linux-kernel - Re: [PATCH v5 14/14] drm/msm/dp: Add support for lane mapping configuration

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Message-ID: <d7b2a904-97de-4716-b179-4ad8d3ecbe58@oss.qualcomm.com>
Date: Tue, 23 Sep 2025 11:38:40 +0800
From: Xiangxu Yin <xiangxu.yin@....qualcomm.com>
To: Dmitry Baryshkov <dmitry.baryshkov@....qualcomm.com>,
        Rob Clark <rob.clark@....qualcomm.com>
Cc: Vinod Koul <vkoul@...nel.org>, Kishon Vijay Abraham I
 <kishon@...nel.org>,
        Rob Herring <robh@...nel.org>,
        Krzysztof Kozlowski <krzk+dt@...nel.org>,
        Conor Dooley
 <conor+dt@...nel.org>,
        Dmitry Baryshkov <lumag@...nel.org>,
        Abhinav Kumar <abhinav.kumar@...ux.dev>,
        Jessica Zhang <jessica.zhang@....qualcomm.com>,
        Sean Paul <sean@...rly.run>,
        Marijn Suijten <marijn.suijten@...ainline.org>,
        David Airlie <airlied@...il.com>, Simona Vetter <simona@...ll.ch>,
        linux-arm-msm@...r.kernel.org, linux-phy@...ts.infradead.org,
        devicetree@...r.kernel.org, linux-kernel@...r.kernel.org,
        dri-devel@...ts.freedesktop.org, freedreno@...ts.freedesktop.org,
        fange.zhang@....qualcomm.com, yongxing.mou@....qualcomm.com,
        li.liu@....qualcomm.com, Bjorn Andersson <andersson@...nel.org>,
        Konrad Dybcio <konradybcio@...nel.org>
Subject: Re: [PATCH v5 14/14] drm/msm/dp: Add support for lane mapping
 configuration


On 9/23/2025 7:39 AM, Dmitry Baryshkov wrote:
> On Mon, Sep 22, 2025 at 08:13:00AM -0700, Rob Clark wrote:
>> On Fri, Sep 19, 2025 at 11:35 AM Dmitry Baryshkov
>> <dmitry.baryshkov@....qualcomm.com> wrote:
>>> On Fri, Sep 19, 2025 at 10:24:31PM +0800, Xiangxu Yin wrote:
>>>> QCS615 platform requires non-default logical-to-physical lane mapping due
>>>> to its unique hardware routing. Unlike the standard mapping sequence
>>>> <0 1 2 3>, QCS615 uses <3 2 0 1>, which necessitates explicit
>>>> configuration via the data-lanes property in the device tree. This ensures
>>>> correct signal routing between the DP controller and PHY.
>>>>
>>>> For partial definitions, fill remaining lanes with unused physical lanes
>>>> in ascending order.
>>>>
>>>> Signed-off-by: Xiangxu Yin <xiangxu.yin@....qualcomm.com>
>>>> ---
>>>>  drivers/gpu/drm/msm/dp/dp_ctrl.c | 10 +++----
>>>>  drivers/gpu/drm/msm/dp/dp_link.c | 60 ++++++++++++++++++++++++++++++++++++++++
>>>>  drivers/gpu/drm/msm/dp/dp_link.h |  1 +
>>>>  3 files changed, 66 insertions(+), 5 deletions(-)
>>>>
>>>> diff --git a/drivers/gpu/drm/msm/dp/dp_link.c b/drivers/gpu/drm/msm/dp/dp_link.c
>>>> index 2aeb3ecf76fab2ee6a9512b785ca5dceebfc3964..34a91e194a124ef5372f13352f7b3513aa88da2a 100644
>>>> --- a/drivers/gpu/drm/msm/dp/dp_link.c
>>>> +++ b/drivers/gpu/drm/msm/dp/dp_link.c
>>>> @@ -1236,6 +1236,61 @@ static u32 msm_dp_link_link_frequencies(struct device_node *of_node)
>>>>       return frequency;
>>>>  }
>>>>
>>>> +/*
>>>> + * Always populate msm_dp_link->lane_map with 4 lanes.
>>>> + * - Use DTS "data-lanes" if present; otherwise fall back to default mapping.
>>>> + * - For partial definitions, fill remaining entries with unused lanes in
>>>> + *   ascending order.
>>>> + */
>>>> +static int msm_dp_link_lane_map(struct device *dev, struct msm_dp_link *msm_dp_link)
>>>> +{
>>>> +     struct device_node *of_node = dev->of_node;
>>>> +     struct device_node *endpoint;
>>>> +     int cnt = msm_dp_link->max_dp_lanes;
>>>> +     u32 tmp[DP_MAX_NUM_DP_LANES];
>>>> +     u32 map[DP_MAX_NUM_DP_LANES] = {0, 1, 2, 3}; /* default 1:1 mapping */
>>>> +     bool used[DP_MAX_NUM_DP_LANES] = {false};
>>>> +     int i, j = 0, ret = -EINVAL;
>>>> +
>>>> +     endpoint = of_graph_get_endpoint_by_regs(of_node, 1, -1);
>>>> +     if (endpoint) {
>>>> +             ret = of_property_read_u32_array(endpoint, "data-lanes", tmp, cnt);
>>>> +             if (ret)
>>>> +                     dev_dbg(dev, "endpoint data-lanes read failed (ret=%d)\n", ret);
>>>> +     }
>>>> +
>>>> +     if (ret) {
>>>> +             ret = of_property_read_u32_array(of_node, "data-lanes", tmp, cnt);
>>>> +             if (ret) {
>>>> +                     dev_info(dev, "data-lanes not defined, set to default\n");
>>>> +                     goto out;
>>>> +             }
>>>> +     }
>>>> +
>>>> +     for (i = 0; i < cnt; i++) {
>>>> +             if (tmp[i] >= DP_MAX_NUM_DP_LANES) {
>>>> +                     dev_err(dev, "data-lanes[%d]=%u out of range\n", i, tmp[i]);
>>>> +                     return -EINVAL;
>>>> +             }
>>>> +             used[tmp[i]] = true;
>>>> +             map[i] = tmp[i];
>>>> +     }
>>>> +
>>>> +     /* Fill the remaining entries with unused physical lanes (ascending) */
>>>> +     for (i = cnt; i < DP_MAX_NUM_DP_LANES && j < DP_MAX_NUM_DP_LANES; j++) {
>>> Nit: i = cnt, j = 0; Don't init loop variables at the top of the
>>> function.
>> These days we can party like it's c99 and declare loop variables
>> inside the for(), instead of at the top of the function.  My
>> preference is to do so, unless the loop variable is used after the
>> loop.
> Ack, works for me too. I think I'm not used for having theese parts of
> C99 allowed in the kernel.


Got it. j is only used in that for loop, so I'll update it to use C99-style declaration as suggested.