sched: Remove double_rq_lock() from __migrate_task()

[linux/fpc-iii.git] / drivers / gpu / drm / drm_dp_helper.c

/*
 * Copyright © 2009 Keith Packard
 *
 * Permission to use, copy, modify, distribute, and sell this software and its
 * documentation for any purpose is hereby granted without fee, provided that
 * the above copyright notice appear in all copies and that both that copyright
 * notice and this permission notice appear in supporting documentation, and
 * that the name of the copyright holders not be used in advertising or
 * publicity pertaining to distribution of the software without specific,
 * written prior permission.  The copyright holders make no representations
 * about the suitability of this software for any purpose.  It is provided "as
 * is" without express or implied warranty.
 *
 * THE COPYRIGHT HOLDERS DISCLAIM ALL WARRANTIES WITH REGARD TO THIS SOFTWARE,
 * INCLUDING ALL IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS, IN NO
 * EVENT SHALL THE COPYRIGHT HOLDERS BE LIABLE FOR ANY SPECIAL, INDIRECT OR
 * CONSEQUENTIAL DAMAGES OR ANY DAMAGES WHATSOEVER RESULTING FROM LOSS OF USE,
 * DATA OR PROFITS, WHETHER IN AN ACTION OF CONTRACT, NEGLIGENCE OR OTHER
 * TORTIOUS ACTION, ARISING OUT OF OR IN CONNECTION WITH THE USE OR PERFORMANCE
 * OF THIS SOFTWARE.
 */

#include <linux/kernel.h>
#include <linux/module.h>
#include <linux/delay.h>
#include <linux/init.h>
#include <linux/errno.h>
#include <linux/sched.h>
#include <linux/i2c.h>
#include <drm/drm_dp_helper.h>
#include <drm/drmP.h>

/**
 * DOC: dp helpers
 *
 * These functions contain some common logic and helpers at various abstraction
 * levels to deal with Display Port sink devices and related things like DP aux
 * channel transfers, EDID reading over DP aux channels, decoding certain DPCD
 * blocks, ...
 */

/* Run a single AUX_CH I2C transaction, writing/reading data as necessary */
static int
i2c_algo_dp_aux_transaction(struct i2c_adapter *adapter, int mode,
                            uint8_t write_byte, uint8_t *read_byte)
{
        struct i2c_algo_dp_aux_data *algo_data = adapter->algo_data;
        int ret;

        ret = (*algo_data->aux_ch)(adapter, mode,
                                   write_byte, read_byte);
        return ret;
}

/*
 * I2C over AUX CH
 */

/*
 * Send the address. If the I2C link is running, this 'restarts'
 * the connection with the new address, this is used for doing
 * a write followed by a read (as needed for DDC)
 */
static int
i2c_algo_dp_aux_address(struct i2c_adapter *adapter, u16 address, bool reading)
{
        struct i2c_algo_dp_aux_data *algo_data = adapter->algo_data;
        int mode = MODE_I2C_START;
        int ret;

        if (reading)
                mode |= MODE_I2C_READ;
        else
                mode |= MODE_I2C_WRITE;
        algo_data->address = address;
        algo_data->running = true;
        ret = i2c_algo_dp_aux_transaction(adapter, mode, 0, NULL);
        return ret;
}

/*
 * Stop the I2C transaction. This closes out the link, sending
 * a bare address packet with the MOT bit turned off
 */
static void
i2c_algo_dp_aux_stop(struct i2c_adapter *adapter, bool reading)
{
        struct i2c_algo_dp_aux_data *algo_data = adapter->algo_data;
        int mode = MODE_I2C_STOP;

        if (reading)
                mode |= MODE_I2C_READ;
        else
                mode |= MODE_I2C_WRITE;
        if (algo_data->running) {
                (void) i2c_algo_dp_aux_transaction(adapter, mode, 0, NULL);
                algo_data->running = false;
        }
}

/*
 * Write a single byte to the current I2C address, the
 * the I2C link must be running or this returns -EIO
 */
static int
i2c_algo_dp_aux_put_byte(struct i2c_adapter *adapter, u8 byte)
{
        struct i2c_algo_dp_aux_data *algo_data = adapter->algo_data;
        int ret;

        if (!algo_data->running)
                return -EIO;

        ret = i2c_algo_dp_aux_transaction(adapter, MODE_I2C_WRITE, byte, NULL);
        return ret;
}

/*
 * Read a single byte from the current I2C address, the
 * I2C link must be running or this returns -EIO
 */
static int
i2c_algo_dp_aux_get_byte(struct i2c_adapter *adapter, u8 *byte_ret)
{
        struct i2c_algo_dp_aux_data *algo_data = adapter->algo_data;
        int ret;

        if (!algo_data->running)
                return -EIO;

        ret = i2c_algo_dp_aux_transaction(adapter, MODE_I2C_READ, 0, byte_ret);
        return ret;
}

static int
i2c_algo_dp_aux_xfer(struct i2c_adapter *adapter,
                     struct i2c_msg *msgs,
                     int num)
{
        int ret = 0;
        bool reading = false;
        int m;
        int b;

        for (m = 0; m < num; m++) {
                u16 len = msgs[m].len;
                u8 *buf = msgs[m].buf;
                reading = (msgs[m].flags & I2C_M_RD) != 0;
                ret = i2c_algo_dp_aux_address(adapter, msgs[m].addr, reading);
                if (ret < 0)
                        break;
                if (reading) {
                        for (b = 0; b < len; b++) {
                                ret = i2c_algo_dp_aux_get_byte(adapter, &buf[b]);
                                if (ret < 0)
                                        break;
                        }
                } else {
                        for (b = 0; b < len; b++) {
                                ret = i2c_algo_dp_aux_put_byte(adapter, buf[b]);
                                if (ret < 0)
                                        break;
                        }
                }
                if (ret < 0)
                        break;
        }
        if (ret >= 0)
                ret = num;
        i2c_algo_dp_aux_stop(adapter, reading);
        DRM_DEBUG_KMS("dp_aux_xfer return %d\n", ret);
        return ret;
}

static u32
i2c_algo_dp_aux_functionality(struct i2c_adapter *adapter)
{
        return I2C_FUNC_I2C | I2C_FUNC_SMBUS_EMUL |
               I2C_FUNC_SMBUS_READ_BLOCK_DATA |
               I2C_FUNC_SMBUS_BLOCK_PROC_CALL |
               I2C_FUNC_10BIT_ADDR;
}

static const struct i2c_algorithm i2c_dp_aux_algo = {
        .master_xfer    = i2c_algo_dp_aux_xfer,
        .functionality  = i2c_algo_dp_aux_functionality,
};

static void
i2c_dp_aux_reset_bus(struct i2c_adapter *adapter)
{
        (void) i2c_algo_dp_aux_address(adapter, 0, false);
        (void) i2c_algo_dp_aux_stop(adapter, false);
}

static int
i2c_dp_aux_prepare_bus(struct i2c_adapter *adapter)
{
        adapter->algo = &i2c_dp_aux_algo;
        adapter->retries = 3;
        i2c_dp_aux_reset_bus(adapter);
        return 0;
}

/**
 * i2c_dp_aux_add_bus() - register an i2c adapter using the aux ch helper
 * @adapter: i2c adapter to register
 *
 * This registers an i2c adapter that uses dp aux channel as it's underlaying
 * transport. The driver needs to fill out the &i2c_algo_dp_aux_data structure
 * and store it in the algo_data member of the @adapter argument. This will be
 * used by the i2c over dp aux algorithm to drive the hardware.
 *
 * RETURNS:
 * 0 on success, -ERRNO on failure.
 *
 * IMPORTANT:
 * This interface is deprecated, please switch to the new dp aux helpers and
 * drm_dp_aux_register().
 */
int
i2c_dp_aux_add_bus(struct i2c_adapter *adapter)
{
        int error;

        error = i2c_dp_aux_prepare_bus(adapter);
        if (error)
                return error;
        error = i2c_add_adapter(adapter);
        return error;
}
EXPORT_SYMBOL(i2c_dp_aux_add_bus);

/* Helpers for DP link training */
static u8 dp_link_status(const u8 link_status[DP_LINK_STATUS_SIZE], int r)
{
        return link_status[r - DP_LANE0_1_STATUS];
}

static u8 dp_get_lane_status(const u8 link_status[DP_LINK_STATUS_SIZE],
                             int lane)
{
        int i = DP_LANE0_1_STATUS + (lane >> 1);
        int s = (lane & 1) * 4;
        u8 l = dp_link_status(link_status, i);
        return (l >> s) & 0xf;
}

bool drm_dp_channel_eq_ok(const u8 link_status[DP_LINK_STATUS_SIZE],
                          int lane_count)
{
        u8 lane_align;
        u8 lane_status;
        int lane;

        lane_align = dp_link_status(link_status,
                                    DP_LANE_ALIGN_STATUS_UPDATED);
        if ((lane_align & DP_INTERLANE_ALIGN_DONE) == 0)
                return false;
        for (lane = 0; lane < lane_count; lane++) {
                lane_status = dp_get_lane_status(link_status, lane);
                if ((lane_status & DP_CHANNEL_EQ_BITS) != DP_CHANNEL_EQ_BITS)
                        return false;
        }
        return true;
}
EXPORT_SYMBOL(drm_dp_channel_eq_ok);

bool drm_dp_clock_recovery_ok(const u8 link_status[DP_LINK_STATUS_SIZE],
                              int lane_count)
{
        int lane;
        u8 lane_status;

        for (lane = 0; lane < lane_count; lane++) {
                lane_status = dp_get_lane_status(link_status, lane);
                if ((lane_status & DP_LANE_CR_DONE) == 0)
                        return false;
        }
        return true;
}
EXPORT_SYMBOL(drm_dp_clock_recovery_ok);

u8 drm_dp_get_adjust_request_voltage(const u8 link_status[DP_LINK_STATUS_SIZE],
                                     int lane)
{
        int i = DP_ADJUST_REQUEST_LANE0_1 + (lane >> 1);
        int s = ((lane & 1) ?
                 DP_ADJUST_VOLTAGE_SWING_LANE1_SHIFT :
                 DP_ADJUST_VOLTAGE_SWING_LANE0_SHIFT);
        u8 l = dp_link_status(link_status, i);

        return ((l >> s) & 0x3) << DP_TRAIN_VOLTAGE_SWING_SHIFT;
}
EXPORT_SYMBOL(drm_dp_get_adjust_request_voltage);

u8 drm_dp_get_adjust_request_pre_emphasis(const u8 link_status[DP_LINK_STATUS_SIZE],
                                          int lane)
{
        int i = DP_ADJUST_REQUEST_LANE0_1 + (lane >> 1);
        int s = ((lane & 1) ?
                 DP_ADJUST_PRE_EMPHASIS_LANE1_SHIFT :
                 DP_ADJUST_PRE_EMPHASIS_LANE0_SHIFT);
        u8 l = dp_link_status(link_status, i);

        return ((l >> s) & 0x3) << DP_TRAIN_PRE_EMPHASIS_SHIFT;
}
EXPORT_SYMBOL(drm_dp_get_adjust_request_pre_emphasis);

void drm_dp_link_train_clock_recovery_delay(const u8 dpcd[DP_RECEIVER_CAP_SIZE]) {
        if (dpcd[DP_TRAINING_AUX_RD_INTERVAL] == 0)
                udelay(100);
        else
                mdelay(dpcd[DP_TRAINING_AUX_RD_INTERVAL] * 4);
}
EXPORT_SYMBOL(drm_dp_link_train_clock_recovery_delay);

void drm_dp_link_train_channel_eq_delay(const u8 dpcd[DP_RECEIVER_CAP_SIZE]) {
        if (dpcd[DP_TRAINING_AUX_RD_INTERVAL] == 0)
                udelay(400);
        else
                mdelay(dpcd[DP_TRAINING_AUX_RD_INTERVAL] * 4);
}
EXPORT_SYMBOL(drm_dp_link_train_channel_eq_delay);

u8 drm_dp_link_rate_to_bw_code(int link_rate)
{
        switch (link_rate) {
        case 162000:
        default:
                return DP_LINK_BW_1_62;
        case 270000:
                return DP_LINK_BW_2_7;
        case 540000:
                return DP_LINK_BW_5_4;
        }
}
EXPORT_SYMBOL(drm_dp_link_rate_to_bw_code);

int drm_dp_bw_code_to_link_rate(u8 link_bw)
{
        switch (link_bw) {
        case DP_LINK_BW_1_62:
        default:
                return 162000;
        case DP_LINK_BW_2_7:
                return 270000;
        case DP_LINK_BW_5_4:
                return 540000;
        }
}
EXPORT_SYMBOL(drm_dp_bw_code_to_link_rate);

/**
 * DOC: dp helpers
 *
 * The DisplayPort AUX channel is an abstraction to allow generic, driver-
 * independent access to AUX functionality. Drivers can take advantage of
 * this by filling in the fields of the drm_dp_aux structure.
 *
 * Transactions are described using a hardware-independent drm_dp_aux_msg
 * structure, which is passed into a driver's .transfer() implementation.
 * Both native and I2C-over-AUX transactions are supported.
 */

static int drm_dp_dpcd_access(struct drm_dp_aux *aux, u8 request,
                              unsigned int offset, void *buffer, size_t size)
{
        struct drm_dp_aux_msg msg;
        unsigned int retry;
        int err;

        memset(&msg, 0, sizeof(msg));
        msg.address = offset;
        msg.request = request;
        msg.buffer = buffer;
        msg.size = size;

        /*
         * The specification doesn't give any recommendation on how often to
         * retry native transactions, so retry 7 times like for I2C-over-AUX
         * transactions.
         */
        for (retry = 0; retry < 7; retry++) {

                mutex_lock(&aux->hw_mutex);
                err = aux->transfer(aux, &msg);
                mutex_unlock(&aux->hw_mutex);
                if (err < 0) {
                        if (err == -EBUSY)
                                continue;

                        return err;
                }


                switch (msg.reply & DP_AUX_NATIVE_REPLY_MASK) {
                case DP_AUX_NATIVE_REPLY_ACK:
                        if (err < size)
                                return -EPROTO;
                        return err;

                case DP_AUX_NATIVE_REPLY_NACK:
                        return -EIO;

                case DP_AUX_NATIVE_REPLY_DEFER:
                        usleep_range(400, 500);
                        break;
                }
        }

        DRM_DEBUG_KMS("too many retries, giving up\n");
        return -EIO;
}

/**
 * drm_dp_dpcd_read() - read a series of bytes from the DPCD
 * @aux: DisplayPort AUX channel
 * @offset: address of the (first) register to read
 * @buffer: buffer to store the register values
 * @size: number of bytes in @buffer
 *
 * Returns the number of bytes transferred on success, or a negative error
 * code on failure. -EIO is returned if the request was NAKed by the sink or
 * if the retry count was exceeded. If not all bytes were transferred, this
 * function returns -EPROTO. Errors from the underlying AUX channel transfer
 * function, with the exception of -EBUSY (which causes the transaction to
 * be retried), are propagated to the caller.
 */
ssize_t drm_dp_dpcd_read(struct drm_dp_aux *aux, unsigned int offset,
                         void *buffer, size_t size)
{
        return drm_dp_dpcd_access(aux, DP_AUX_NATIVE_READ, offset, buffer,
                                  size);
}
EXPORT_SYMBOL(drm_dp_dpcd_read);

/**
 * drm_dp_dpcd_write() - write a series of bytes to the DPCD
 * @aux: DisplayPort AUX channel
 * @offset: address of the (first) register to write
 * @buffer: buffer containing the values to write
 * @size: number of bytes in @buffer
 *
 * Returns the number of bytes transferred on success, or a negative error
 * code on failure. -EIO is returned if the request was NAKed by the sink or
 * if the retry count was exceeded. If not all bytes were transferred, this
 * function returns -EPROTO. Errors from the underlying AUX channel transfer
 * function, with the exception of -EBUSY (which causes the transaction to
 * be retried), are propagated to the caller.
 */
ssize_t drm_dp_dpcd_write(struct drm_dp_aux *aux, unsigned int offset,
                          void *buffer, size_t size)
{
        return drm_dp_dpcd_access(aux, DP_AUX_NATIVE_WRITE, offset, buffer,
                                  size);
}
EXPORT_SYMBOL(drm_dp_dpcd_write);

/**
 * drm_dp_dpcd_read_link_status() - read DPCD link status (bytes 0x202-0x207)
 * @aux: DisplayPort AUX channel
 * @status: buffer to store the link status in (must be at least 6 bytes)
 *
 * Returns the number of bytes transferred on success or a negative error
 * code on failure.
 */
int drm_dp_dpcd_read_link_status(struct drm_dp_aux *aux,
                                 u8 status[DP_LINK_STATUS_SIZE])
{
        return drm_dp_dpcd_read(aux, DP_LANE0_1_STATUS, status,
                                DP_LINK_STATUS_SIZE);
}
EXPORT_SYMBOL(drm_dp_dpcd_read_link_status);

/**
 * 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 values[3];
        int err;

        memset(link, 0, sizeof(*link));

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

        link->revision = values[0];
        link->rate = drm_dp_bw_code_to_link_rate(values[1]);
        link->num_lanes = values[2] & DP_MAX_LANE_COUNT_MASK;

        if (values[2] & DP_ENHANCED_FRAME_CAP)
                link->capabilities |= DP_LINK_CAP_ENHANCED_FRAMING;

        return 0;
}
EXPORT_SYMBOL(drm_dp_link_probe);

/**
 * 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;
}
EXPORT_SYMBOL(drm_dp_link_power_up);

/**
 * 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];
        int err;

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

        if (link->capabilities & DP_LINK_CAP_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;

        return 0;
}
EXPORT_SYMBOL(drm_dp_link_configure);

/*
 * I2C-over-AUX implementation
 */

static u32 drm_dp_i2c_functionality(struct i2c_adapter *adapter)
{
        return I2C_FUNC_I2C | I2C_FUNC_SMBUS_EMUL |
               I2C_FUNC_SMBUS_READ_BLOCK_DATA |
               I2C_FUNC_SMBUS_BLOCK_PROC_CALL |
               I2C_FUNC_10BIT_ADDR;
}

/*
 * Transfer a single I2C-over-AUX message and handle various error conditions,
 * retrying the transaction as appropriate.  It is assumed that the
 * aux->transfer function does not modify anything in the msg other than the
 * reply field.
 */
static int drm_dp_i2c_do_msg(struct drm_dp_aux *aux, struct drm_dp_aux_msg *msg)
{
        unsigned int retry;
        int err;

        /*
         * DP1.2 sections 2.7.7.1.5.6.1 and 2.7.7.1.6.6.1: A DP Source device
         * is required to retry at least seven times upon receiving AUX_DEFER
         * before giving up the AUX transaction.
         */
        for (retry = 0; retry < 7; retry++) {
                mutex_lock(&aux->hw_mutex);
                err = aux->transfer(aux, msg);
                mutex_unlock(&aux->hw_mutex);
                if (err < 0) {
                        if (err == -EBUSY)
                                continue;

                        DRM_DEBUG_KMS("transaction failed: %d\n", err);
                        return err;
                }


                switch (msg->reply & DP_AUX_NATIVE_REPLY_MASK) {
                case DP_AUX_NATIVE_REPLY_ACK:
                        /*
                         * For I2C-over-AUX transactions this isn't enough, we
                         * need to check for the I2C ACK reply.
                         */
                        break;

                case DP_AUX_NATIVE_REPLY_NACK:
                        DRM_DEBUG_KMS("native nack\n");
                        return -EREMOTEIO;

                case DP_AUX_NATIVE_REPLY_DEFER:
                        DRM_DEBUG_KMS("native defer");
                        /*
                         * We could check for I2C bit rate capabilities and if
                         * available adjust this interval. We could also be
                         * more careful with DP-to-legacy adapters where a
                         * long legacy cable may force very low I2C bit rates.
                         *
                         * For now just defer for long enough to hopefully be
                         * safe for all use-cases.
                         */
                        usleep_range(500, 600);
                        continue;

                default:
                        DRM_ERROR("invalid native reply %#04x\n", msg->reply);
                        return -EREMOTEIO;
                }

                switch (msg->reply & DP_AUX_I2C_REPLY_MASK) {
                case DP_AUX_I2C_REPLY_ACK:
                        /*
                         * Both native ACK and I2C ACK replies received. We
                         * can assume the transfer was successful.
                         */
                        if (err < msg->size)
                                return -EPROTO;
                        return 0;

                case DP_AUX_I2C_REPLY_NACK:
                        DRM_DEBUG_KMS("I2C nack\n");
                        return -EREMOTEIO;

                case DP_AUX_I2C_REPLY_DEFER:
                        DRM_DEBUG_KMS("I2C defer\n");
                        usleep_range(400, 500);
                        continue;

                default:
                        DRM_ERROR("invalid I2C reply %#04x\n", msg->reply);
                        return -EREMOTEIO;
                }
        }

        DRM_DEBUG_KMS("too many retries, giving up\n");
        return -EREMOTEIO;
}

static int drm_dp_i2c_xfer(struct i2c_adapter *adapter, struct i2c_msg *msgs,
                           int num)
{
        struct drm_dp_aux *aux = adapter->algo_data;
        unsigned int i, j;
        struct drm_dp_aux_msg msg;
        int err = 0;

        memset(&msg, 0, sizeof(msg));

        for (i = 0; i < num; i++) {
                msg.address = msgs[i].addr;
                msg.request = (msgs[i].flags & I2C_M_RD) ?
                        DP_AUX_I2C_READ :
                        DP_AUX_I2C_WRITE;
                msg.request |= DP_AUX_I2C_MOT;
                /* Send a bare address packet to start the transaction.
                 * Zero sized messages specify an address only (bare
                 * address) transaction.
                 */
                msg.buffer = NULL;
                msg.size = 0;
                err = drm_dp_i2c_do_msg(aux, &msg);
                if (err < 0)
                        break;
                /*
                 * Many hardware implementations support FIFOs larger than a
                 * single byte, but it has been empirically determined that
                 * transferring data in larger chunks can actually lead to
                 * decreased performance. Therefore each message is simply
                 * transferred byte-by-byte.
                 */
                for (j = 0; j < msgs[i].len; j++) {
                        msg.buffer = msgs[i].buf + j;
                        msg.size = 1;

                        err = drm_dp_i2c_do_msg(aux, &msg);
                        if (err < 0)
                                break;
                }
                if (err < 0)
                        break;
        }
        if (err >= 0)
                err = num;
        /* Send a bare address packet to close out the transaction.
         * Zero sized messages specify an address only (bare
         * address) transaction.
         */
        msg.request &= ~DP_AUX_I2C_MOT;
        msg.buffer = NULL;
        msg.size = 0;
        (void)drm_dp_i2c_do_msg(aux, &msg);

        return err;
}

static const struct i2c_algorithm drm_dp_i2c_algo = {
        .functionality = drm_dp_i2c_functionality,
        .master_xfer = drm_dp_i2c_xfer,
};

/**
 * drm_dp_aux_register() - initialise and register aux channel
 * @aux: DisplayPort AUX channel
 *
 * Returns 0 on success or a negative error code on failure.
 */
int drm_dp_aux_register(struct drm_dp_aux *aux)
{
        mutex_init(&aux->hw_mutex);

        aux->ddc.algo = &drm_dp_i2c_algo;
        aux->ddc.algo_data = aux;
        aux->ddc.retries = 3;

        aux->ddc.class = I2C_CLASS_DDC;
        aux->ddc.owner = THIS_MODULE;
        aux->ddc.dev.parent = aux->dev;
        aux->ddc.dev.of_node = aux->dev->of_node;

        strlcpy(aux->ddc.name, aux->name ? aux->name : dev_name(aux->dev),
                sizeof(aux->ddc.name));

        return i2c_add_adapter(&aux->ddc);
}
EXPORT_SYMBOL(drm_dp_aux_register);

/**
 * drm_dp_aux_unregister() - unregister an AUX adapter
 * @aux: DisplayPort AUX channel
 */
void drm_dp_aux_unregister(struct drm_dp_aux *aux)
{
        i2c_del_adapter(&aux->ddc);
}
EXPORT_SYMBOL(drm_dp_aux_unregister);