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

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Message-ID: <155e4171-187c-4ecf-5a9b-12f0c2207524@linaro.org>
Date:   Tue, 1 Nov 2022 03:15:51 +0300
From:   Dmitry Baryshkov <dmitry.baryshkov@...aro.org>
To:     Doug Anderson <dianders@...omium.org>,
        Kuogee Hsieh <quic_khsieh@...cinc.com>
Cc:     robdclark@...il.com, sean@...rly.run, swboyd@...omium.org,
        vkoul@...nel.org, daniel@...ll.ch, airlied@...ux.ie,
        agross@...nel.org, bjorn.andersson@...aro.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
Subject: Re: [PATCH] drm/msm/dp: remove limitation of link rate at 5.4G to
 support HBR3

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.

-- 
With best wishes
Dmitry