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Message-ID: <qsmcpai2uqfhmy6y5wks55p4fyqigndfzqfnapvdjf35gu4wg3@5aowfh3xaydn>
Date: Fri, 3 Jan 2025 20:12:13 +0200
From: Dmitry Baryshkov <dmitry.baryshkov@...aro.org>
To: Abel Vesa <abel.vesa@...aro.org>
Cc: 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>,
Abhinav Kumar <quic_abhinavk@...cinc.com>, Sean Paul <sean@...rly.run>,
Marijn Suijten <marijn.suijten@...ainline.org>, 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 v3 4/4] drm/msm/dp: Add support for LTTPR handling
On Fri, Jan 03, 2025 at 02:58:18PM +0200, 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 this 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>
> Signed-off-by: Abel Vesa <abel.vesa@...aro.org>
> ---
> drivers/gpu/drm/msm/dp/dp_display.c | 17 +++++++++++++++++
> 1 file changed, 17 insertions(+)
Reviewed-by: Dmitry Baryshkov <dmitry.baryshkov@...aro.org>
--
With best wishes
Dmitry
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