// 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;
}