linux-kernel - Re: [PATCH v5 4/4] drm/msm/dp: Add support for LTTPR handling

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Message-ID: <e88c821f-3103-4d16-9605-bb88842e8911@quicinc.com>
Date: Mon, 3 Feb 2025 16:32:38 -0800
From: Abhinav Kumar <quic_abhinavk@...cinc.com>
To: Abel Vesa <abel.vesa@...aro.org>,
        Maarten Lankhorst
	<maarten.lankhorst@...ux.intel.com>,
        Maxime Ripard <mripard@...nel.org>,
        Thomas Zimmermann <tzimmermann@...e.de>,
        David Airlie <airlied@...il.com>, Simona Vetter <simona@...ll.ch>,
        Karol Herbst <kherbst@...hat.com>, Lyude
 Paul <lyude@...hat.com>,
        Danilo Krummrich <dakr@...hat.com>,
        Jani Nikula
	<jani.nikula@...ux.intel.com>,
        Rodrigo Vivi <rodrigo.vivi@...el.com>,
        Joonas
 Lahtinen <joonas.lahtinen@...ux.intel.com>,
        Tvrtko Ursulin
	<tursulin@...ulin.net>,
        Rob Clark <robdclark@...il.com>,
        Dmitry Baryshkov
	<dmitry.baryshkov@...aro.org>,
        Sean Paul <sean@...rly.run>,
        Marijn Suijten
	<marijn.suijten@...ainline.org>
CC: Bjorn Andersson <andersson@...nel.org>,
        Konrad Dybcio
	<konradybcio@...nel.org>,
        Johan Hovold <johan@...nel.org>, <dri-devel@...ts.freedesktop.org>,
        <linux-kernel@...r.kernel.org>, <nouveau@...ts.freedesktop.org>,
        <intel-gfx@...ts.freedesktop.org>, <intel-xe@...ts.freedesktop.org>,
        <linux-arm-msm@...r.kernel.org>, <freedreno@...ts.freedesktop.org>,
        Johan Hovold <johan+linaro@...nel.org>
Subject: Re: [PATCH v5 4/4] drm/msm/dp: Add support for LTTPR handling



On 2/3/2025 2:57 AM, Abel Vesa wrote:
> Link Training Tunable PHY Repeaters (LTTPRs) are defined in DisplayPort
> 1.4a specification. As the name suggests, these PHY repeaters are
> capable of adjusting their output for link training purposes.
> 
> According to the DisplayPort standard, LTTPRs have two operating
> modes:
>   - non-transparent - it replies to DPCD LTTPR field specific AUX
>     requests, while passes through all other AUX requests
>   - transparent - it passes through all AUX requests.
> 
> Switching between these two modes is done by the DPTX by issuing
> an AUX write to the DPCD PHY_REPEATER_MODE register.
> 
> The msm DP driver is currently lacking any handling of LTTPRs.
> This means that if at least one LTTPR is found between DPTX and DPRX,
> the link training would fail if that LTTPR was not already configured
> in transparent mode.
> 
> The section 3.6.6.1 from the DisplayPort v2.0 specification mandates
> that before link training with the LTTPR is started, the DPTX may place
> the LTTPR in non-transparent mode by first switching to transparent mode
> and then to non-transparent mode. This operation seems to be needed only
> on first link training and doesn't need to be done again until device is
> unplugged.
> 
> It has been observed on a few X Elite-based platforms which have
> such LTTPRs in their board design that the DPTX needs to follow the
> procedure described above in order for the link training to be successful.
> 
> So add support for reading the LTTPR DPCD caps to figure out the number
> of such LTTPRs first. Then, for platforms (or Type-C dongles) that have
> at least one such an LTTPR, set its operation mode to transparent mode
> first and then to non-transparent, just like the mentioned section of
> the specification mandates.
> 
> Tested-by: Johan Hovold <johan+linaro@...nel.org>
> Reviewed-by: Dmitry Baryshkov <dmitry.baryshkov@...aro.org>
> Reviewed-by: Johan Hovold <johan+linaro@...nel.org>
> Signed-off-by: Abel Vesa <abel.vesa@...aro.org>
> ---
>   drivers/gpu/drm/msm/dp/dp_display.c | 15 +++++++++++++++
>   1 file changed, 15 insertions(+)
> 

Reviewed-by: Abhinav Kumar <quic_abhinavk@...cinc.com>