including sdm845 and can
>>>>> support, if I'm not mistaken, platforms up to msm8998/sdm630/660.
>>>>> Could you please confirm that all these SoCs have support for HBR3?
>>>>>
>>>>> With that fact being confirmed:
>>>>>
>>>>> Reviewed-by: Dmitry Baryshkov <dmitry.baryshkov@...aro.org>
>>>>>
>>>>>
>>>>>>
>>>>>> Signed-off-by: Kuogee Hsieh <quic_khsieh@...cinc.com>
>>>>>> ---
>>>>>>     drivers/gpu/drm/msm/dp/dp_panel.c | 4 ----
>>>>>>     1 file changed, 4 deletions(-)
>>>>>>
>>>>>> diff --git a/drivers/gpu/drm/msm/dp/dp_panel.c
>>>>>> b/drivers/gpu/drm/msm/dp/dp_panel.c
>>>>>> index 5149ceb..3344f5a 100644
>>>>>> --- a/drivers/gpu/drm/msm/dp/dp_panel.c
>>>>>> +++ b/drivers/gpu/drm/msm/dp/dp_panel.c
>>>>>> @@ -78,10 +78,6 @@ static int dp_panel_read_dpcd(struct dp_panel
>>>>>> *dp_panel)
>>>>>>         if (link_info->num_lanes > dp_panel->max_dp_lanes)
>>>>>>             link_info->num_lanes = dp_panel->max_dp_lanes;
>>>>>>     -    /* Limit support upto HBR2 until HBR3 support is added */
>>>>>> -    if (link_info->rate >=
>>>>>> (drm_dp_bw_code_to_link_rate(DP_LINK_BW_5_4)))
>>>>>> -        link_info->rate = drm_dp_bw_code_to_link_rate(DP_LINK_BW_5_4);
>>>>>> -
>>>>>>         drm_dbg_dp(panel->drm_dev, "version: %d.%d\n", major, minor);
>>>>>>         drm_dbg_dp(panel->drm_dev, "link_rate=%d\n", link_info->rate);
>>>>>>         drm_dbg_dp(panel->drm_dev, "lane_count=%d\n",
>>>>>> link_info->num_lanes);
>>>
>>> Stephen might remember better, but I could have sworn that the problem
>>> was that there might be something in the middle that couldn't support
>>> the higher link rate. In other words, I think we have:
>>>
>>> SoC <--> TypeC Port Controller <--> Display
>>>
>>> The SoC might support HBR3 and the display might support HBR3, but the
>>> TCPC (Type C Port Controller) might not. I think that the TCPC is a
>>> silent/passive component so it can't really let anyone know about its
>>> limitations.
>>>
>>> In theory I guess you could rely on link training to just happen to
>>> fail if you drive the link too fast for the TCPC to handle. Does this
>>> actually work reliably?
>>>
>>> I think the other option that was discussed in the past was to add
>>> something in the device tree for this. Either you could somehow model
>>> the TCPC in DRM and thus know that a given model of TCPC limits the
>>> link rate or you could hack in a property in the DP controller to
>>> limit it.
>>
>> Latest pmic_glink proposal from Bjorn include adding the drm_bridge for
>> the TCPC. Such bridge can in theory limit supported modes and rates.
> 
> Excellent! Even so, I think this isn't totally a solved problem,
> right? Even though a bridge seems like a good place for this, last I
> remember checking the bridge API wasn't expressive enough to solve
> this problem. A bridge could limit pixel clocks just fine, but here we
> need to take into account other considerations to know if a given
> pixel clock can work at 5.4 GHz or not. For instance, if we're at 4
> lanes we could maybe make a given pixel clock at 5.4 GHz but not if we
> only have 2 lanes. I don't think that the DP controller passes the
> number of lanes to other parts of the bridge chain, though maybe
> there's some trick for it?

I hope that somebody would fix MSM DP's data-lanes property usage to 
follow the usual way (a part of DT graph). Then it would be possible to 
query the amount of the lanes from the bridge.

> ...I guess the other problem is that all existing users aren't
> currently modeling their TCPC in this way. What happens to them?

There are no existing users. Bryan implemented TCPM support at some 
point, but we never pushed this upstream.

-- 
With best wishes
Dmitry