kernel/drivers/gpu/drm/tegra/dp.c

// SPDX-License-Identifier: MIT
/*
 * Copyright (C) 2013-2019 NVIDIA Corporation
 * Copyright (C) 2015 Rob Clark
 */

#include <drm/drm_crtc.h>
#include <drm/drm_dp_helper.h>
#include <drm/drm_print.h>

#include "dp.h"

static const u8 drm_dp_edp_revisions[] = { 0x11, 0x12, 0x13, 0x14 };

static void drm_dp_link_caps_reset(struct drm_dp_link_caps *caps)
{
	caps->enhanced_framing = false;
	caps->tps3_supported = false;
	caps->fast_training = false;
	caps->channel_coding = false;
	caps->alternate_scrambler_reset = false;
}

void drm_dp_link_caps_copy(struct drm_dp_link_caps *dest,
			   const struct drm_dp_link_caps *src)
{
	dest->enhanced_framing = src->enhanced_framing;
	dest->tps3_supported = src->tps3_supported;
	dest->fast_training = src->fast_training;
	dest->channel_coding = src->channel_coding;
	dest->alternate_scrambler_reset = src->alternate_scrambler_reset;
}

static void drm_dp_link_reset(struct drm_dp_link *link)
{
	unsigned int i;

	if (!link)
		return;

	link->revision = 0;
	link->max_rate = 0;
	link->max_lanes = 0;

	drm_dp_link_caps_reset(&link->caps);
	link->aux_rd_interval.cr = 0;
	link->aux_rd_interval.ce = 0;
	link->edp = 0;

	link->rate = 0;
	link->lanes = 0;

	for (i = 0; i < DP_MAX_SUPPORTED_RATES; i++)
		link->rates[i] = 0;

	link->num_rates = 0;
}

/**
 * drm_dp_link_add_rate() - add a rate to the list of supported rates
 * @link: the link to add the rate to
 * @rate: the rate to add
 *
 * Add a link rate to the list of supported link rates.
 *
 * Returns:
 * 0 on success or one of the following negative error codes on failure:
 * - ENOSPC if the maximum number of supported rates has been reached
 * - EEXISTS if the link already supports this rate
 *
 * See also:
 * drm_dp_link_remove_rate()
 */
int drm_dp_link_add_rate(struct drm_dp_link *link, unsigned long rate)
{
	unsigned int i, pivot;

	if (link->num_rates == DP_MAX_SUPPORTED_RATES)
		return -ENOSPC;

	for (pivot = 0; pivot < link->num_rates; pivot++)
		if (rate <= link->rates[pivot])
			break;

	if (pivot != link->num_rates && rate == link->rates[pivot])
		return -EEXIST;

	for (i = link->num_rates; i > pivot; i--)
		link->rates[i] = link->rates[i - 1];

	link->rates[pivot] = rate;
	link->num_rates++;

	return 0;
}

/**
 * drm_dp_link_remove_rate() - remove a rate from the list of supported rates
 * @link: the link from which to remove the rate
 * @rate: the rate to remove
 *
 * Removes a link rate from the list of supported link rates.
 *
 * Returns:
 * 0 on success or one of the following negative error codes on failure:
 * - EINVAL if the specified rate is not among the supported rates
 *
 * See also:
 * drm_dp_link_add_rate()
 */
int drm_dp_link_remove_rate(struct drm_dp_link *link, unsigned long rate)
{
	unsigned int i;

	for (i = 0; i < link->num_rates; i++)
		if (rate == link->rates[i])
			break;

	if (i == link->num_rates)
		return -EINVAL;

	link->num_rates--;

	while (i < link->num_rates) {
		link->rates[i] = link->rates[i + 1];
		i++;
	}

	return 0;
}

/**
 * drm_dp_link_update_rates() - normalize the supported link rates array
 * @link: the link for which to normalize the supported link rates
 *
 * Users should call this function after they've manually modified the array
 * of supported link rates. This function removes any stale entries, compacts
 * the array and updates the supported link rate count. Note that calling the
 * drm_dp_link_remove_rate() function already does this janitorial work.
 *
 * See also:
 * drm_dp_link_add_rate(), drm_dp_link_remove_rate()
 */
void drm_dp_link_update_rates(struct drm_dp_link *link)
{
	unsigned int i, count = 0;

	for (i = 0; i < link->num_rates; i++) {
		if (link->rates[i] != 0)
			link->rates[count++] = link->rates[i];
	}

	for (i = count; i < link->num_rates; i++)
		link->rates[i] = 0;

	link->num_rates = count;
}

/**
 * drm_dp_link_probe() - probe a DisplayPort link for capabilities
 * @aux: DisplayPort AUX channel
 * @link: pointer to structure in which to return link capabilities
 *
 * The structure filled in by this function can usually be passed directly
 * into drm_dp_link_power_up() and drm_dp_link_configure() to power up and
 * configure the link based on the link's capabilities.
 *
 * Returns 0 on success or a negative error code on failure.
 */
int drm_dp_link_probe(struct drm_dp_aux *aux, struct drm_dp_link *link)
{
	u8 dpcd[DP_RECEIVER_CAP_SIZE], value;
	unsigned int rd_interval;
	int err;

	drm_dp_link_reset(link);

	err = drm_dp_dpcd_read(aux, DP_DPCD_REV, dpcd, sizeof(dpcd));
	if (err < 0)
		return err;

	link->revision = dpcd[DP_DPCD_REV];
	link->max_rate = drm_dp_max_link_rate(dpcd);
	link->max_lanes = drm_dp_max_lane_count(dpcd);

	link->caps.enhanced_framing = drm_dp_enhanced_frame_cap(dpcd);
	link->caps.tps3_supported = drm_dp_tps3_supported(dpcd);
	link->caps.fast_training = drm_dp_fast_training_cap(dpcd);
	link->caps.channel_coding = drm_dp_channel_coding_supported(dpcd);

	if (drm_dp_alternate_scrambler_reset_cap(dpcd)) {
		link->caps.alternate_scrambler_reset = true;

		err = drm_dp_dpcd_readb(aux, DP_EDP_DPCD_REV, &value);
		if (err < 0)
			return err;

		if (value >= ARRAY_SIZE(drm_dp_edp_revisions))
			DRM_ERROR("unsupported eDP version: %02x\n", value);
		else
			link->edp = drm_dp_edp_revisions[value];
	}

	/*
	 * The DPCD stores the AUX read interval in units of 4 ms. There are
	 * two special cases:
	 *
	 *   1) if the TRAINING_AUX_RD_INTERVAL field is 0, the clock recovery
	 *      and channel equalization should use 100 us or 400 us AUX read
	 *      intervals, respectively
	 *
	 *   2) for DP v1.4 and above, clock recovery should always use 100 us
	 *      AUX read intervals
	 */
	rd_interval = dpcd[DP_TRAINING_AUX_RD_INTERVAL] &
			   DP_TRAINING_AUX_RD_MASK;

	if (rd_interval > 4) {
		DRM_DEBUG_KMS("AUX interval %u out of range (max. 4)\n",
			      rd_interval);
		rd_interval = 4;
	}

	rd_interval *= 4 * USEC_PER_MSEC;

	if (rd_interval == 0 || link->revision >= DP_DPCD_REV_14)
		link->aux_rd_interval.cr = 100;

	if (rd_interval == 0)
		link->aux_rd_interval.ce = 400;

	link->rate = link->max_rate;
	link->lanes = link->max_lanes;

	/* Parse SUPPORTED_LINK_RATES from eDP 1.4 */
	if (link->edp >= 0x14) {
		u8 supported_rates[DP_MAX_SUPPORTED_RATES * 2];
		unsigned int i;
		u16 rate;

		err = drm_dp_dpcd_read(aux, DP_SUPPORTED_LINK_RATES,
				       supported_rates,
				       sizeof(supported_rates));
		if (err < 0)
			return err;

		for (i = 0; i < DP_MAX_SUPPORTED_RATES; i++) {
			rate = supported_rates[i * 2 + 1] << 8 |
			       supported_rates[i * 2 + 0];

			drm_dp_link_add_rate(link, rate * 200);
		}
	}

	return 0;
}

/**
 * drm_dp_link_power_up() - power up a DisplayPort link
 * @aux: DisplayPort AUX channel
 * @link: pointer to a structure containing the link configuration
 *
 * Returns 0 on success or a negative error code on failure.
 */
int drm_dp_link_power_up(struct drm_dp_aux *aux, struct drm_dp_link *link)
{
	u8 value;
	int err;

	/* DP_SET_POWER register is only available on DPCD v1.1 and later */
	if (link->revision < 0x11)
		return 0;

	err = drm_dp_dpcd_readb(aux, DP_SET_POWER, &value);
	if (err < 0)
		return err;

	value &= ~DP_SET_POWER_MASK;
	value |= DP_SET_POWER_D0;

	err = drm_dp_dpcd_writeb(aux, DP_SET_POWER, value);
	if (err < 0)
		return err;

	/*
	 * According to the DP 1.1 specification, a "Sink Device must exit the
	 * power saving state within 1 ms" (Section 2.5.3.1, Table 5-52, "Sink
	 * Control Field" (register 0x600).
	 */
	usleep_range(1000, 2000);

	return 0;
}

/**
 * drm_dp_link_power_down() - power down a DisplayPort link
 * @aux: DisplayPort AUX channel
 * @link: pointer to a structure containing the link configuration
 *
 * Returns 0 on success or a negative error code on failure.
 */
int drm_dp_link_power_down(struct drm_dp_aux *aux, struct drm_dp_link *link)
{
	u8 value;
	int err;

	/* DP_SET_POWER register is only available on DPCD v1.1 and later */
	if (link->revision < 0x11)
		return 0;

	err = drm_dp_dpcd_readb(aux, DP_SET_POWER, &value);
	if (err < 0)
		return err;

	value &= ~DP_SET_POWER_MASK;
	value |= DP_SET_POWER_D3;

	err = drm_dp_dpcd_writeb(aux, DP_SET_POWER, value);
	if (err < 0)
		return err;

	return 0;
}

/**
 * drm_dp_link_configure() - configure a DisplayPort link
 * @aux: DisplayPort AUX channel
 * @link: pointer to a structure containing the link configuration
 *
 * Returns 0 on success or a negative error code on failure.
 */
int drm_dp_link_configure(struct drm_dp_aux *aux, struct drm_dp_link *link)
{
	u8 values[2], value;
	int err;

	if (link->ops && link->ops->configure) {
		err = link->ops->configure(link);
		if (err < 0) {
			DRM_ERROR("failed to configure DP link: %d\n", err);
			return err;
		}
	}

	values[0] = drm_dp_link_rate_to_bw_code(link->rate);
	values[1] = link->lanes;

	if (link->caps.enhanced_framing)
		values[1] |= DP_LANE_COUNT_ENHANCED_FRAME_EN;

	err = drm_dp_dpcd_write(aux, DP_LINK_BW_SET, values, sizeof(values));
	if (err < 0)
		return err;

	if (link->caps.channel_coding)
		value = DP_SET_ANSI_8B10B;
	else
		value = 0;

	err = drm_dp_dpcd_writeb(aux, DP_MAIN_LINK_CHANNEL_CODING_SET, value);
	if (err < 0)
		return err;

	if (link->caps.alternate_scrambler_reset) {
		err = drm_dp_dpcd_writeb(aux, DP_EDP_CONFIGURATION_SET,
					 DP_ALTERNATE_SCRAMBLER_RESET_ENABLE);
		if (err < 0)
			return err;
	}

	return 0;
}

/**
 * drm_dp_link_choose() - choose the lowest possible configuration for a mode
 * @link: DRM DP link object
 * @mode: DRM display mode
 * @info: DRM display information
 *
 * According to the eDP specification, a source should select a configuration
 * with the lowest number of lanes and the lowest possible link rate that can
 * match the bitrate requirements of a video mode. However it must ensure not
 * to exceed the capabilities of the sink.
 *
 * Returns: 0 on success or a negative error code on failure.
 */
int drm_dp_link_choose(struct drm_dp_link *link,
		       const struct drm_display_mode *mode,
		       const struct drm_display_info *info)
{
	/* available link symbol clock rates */
	static const unsigned int rates[3] = { 162000, 270000, 540000 };
	/* available number of lanes */
	static const unsigned int lanes[3] = { 1, 2, 4 };
	unsigned long requirement, capacity;
	unsigned int rate = link->max_rate;
	unsigned int i, j;

	/* bandwidth requirement */
	requirement = mode->clock * info->bpc * 3;

	for (i = 0; i < ARRAY_SIZE(lanes) && lanes[i] <= link->max_lanes; i++) {
		for (j = 0; j < ARRAY_SIZE(rates) && rates[j] <= rate; j++) {
			/*
			 * Capacity for this combination of lanes and rate,
			 * factoring in the ANSI 8B/10B encoding.
			 *
			 * Link rates in the DRM DP helpers are really link
			 * symbol frequencies, so a tenth of the actual rate
			 * of the link.
			 */
			capacity = lanes[i] * (rates[j] * 10) * 8 / 10;

			if (capacity >= requirement) {
				DRM_DEBUG_KMS("using %u lanes at %u kHz (%lu/%lu kbps)\n",
					      lanes[i], rates[j], requirement,
					      capacity);
				link->lanes = lanes[i];
				link->rate = rates[j];
				return 0;
			}
		}
	}

	return -ERANGE;
}

/**
 * DOC: Link training
 *
 * These functions contain common logic and helpers to implement DisplayPort
 * link training.
 */

/**
 * drm_dp_link_train_init() - initialize DisplayPort link training state
 * @train: DisplayPort link training state
 */
void drm_dp_link_train_init(struct drm_dp_link_train *train)
{
	struct drm_dp_link_train_set *request = &train->request;
	struct drm_dp_link_train_set *adjust = &train->adjust;
	unsigned int i;

	for (i = 0; i < 4; i++) {
		request->voltage_swing[i] = 0;
		adjust->voltage_swing[i] = 0;

		request->pre_emphasis[i] = 0;
		adjust->pre_emphasis[i] = 0;

		request->post_cursor[i] = 0;
		adjust->post_cursor[i] = 0;
	}

	train->pattern = DP_TRAINING_PATTERN_DISABLE;
	train->clock_recovered = false;
	train->channel_equalized = false;
}

static bool drm_dp_link_train_valid(const struct drm_dp_link_train *train)
{
	return train->clock_recovered && train->channel_equalized;
}

static int drm_dp_link_apply_training(struct drm_dp_link *link)
{
	struct drm_dp_link_train_set *request = &link->train.request;
	unsigned int lanes = link->lanes, *vs, *pe, *pc, i;
	struct drm_dp_aux *aux = link->aux;
	u8 values[4], pattern = 0;
	int err;

	err = link->ops->apply_training(link);
	if (err < 0) {
		DRM_ERROR("failed to apply link training: %d\n", err);
		return err;
	}

	vs = request->voltage_swing;
	pe = request->pre_emphasis;
	pc = request->post_cursor;

	/* write currently selected voltage-swing and pre-emphasis levels */
	for (i = 0; i < lanes; i++)
		values[i] = DP_TRAIN_VOLTAGE_SWING_LEVEL(vs[i]) |
			    DP_TRAIN_PRE_EMPHASIS_LEVEL(pe[i]);

	err = drm_dp_dpcd_write(aux, DP_TRAINING_LANE0_SET, values, lanes);
	if (err < 0) {
		DRM_ERROR("failed to set training parameters: %d\n", err);
		return err;
	}

	/* write currently selected post-cursor level (if supported) */
	if (link->revision >= 0x12 && link->rate == 540000) {
		values[0] = values[1] = 0;

		for (i = 0; i < lanes; i++)
			values[i / 2] |= DP_LANE_POST_CURSOR(i, pc[i]);

		err = drm_dp_dpcd_write(aux, DP_TRAINING_LANE0_1_SET2, values,
					DIV_ROUND_UP(lanes, 2));
		if (err < 0) {
			DRM_ERROR("failed to set post-cursor: %d\n", err);
			return err;
		}
	}

	/* write link pattern */
	if (link->train.pattern != DP_TRAINING_PATTERN_DISABLE)
		pattern |= DP_LINK_SCRAMBLING_DISABLE;

	pattern |= link->train.pattern;

	err = drm_dp_dpcd_writeb(aux, DP_TRAINING_PATTERN_SET, pattern);
	if (err < 0) {
		DRM_ERROR("failed to set training pattern: %d\n", err);
		return err;
	}

	return 0;
}

static void drm_dp_link_train_wait(struct drm_dp_link *link)
{
	unsigned long min = 0;

	switch (link->train.pattern) {
	case DP_TRAINING_PATTERN_1:
		min = link->aux_rd_interval.cr;
		break;

	case DP_TRAINING_PATTERN_2:
	case DP_TRAINING_PATTERN_3:
		min = link->aux_rd_interval.ce;
		break;

	default:
		break;
	}

	if (min > 0)
		usleep_range(min, 2 * min);
}

static void drm_dp_link_get_adjustments(struct drm_dp_link *link,
					u8 status[DP_LINK_STATUS_SIZE])
{
	struct drm_dp_link_train_set *adjust = &link->train.adjust;
	unsigned int i;

	for (i = 0; i < link->lanes; i++) {
		adjust->voltage_swing[i] =
			drm_dp_get_adjust_request_voltage(status, i) >>
				DP_TRAIN_VOLTAGE_SWING_SHIFT;

		adjust->pre_emphasis[i] =
			drm_dp_get_adjust_request_pre_emphasis(status, i) >>
				DP_TRAIN_PRE_EMPHASIS_SHIFT;

		adjust->post_cursor[i] =
			drm_dp_get_adjust_request_post_cursor(status, i);
	}
}

static void drm_dp_link_train_adjust(struct drm_dp_link_train *train)
{
	struct drm_dp_link_train_set *request = &train->request;
	struct drm_dp_link_train_set *adjust = &train->adjust;
	unsigned int i;

	for (i = 0; i < 4; i++)
		if (request->voltage_swing[i] != adjust->voltage_swing[i])
			request->voltage_swing[i] = adjust->voltage_swing[i];

	for (i = 0; i < 4; i++)
		if (request->pre_emphasis[i] != adjust->pre_emphasis[i])
			request->pre_emphasis[i] = adjust->pre_emphasis[i];

	for (i = 0; i < 4; i++)
		if (request->post_cursor[i] != adjust->post_cursor[i])
			request->post_cursor[i] = adjust->post_cursor[i];
}

static int drm_dp_link_recover_clock(struct drm_dp_link *link)
{
	u8 status[DP_LINK_STATUS_SIZE];
	int err;

	err = drm_dp_link_apply_training(link);
	if (err < 0)
		return err;

	drm_dp_link_train_wait(link);

	err = drm_dp_dpcd_read_link_status(link->aux, status);
	if (err < 0) {
		DRM_ERROR("failed to read link status: %d\n", err);
		return err;
	}

	if (!drm_dp_clock_recovery_ok(status, link->lanes))
		drm_dp_link_get_adjustments(link, status);
	else
		link->train.clock_recovered = true;

	return 0;
}

static int drm_dp_link_clock_recovery(struct drm_dp_link *link)
{
	unsigned int repeat;
	int err;

	/* start clock recovery using training pattern 1 */
	link->train.pattern = DP_TRAINING_PATTERN_1;

	for (repeat = 1; repeat < 5; repeat++) {
		err = drm_dp_link_recover_clock(link);
		if (err < 0) {
			DRM_ERROR("failed to recover clock: %d\n", err);
			return err;
		}

		if (link->train.clock_recovered)
			break;

		drm_dp_link_train_adjust(&link->train);
	}

	return 0;
}

static int drm_dp_link_equalize_channel(struct drm_dp_link *link)
{
	struct drm_dp_aux *aux = link->aux;
	u8 status[DP_LINK_STATUS_SIZE];
	int err;

	err = drm_dp_link_apply_training(link);
	if (err < 0)
		return err;

	drm_dp_link_train_wait(link);

	err = drm_dp_dpcd_read_link_status(aux, status);
	if (err < 0) {
		DRM_ERROR("failed to read link status: %d\n", err);
		return err;
	}

	if (!drm_dp_clock_recovery_ok(status, link->lanes)) {
		DRM_ERROR("clock recovery lost while equalizing channel\n");
		link->train.clock_recovered = false;
		return 0;
	}

	if (!drm_dp_channel_eq_ok(status, link->lanes))
		drm_dp_link_get_adjustments(link, status);
	else
		link->train.channel_equalized = true;

	return 0;
}

static int drm_dp_link_channel_equalization(struct drm_dp_link *link)
{
	unsigned int repeat;
	int err;

	/* start channel equalization using pattern 2 or 3 */
	if (link->caps.tps3_supported)
		link->train.pattern = DP_TRAINING_PATTERN_3;
	else
		link->train.pattern = DP_TRAINING_PATTERN_2;

	for (repeat = 1; repeat < 5; repeat++) {
		err = drm_dp_link_equalize_channel(link);
		if (err < 0) {
			DRM_ERROR("failed to equalize channel: %d\n", err);
			return err;
		}

		if (link->train.channel_equalized)
			break;

		drm_dp_link_train_adjust(&link->train);
	}

	return 0;
}

static int drm_dp_link_downgrade(struct drm_dp_link *link)
{
	switch (link->rate) {
	case 162000:
		return -EINVAL;

	case 270000:
		link->rate = 162000;
		break;

	case 540000:
		link->rate = 270000;
		return 0;
	}

	return 0;
}

static void drm_dp_link_train_disable(struct drm_dp_link *link)
{
	int err;

	link->train.pattern = DP_TRAINING_PATTERN_DISABLE;

	err = drm_dp_link_apply_training(link);
	if (err < 0)
		DRM_ERROR("failed to disable link training: %d\n", err);
}

static int drm_dp_link_train_full(struct drm_dp_link *link)
{
	int err;

retry:
	DRM_DEBUG_KMS("full-training link: %u lane%s at %u MHz\n",
		      link->lanes, (link->lanes > 1) ? "s" : "",
		      link->rate / 100);

	err = drm_dp_link_configure(link->aux, link);
	if (err < 0) {
		DRM_ERROR("failed to configure DP link: %d\n", err);
		return err;
	}

	err = drm_dp_link_clock_recovery(link);
	if (err < 0) {
		DRM_ERROR("clock recovery failed: %d\n", err);
		goto out;
	}

	if (!link->train.clock_recovered) {
		DRM_ERROR("clock recovery failed, downgrading link\n");

		err = drm_dp_link_downgrade(link);
		if (err < 0)
			goto out;

		goto retry;
	}

	DRM_DEBUG_KMS("clock recovery succeeded\n");

	err = drm_dp_link_channel_equalization(link);
	if (err < 0) {
		DRM_ERROR("channel equalization failed: %d\n", err);
		goto out;
	}

	if (!link->train.channel_equalized) {
		DRM_ERROR("channel equalization failed, downgrading link\n");

		err = drm_dp_link_downgrade(link);
		if (err < 0)
			goto out;

		goto retry;
	}

	DRM_DEBUG_KMS("channel equalization succeeded\n");

out:
	drm_dp_link_train_disable(link);
	return err;
}

static int drm_dp_link_train_fast(struct drm_dp_link *link)
{
	u8 status[DP_LINK_STATUS_SIZE];
	int err;

	DRM_DEBUG_KMS("fast-training link: %u lane%s at %u MHz\n",
		      link->lanes, (link->lanes > 1) ? "s" : "",
		      link->rate / 100);

	err = drm_dp_link_configure(link->aux, link);
	if (err < 0) {
		DRM_ERROR("failed to configure DP link: %d\n", err);
		return err;
	}

	/* transmit training pattern 1 for 500 microseconds */
	link->train.pattern = DP_TRAINING_PATTERN_1;

	err = drm_dp_link_apply_training(link);
	if (err < 0)
		goto out;

	usleep_range(500, 1000);

	/* transmit training pattern 2 or 3 for 500 microseconds */
	if (link->caps.tps3_supported)
		link->train.pattern = DP_TRAINING_PATTERN_3;
	else
		link->train.pattern = DP_TRAINING_PATTERN_2;

	err = drm_dp_link_apply_training(link);
	if (err < 0)
		goto out;

	usleep_range(500, 1000);

	err = drm_dp_dpcd_read_link_status(link->aux, status);
	if (err < 0) {
		DRM_ERROR("failed to read link status: %d\n", err);
		goto out;
	}

	if (!drm_dp_clock_recovery_ok(status, link->lanes)) {
		DRM_ERROR("clock recovery failed\n");
		err = -EIO;
	}

	if (!drm_dp_channel_eq_ok(status, link->lanes)) {
		DRM_ERROR("channel equalization failed\n");
		err = -EIO;
	}

out:
	drm_dp_link_train_disable(link);
	return err;
}

/**
 * drm_dp_link_train() - perform DisplayPort link training
 * @link: a DP link object
 *
 * Uses the context stored in the DP link object to perform link training. It
 * is expected that drivers will call drm_dp_link_probe() to obtain the link
 * capabilities before performing link training.
 *
 * If the sink supports fast link training (no AUX CH handshake) and valid
 * training settings are available, this function will try to perform fast
 * link training and fall back to full link training on failure.
 *
 * Returns: 0 on success or a negative error code on failure.
 */
int drm_dp_link_train(struct drm_dp_link *link)
{
	int err;

	drm_dp_link_train_init(&link->train);

	if (link->caps.fast_training) {
		if (drm_dp_link_train_valid(&link->train)) {
			err = drm_dp_link_train_fast(link);
			if (err < 0)
				DRM_ERROR("fast link training failed: %d\n",
					  err);
			else
				return 0;
		} else {
			DRM_DEBUG_KMS("training parameters not available\n");
		}
	} else {
		DRM_DEBUG_KMS("fast link training not supported\n");
	}

	err = drm_dp_link_train_full(link);
	if (err < 0)
		DRM_ERROR("full link training failed: %d\n", err);

	return err;
}