linux-kernel - Re: [PATCH] drm/msm/dp: delete DP_RECOVERED_CLOCK_OUT

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Date:   Mon, 1 Aug 2022 12:51:43 -0700
From:   Stephen Boyd <swboyd@...omium.org>
To:     Kuogee Hsieh <quic_khsieh@...cinc.com>, agross@...nel.org,
        airlied@...ux.ie, bjorn.andersson@...aro.org, daniel@...ll.ch,
        dianders@...omium.org, dmitry.baryshkov@...aro.org,
        robdclark@...il.com, sean@...rly.run, vkoul@...nel.org
Cc:     quic_aravindh@...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: delete DP_RECOVERED_CLOCK_OUT_EN to fix tps4

Quoting Kuogee Hsieh (2022-08-01 12:42:50)
> Data Symbols scrambled is required for tps4 at link training 2.
> Therefore SCRAMBLING_DISABLE bit should not be set for tps4 to
> work.
> RECOVERED_CLOCK_OUT_EN is for enable simple EYE test for jitter
> measurement with minimal equipment for embedded applications purpose
> and is not required to be set during normal operation.
> Current implementation always have RECOVERED_CLOCK_OUT_EN bit set
> which cause SCRAMBLING_DISABLE bit wrongly set at tps4 which prevent
> tps4 from working.
> This patch delete setting RECOVERED_CLOCK_OUT_EN to fix SCRAMBLING_DISABLE
> be wrongly set at tps4.
>
> Fixes: 956653250b21 ("drm/msm/dp: add support of tps4 (training pattern 4) for HBR3")
>
> Signed-off-by: Kuogee Hsieh <quic_khsieh@...cinc.com>
> ---
>  drivers/gpu/drm/msm/dp/dp_ctrl.c | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
>
> diff --git a/drivers/gpu/drm/msm/dp/dp_ctrl.c b/drivers/gpu/drm/msm/dp/dp_ctrl.c
> index ab6aa13..013ca02 100644
> --- a/drivers/gpu/drm/msm/dp/dp_ctrl.c
> +++ b/drivers/gpu/drm/msm/dp/dp_ctrl.c
> @@ -1214,7 +1214,7 @@ static int dp_ctrl_link_train_2(struct dp_ctrl_private *ctrl,
>         if (ret)
>                 return ret;
>
> -       dp_ctrl_train_pattern_set(ctrl, pattern | DP_RECOVERED_CLOCK_OUT_EN);
> +       dp_ctrl_train_pattern_set(ctrl, pattern);

This line is from the first patch introducing this driver. Even if this
is fixing tps4 support, it sounds like the bit should never have been
enabled in the first place. Why isn't the fixes tag targeted at the
first commit? Does it hurt to apply it without commit 956653250b21?