linux-kernel - Re: [PATCH v2 1/4] drm/dp: Add helper to set LTTPRs in transparent mode

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Message-ID: <o6xcm7jdcay77b6kltj7zownk6je6umqlmxsuscbbubw4jlr5v@w4zuusufajwx>
Date: Wed, 11 Dec 2024 21:22:00 +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
Subject: Re: [PATCH v2 1/4] drm/dp: Add helper to set LTTPRs in transparent
 mode

On Wed, Dec 11, 2024 at 03:04:12PM +0200, Abel Vesa wrote:
> 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.
> 
> Add a generic helper that allows switching between these modes.
> 
> Also add a generic wrapper for the helper that handles the explicit
> disabling of non-transparent mode and its disable->enable sequence
> mentioned in the DP Standard v2.0 section 3.6.6.1. Do this in order
> to move this handling out of the vendor specific driver implementation
> into the generic framework.
> 
> Signed-off-by: Abel Vesa <abel.vesa@...aro.org>
> ---
>  drivers/gpu/drm/display/drm_dp_helper.c | 50 +++++++++++++++++++++++++++++++++
>  include/drm/display/drm_dp_helper.h     |  2 ++
>  2 files changed, 52 insertions(+)
> 


> +/**
> + * drm_dp_lttpr_init - init LTTPR transparency mode according to DP standard
> + *
> + * @aux: DisplayPort AUX channel
> + * @lttpr_count: Number of LTTPRs
> + *
> + * Returns 0 on success or a negative error code on failure.
> + */
> +int drm_dp_lttpr_init(struct drm_dp_aux *aux, int lttpr_count)
> +{
> +	if (!lttpr_count)
> +		return 0;
> +
> +	/*
> +	 * See DP Standard v2.0 3.6.6.1 about the explicit disabling of
> +	 * non-transparent mode and the disable->enable non-transparent mode
> +	 * sequence.
> +	 */
> +	drm_dp_lttpr_set_transparent_mode(aux, true);
> +
> +	if (lttpr_count > 0 && !drm_dp_lttpr_set_transparent_mode(aux, false))
> +		return 0;
> +
> +	/*
> +	 * Roll-back to tranparent mode if setting non-tranparent mode failed or
> +	 * the number of LTTPRs is invalid
> +	 */
> +	drm_dp_lttpr_set_transparent_mode(aux, true);

This means that if lttpr_count < 0, then there will be two requests to
set LTTPRs to a transparent mode. Is that expected?

> +
> +	return -EINVAL;
> +}
> +EXPORT_SYMBOL(drm_dp_lttpr_init);
> +

-- 
With best wishes
Dmitry