iw_send.c

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/*
 * Copyright (c) 2006 Oracle.  All rights reserved.
 *
 * This software is available to you under a choice of one of two
 * licenses.  You may choose to be licensed under the terms of the GNU
 * General Public License (GPL) Version 2, available from the file
 * COPYING in the main directory of this source tree, or the
 * OpenIB.org BSD license below:
 *
 *     Redistribution and use in source and binary forms, with or
 *     without modification, are permitted provided that the following
 *     conditions are met:
 *
 *      - Redistributions of source code must retain the above
 *        copyright notice, this list of conditions and the following
 *        disclaimer.
 *
 *      - Redistributions in binary form must reproduce the above
 *        copyright notice, this list of conditions and the following
 *        disclaimer in the documentation and/or other materials
 *        provided with the distribution.
 *
 * THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND,
 * EXPRESS OR IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF
 * MERCHANTABILITY, FITNESS FOR A PARTICULAR PURPOSE AND
 * NONINFRINGEMENT. IN NO EVENT SHALL THE AUTHORS OR COPYRIGHT HOLDERS
 * BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER LIABILITY, WHETHER IN AN
 * ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM, OUT OF OR IN
 * CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN THE
 * SOFTWARE.
 *
 */
#include <linux/kernel.h>
#include <linux/in.h>
#include <linux/device.h>
#include <linux/dmapool.h>
#include <linux/ratelimit.h>

#include "rds.h"
#include "iw.h"

static void rds_iw_send_rdma_complete(struct rds_message *rm,
                      int wc_status)
{
    int notify_status;

    switch (wc_status) {
    case IB_WC_WR_FLUSH_ERR:
        return;

    case IB_WC_SUCCESS:
        notify_status = RDS_RDMA_SUCCESS;
        break;

    case IB_WC_REM_ACCESS_ERR:
        notify_status = RDS_RDMA_REMOTE_ERROR;
        break;

    default:
        notify_status = RDS_RDMA_OTHER_ERROR;
        break;
    }
    rds_rdma_send_complete(rm, notify_status);
}

static void rds_iw_send_unmap_rdma(struct rds_iw_connection *ic,
                   struct rm_rdma_op *op)
{
    if (op->op_mapped) {
        ib_dma_unmap_sg(ic->i_cm_id->device,
            op->op_sg, op->op_nents,
            op->op_write ? DMA_TO_DEVICE : DMA_FROM_DEVICE);
        op->op_mapped = 0;
    }
}

static void rds_iw_send_unmap_rm(struct rds_iw_connection *ic,
              struct rds_iw_send_work *send,
              int wc_status)
{
    struct rds_message *rm = send->s_rm;

    rdsdebug("ic %p send %p rm %p\n", ic, send, rm);

    ib_dma_unmap_sg(ic->i_cm_id->device,
             rm->data.op_sg, rm->data.op_nents,
             DMA_TO_DEVICE);

    if (rm->rdma.op_active) {
        rds_iw_send_unmap_rdma(ic, &rm->rdma);

        /* If the user asked for a completion notification on this
         * message, we can implement three different semantics:
         *  1.  Notify when we received the ACK on the RDS message
         *  that was queued with the RDMA. This provides reliable
         *  notification of RDMA status at the expense of a one-way
         *  packet delay.
         *  2.  Notify when the IB stack gives us the completion event for
         *  the RDMA operation.
         *  3.  Notify when the IB stack gives us the completion event for
         *  the accompanying RDS messages.
         * Here, we implement approach #3. To implement approach #2,
         * call rds_rdma_send_complete from the cq_handler. To implement #1,
         * don't call rds_rdma_send_complete at all, and fall back to the notify
         * handling in the ACK processing code.
         *
         * Note: There's no need to explicitly sync any RDMA buffers using
         * ib_dma_sync_sg_for_cpu - the completion for the RDMA
         * operation itself unmapped the RDMA buffers, which takes care
         * of synching.
         */
        rds_iw_send_rdma_complete(rm, wc_status);

        if (rm->rdma.op_write)
            rds_stats_add(s_send_rdma_bytes, rm->rdma.op_bytes);
        else
            rds_stats_add(s_recv_rdma_bytes, rm->rdma.op_bytes);
    }

    /* If anyone waited for this message to get flushed out, wake
     * them up now */
    rds_message_unmapped(rm);

    rds_message_put(rm);
    send->s_rm = NULL;
}

void rds_iw_send_init_ring(struct rds_iw_connection *ic)
{
    struct rds_iw_send_work *send;
    u32 i;

    for (i = 0, send = ic->i_sends; i < ic->i_send_ring.w_nr; i++, send++) {
        struct ib_sge *sge;

        send->s_rm = NULL;
        send->s_op = NULL;
        send->s_mapping = NULL;

        send->s_wr.next = NULL;
        send->s_wr.wr_id = i;
        send->s_wr.sg_list = send->s_sge;
        send->s_wr.num_sge = 1;
        send->s_wr.opcode = IB_WR_SEND;
        send->s_wr.send_flags = 0;
        send->s_wr.ex.imm_data = 0;

        sge = rds_iw_data_sge(ic, send->s_sge);
        sge->lkey = 0;

        sge = rds_iw_header_sge(ic, send->s_sge);
        sge->addr = ic->i_send_hdrs_dma + (i * sizeof(struct rds_header));
        sge->length = sizeof(struct rds_header);
        sge->lkey = 0;

        send->s_mr = ib_alloc_fast_reg_mr(ic->i_pd, fastreg_message_size);
        if (IS_ERR(send->s_mr)) {
            printk(KERN_WARNING "RDS/IW: ib_alloc_fast_reg_mr failed\n");
            break;
        }

        send->s_page_list = ib_alloc_fast_reg_page_list(
            ic->i_cm_id->device, fastreg_message_size);
        if (IS_ERR(send->s_page_list)) {
            printk(KERN_WARNING "RDS/IW: ib_alloc_fast_reg_page_list failed\n");
            break;
        }
    }
}

void rds_iw_send_clear_ring(struct rds_iw_connection *ic)
{
    struct rds_iw_send_work *send;
    u32 i;

    for (i = 0, send = ic->i_sends; i < ic->i_send_ring.w_nr; i++, send++) {
        BUG_ON(!send->s_mr);
        ib_dereg_mr(send->s_mr);
        BUG_ON(!send->s_page_list);
        ib_free_fast_reg_page_list(send->s_page_list);
        if (send->s_wr.opcode == 0xdead)
            continue;
        if (send->s_rm)
            rds_iw_send_unmap_rm(ic, send, IB_WC_WR_FLUSH_ERR);
        if (send->s_op)
            rds_iw_send_unmap_rdma(ic, send->s_op);
    }
}

/*
 * The _oldest/_free ring operations here race cleanly with the alloc/unalloc
 * operations performed in the send path.  As the sender allocs and potentially
 * unallocs the next free entry in the ring it doesn't alter which is
 * the next to be freed, which is what this is concerned with.
 */
void rds_iw_send_cq_comp_handler(struct ib_cq *cq, void *context)
{
    struct rds_connection *conn = context;
    struct rds_iw_connection *ic = conn->c_transport_data;
    struct ib_wc wc;
    struct rds_iw_send_work *send;
    u32 completed;
    u32 oldest;
    u32 i;
    int ret;

    rdsdebug("cq %p conn %p\n", cq, conn);
    rds_iw_stats_inc(s_iw_tx_cq_call);
    ret = ib_req_notify_cq(cq, IB_CQ_NEXT_COMP);
    if (ret)
        rdsdebug("ib_req_notify_cq send failed: %d\n", ret);

    while (ib_poll_cq(cq, 1, &wc) > 0) {
        rdsdebug("wc wr_id 0x%llx status %u byte_len %u imm_data %u\n",
             (unsigned long long)wc.wr_id, wc.status, wc.byte_len,
             be32_to_cpu(wc.ex.imm_data));
        rds_iw_stats_inc(s_iw_tx_cq_event);

        if (wc.status != IB_WC_SUCCESS) {
            printk(KERN_ERR "WC Error:  status = %d opcode = %d\n", wc.status, wc.opcode);
            break;
        }

        if (wc.opcode == IB_WC_LOCAL_INV && wc.wr_id == RDS_IW_LOCAL_INV_WR_ID) {
            ic->i_fastreg_posted = 0;
            continue;
        }

        if (wc.opcode == IB_WC_FAST_REG_MR && wc.wr_id == RDS_IW_FAST_REG_WR_ID) {
            ic->i_fastreg_posted = 1;
            continue;
        }

        if (wc.wr_id == RDS_IW_ACK_WR_ID) {
            if (ic->i_ack_queued + HZ/2 < jiffies)
                rds_iw_stats_inc(s_iw_tx_stalled);
            rds_iw_ack_send_complete(ic);
            continue;
        }

        oldest = rds_iw_ring_oldest(&ic->i_send_ring);

        completed = rds_iw_ring_completed(&ic->i_send_ring, wc.wr_id, oldest);

        for (i = 0; i < completed; i++) {
            send = &ic->i_sends[oldest];

            /* In the error case, wc.opcode sometimes contains garbage */
            switch (send->s_wr.opcode) {
            case IB_WR_SEND:
                if (send->s_rm)
                    rds_iw_send_unmap_rm(ic, send, wc.status);
                break;
            case IB_WR_FAST_REG_MR:
            case IB_WR_RDMA_WRITE:
            case IB_WR_RDMA_READ:
            case IB_WR_RDMA_READ_WITH_INV:
                /* Nothing to be done - the SG list will be unmapped
                 * when the SEND completes. */
                break;
            default:
                printk_ratelimited(KERN_NOTICE
                        "RDS/IW: %s: unexpected opcode 0x%x in WR!\n",
                        __func__, send->s_wr.opcode);
                break;
            }

            send->s_wr.opcode = 0xdead;
            send->s_wr.num_sge = 1;
            if (send->s_queued + HZ/2 < jiffies)
                rds_iw_stats_inc(s_iw_tx_stalled);

            /* If a RDMA operation produced an error, signal this right
             * away. If we don't, the subsequent SEND that goes with this
             * RDMA will be canceled with ERR_WFLUSH, and the application
             * never learn that the RDMA failed. */
            if (unlikely(wc.status == IB_WC_REM_ACCESS_ERR && send->s_op)) {
                struct rds_message *rm;

                rm = rds_send_get_message(conn, send->s_op);
                if (rm)
                    rds_iw_send_rdma_complete(rm, wc.status);
            }

            oldest = (oldest + 1) % ic->i_send_ring.w_nr;
        }

        rds_iw_ring_free(&ic->i_send_ring, completed);

        if (test_and_clear_bit(RDS_LL_SEND_FULL, &conn->c_flags) ||
            test_bit(0, &conn->c_map_queued))
            queue_delayed_work(rds_wq, &conn->c_send_w, 0);

        /* We expect errors as the qp is drained during shutdown */
        if (wc.status != IB_WC_SUCCESS && rds_conn_up(conn)) {
            rds_iw_conn_error(conn,
                "send completion on %pI4 "
                "had status %u, disconnecting and reconnecting\n",
                &conn->c_faddr, wc.status);
        }
    }
}

/*
 * This is the main function for allocating credits when sending
 * messages.
 *
 * Conceptually, we have two counters:
 *  -   send credits: this tells us how many WRs we're allowed
 *  to submit without overruning the receiver's queue. For
 *  each SEND WR we post, we decrement this by one.
 *
 *  -   posted credits: this tells us how many WRs we recently
 *  posted to the receive queue. This value is transferred
 *  to the peer as a "credit update" in a RDS header field.
 *  Every time we transmit credits to the peer, we subtract
 *  the amount of transferred credits from this counter.
 *
 * It is essential that we avoid situations where both sides have
 * exhausted their send credits, and are unable to send new credits
 * to the peer. We achieve this by requiring that we send at least
 * one credit update to the peer before exhausting our credits.
 * When new credits arrive, we subtract one credit that is withheld
 * until we've posted new buffers and are ready to transmit these
 * credits (see rds_iw_send_add_credits below).
 *
 * The RDS send code is essentially single-threaded; rds_send_xmit
 * grabs c_send_lock to ensure exclusive access to the send ring.
 * However, the ACK sending code is independent and can race with
 * message SENDs.
 *
 * In the send path, we need to update the counters for send credits
 * and the counter of posted buffers atomically - when we use the
 * last available credit, we cannot allow another thread to race us
 * and grab the posted credits counter.  Hence, we have to use a
 * spinlock to protect the credit counter, or use atomics.
 *
 * Spinlocks shared between the send and the receive path are bad,
 * because they create unnecessary delays. An early implementation
 * using a spinlock showed a 5% degradation in throughput at some
 * loads.
 *
 * This implementation avoids spinlocks completely, putting both
 * counters into a single atomic, and updating that atomic using
 * atomic_add (in the receive path, when receiving fresh credits),
 * and using atomic_cmpxchg when updating the two counters.
 */
int rds_iw_send_grab_credits(struct rds_iw_connection *ic,
                 u32 wanted, u32 *adv_credits, int need_posted, int max_posted)
{
    unsigned int avail, posted, got = 0, advertise;
    long oldval, newval;

    *adv_credits = 0;
    if (!ic->i_flowctl)
        return wanted;

try_again:
    advertise = 0;
    oldval = newval = atomic_read(&ic->i_credits);
    posted = IB_GET_POST_CREDITS(oldval);
    avail = IB_GET_SEND_CREDITS(oldval);

    rdsdebug("rds_iw_send_grab_credits(%u): credits=%u posted=%u\n",
            wanted, avail, posted);

    /* The last credit must be used to send a credit update. */
    if (avail && !posted)
        avail--;

    if (avail < wanted) {
        struct rds_connection *conn = ic->i_cm_id->context;

        /* Oops, there aren't that many credits left! */
        set_bit(RDS_LL_SEND_FULL, &conn->c_flags);
        got = avail;
    } else {
        /* Sometimes you get what you want, lalala. */
        got = wanted;
    }
    newval -= IB_SET_SEND_CREDITS(got);

    /*
     * If need_posted is non-zero, then the caller wants
     * the posted regardless of whether any send credits are
     * available.
     */
    if (posted && (got || need_posted)) {
        advertise = min_t(unsigned int, posted, max_posted);
        newval -= IB_SET_POST_CREDITS(advertise);
    }

    /* Finally bill everything */
    if (atomic_cmpxchg(&ic->i_credits, oldval, newval) != oldval)
        goto try_again;

    *adv_credits = advertise;
    return got;
}

void rds_iw_send_add_credits(struct rds_connection *conn, unsigned int credits)
{
    struct rds_iw_connection *ic = conn->c_transport_data;

    if (credits == 0)
        return;

    rdsdebug("rds_iw_send_add_credits(%u): current=%u%s\n",
            credits,
            IB_GET_SEND_CREDITS(atomic_read(&ic->i_credits)),
            test_bit(RDS_LL_SEND_FULL, &conn->c_flags) ? ", ll_send_full" : "");

    atomic_add(IB_SET_SEND_CREDITS(credits), &ic->i_credits);
    if (test_and_clear_bit(RDS_LL_SEND_FULL, &conn->c_flags))
        queue_delayed_work(rds_wq, &conn->c_send_w, 0);

    WARN_ON(IB_GET_SEND_CREDITS(credits) >= 16384);

    rds_iw_stats_inc(s_iw_rx_credit_updates);
}

void rds_iw_advertise_credits(struct rds_connection *conn, unsigned int posted)
{
    struct rds_iw_connection *ic = conn->c_transport_data;

    if (posted == 0)
        return;

    atomic_add(IB_SET_POST_CREDITS(posted), &ic->i_credits);

    /* Decide whether to send an update to the peer now.
     * If we would send a credit update for every single buffer we
     * post, we would end up with an ACK storm (ACK arrives,
     * consumes buffer, we refill the ring, send ACK to remote
     * advertising the newly posted buffer... ad inf)
     *
     * Performance pretty much depends on how often we send
     * credit updates - too frequent updates mean lots of ACKs.
     * Too infrequent updates, and the peer will run out of
     * credits and has to throttle.
     * For the time being, 16 seems to be a good compromise.
     */
    if (IB_GET_POST_CREDITS(atomic_read(&ic->i_credits)) >= 16)
        set_bit(IB_ACK_REQUESTED, &ic->i_ack_flags);
}

static inline void
rds_iw_xmit_populate_wr(struct rds_iw_connection *ic,
        struct rds_iw_send_work *send, unsigned int pos,
        unsigned long buffer, unsigned int length,
        int send_flags)
{
    struct ib_sge *sge;

    WARN_ON(pos != send - ic->i_sends);

    send->s_wr.send_flags = send_flags;
    send->s_wr.opcode = IB_WR_SEND;
    send->s_wr.num_sge = 2;
    send->s_wr.next = NULL;
    send->s_queued = jiffies;
    send->s_op = NULL;

    if (length != 0) {
        sge = rds_iw_data_sge(ic, send->s_sge);
        sge->addr = buffer;
        sge->length = length;
        sge->lkey = rds_iw_local_dma_lkey(ic);

        sge = rds_iw_header_sge(ic, send->s_sge);
    } else {
        /* We're sending a packet with no payload. There is only
         * one SGE */
        send->s_wr.num_sge = 1;
        sge = &send->s_sge[0];
    }

    sge->addr = ic->i_send_hdrs_dma + (pos * sizeof(struct rds_header));
    sge->length = sizeof(struct rds_header);
    sge->lkey = rds_iw_local_dma_lkey(ic);
}

/*
 * This can be called multiple times for a given message.  The first time
 * we see a message we map its scatterlist into the IB device so that
 * we can provide that mapped address to the IB scatter gather entries
 * in the IB work requests.  We translate the scatterlist into a series
 * of work requests that fragment the message.  These work requests complete
 * in order so we pass ownership of the message to the completion handler
 * once we send the final fragment.
 *
 * The RDS core uses the c_send_lock to only enter this function once
 * per connection.  This makes sure that the tx ring alloc/unalloc pairs
 * don't get out of sync and confuse the ring.
 */
int rds_iw_xmit(struct rds_connection *conn, struct rds_message *rm,
        unsigned int hdr_off, unsigned int sg, unsigned int off)
{
    struct rds_iw_connection *ic = conn->c_transport_data;
    struct ib_device *dev = ic->i_cm_id->device;
    struct rds_iw_send_work *send = NULL;
    struct rds_iw_send_work *first;
    struct rds_iw_send_work *prev;
    struct ib_send_wr *failed_wr;
    struct scatterlist *scat;
    u32 pos;
    u32 i;
    u32 work_alloc;
    u32 credit_alloc;
    u32 posted;
    u32 adv_credits = 0;
    int send_flags = 0;
    int sent;
    int ret;
    int flow_controlled = 0;

    BUG_ON(off % RDS_FRAG_SIZE);
    BUG_ON(hdr_off != 0 && hdr_off != sizeof(struct rds_header));

    /* Fastreg support */
    if (rds_rdma_cookie_key(rm->m_rdma_cookie) && !ic->i_fastreg_posted) {
        ret = -EAGAIN;
        goto out;
    }

    /* FIXME we may overallocate here */
    if (be32_to_cpu(rm->m_inc.i_hdr.h_len) == 0)
        i = 1;
    else
        i = ceil(be32_to_cpu(rm->m_inc.i_hdr.h_len), RDS_FRAG_SIZE);

    work_alloc = rds_iw_ring_alloc(&ic->i_send_ring, i, &pos);
    if (work_alloc == 0) {
        set_bit(RDS_LL_SEND_FULL, &conn->c_flags);
        rds_iw_stats_inc(s_iw_tx_ring_full);
        ret = -ENOMEM;
        goto out;
    }

    credit_alloc = work_alloc;
    if (ic->i_flowctl) {
        credit_alloc = rds_iw_send_grab_credits(ic, work_alloc, &posted, 0, RDS_MAX_ADV_CREDIT);
        adv_credits += posted;
        if (credit_alloc < work_alloc) {
            rds_iw_ring_unalloc(&ic->i_send_ring, work_alloc - credit_alloc);
            work_alloc = credit_alloc;
            flow_controlled++;
        }
        if (work_alloc == 0) {
            set_bit(RDS_LL_SEND_FULL, &conn->c_flags);
            rds_iw_stats_inc(s_iw_tx_throttle);
            ret = -ENOMEM;
            goto out;
        }
    }

    /* map the message the first time we see it */
    if (!ic->i_rm) {
        /*
        printk(KERN_NOTICE "rds_iw_xmit prep msg dport=%u flags=0x%x len=%d\n",
                be16_to_cpu(rm->m_inc.i_hdr.h_dport),
                rm->m_inc.i_hdr.h_flags,
                be32_to_cpu(rm->m_inc.i_hdr.h_len));
           */
        if (rm->data.op_nents) {
            rm->data.op_count = ib_dma_map_sg(dev,
                              rm->data.op_sg,
                              rm->data.op_nents,
                              DMA_TO_DEVICE);
            rdsdebug("ic %p mapping rm %p: %d\n", ic, rm, rm->data.op_count);
            if (rm->data.op_count == 0) {
                rds_iw_stats_inc(s_iw_tx_sg_mapping_failure);
                rds_iw_ring_unalloc(&ic->i_send_ring, work_alloc);
                ret = -ENOMEM; /* XXX ? */
                goto out;
            }
        } else {
            rm->data.op_count = 0;
        }

        ic->i_unsignaled_wrs = rds_iw_sysctl_max_unsig_wrs;
        ic->i_unsignaled_bytes = rds_iw_sysctl_max_unsig_bytes;
        rds_message_addref(rm);
        ic->i_rm = rm;

        /* Finalize the header */
        if (test_bit(RDS_MSG_ACK_REQUIRED, &rm->m_flags))
            rm->m_inc.i_hdr.h_flags |= RDS_FLAG_ACK_REQUIRED;
        if (test_bit(RDS_MSG_RETRANSMITTED, &rm->m_flags))
            rm->m_inc.i_hdr.h_flags |= RDS_FLAG_RETRANSMITTED;

        /* If it has a RDMA op, tell the peer we did it. This is
         * used by the peer to release use-once RDMA MRs. */
        if (rm->rdma.op_active) {
            struct rds_ext_header_rdma ext_hdr;

            ext_hdr.h_rdma_rkey = cpu_to_be32(rm->rdma.op_rkey);
            rds_message_add_extension(&rm->m_inc.i_hdr,
                    RDS_EXTHDR_RDMA, &ext_hdr, sizeof(ext_hdr));
        }
        if (rm->m_rdma_cookie) {
            rds_message_add_rdma_dest_extension(&rm->m_inc.i_hdr,
                    rds_rdma_cookie_key(rm->m_rdma_cookie),
                    rds_rdma_cookie_offset(rm->m_rdma_cookie));
        }

        /* Note - rds_iw_piggyb_ack clears the ACK_REQUIRED bit, so
         * we should not do this unless we have a chance of at least
         * sticking the header into the send ring. Which is why we
         * should call rds_iw_ring_alloc first. */
        rm->m_inc.i_hdr.h_ack = cpu_to_be64(rds_iw_piggyb_ack(ic));
        rds_message_make_checksum(&rm->m_inc.i_hdr);

        /*
         * Update adv_credits since we reset the ACK_REQUIRED bit.
         */
        rds_iw_send_grab_credits(ic, 0, &posted, 1, RDS_MAX_ADV_CREDIT - adv_credits);
        adv_credits += posted;
        BUG_ON(adv_credits > 255);
    }

    send = &ic->i_sends[pos];
    first = send;
    prev = NULL;
    scat = &rm->data.op_sg[sg];
    sent = 0;
    i = 0;

    /* Sometimes you want to put a fence between an RDMA
     * READ and the following SEND.
     * We could either do this all the time
     * or when requested by the user. Right now, we let
     * the application choose.
     */
    if (rm->rdma.op_active && rm->rdma.op_fence)
        send_flags = IB_SEND_FENCE;

    /*
     * We could be copying the header into the unused tail of the page.
     * That would need to be changed in the future when those pages might
     * be mapped userspace pages or page cache pages.  So instead we always
     * use a second sge and our long-lived ring of mapped headers.  We send
     * the header after the data so that the data payload can be aligned on
     * the receiver.
     */

    /* handle a 0-len message */
    if (be32_to_cpu(rm->m_inc.i_hdr.h_len) == 0) {
        rds_iw_xmit_populate_wr(ic, send, pos, 0, 0, send_flags);
        goto add_header;
    }

    /* if there's data reference it with a chain of work reqs */
    for (; i < work_alloc && scat != &rm->data.op_sg[rm->data.op_count]; i++) {
        unsigned int len;

        send = &ic->i_sends[pos];

        len = min(RDS_FRAG_SIZE, ib_sg_dma_len(dev, scat) - off);
        rds_iw_xmit_populate_wr(ic, send, pos,
                ib_sg_dma_address(dev, scat) + off, len,
                send_flags);

        /*
         * We want to delay signaling completions just enough to get
         * the batching benefits but not so much that we create dead time
         * on the wire.
         */
        if (ic->i_unsignaled_wrs-- == 0) {
            ic->i_unsignaled_wrs = rds_iw_sysctl_max_unsig_wrs;
            send->s_wr.send_flags |= IB_SEND_SIGNALED | IB_SEND_SOLICITED;
        }

        ic->i_unsignaled_bytes -= len;
        if (ic->i_unsignaled_bytes <= 0) {
            ic->i_unsignaled_bytes = rds_iw_sysctl_max_unsig_bytes;
            send->s_wr.send_flags |= IB_SEND_SIGNALED | IB_SEND_SOLICITED;
        }

        /*
         * Always signal the last one if we're stopping due to flow control.
         */
        if (flow_controlled && i == (work_alloc-1))
            send->s_wr.send_flags |= IB_SEND_SIGNALED | IB_SEND_SOLICITED;

        rdsdebug("send %p wr %p num_sge %u next %p\n", send,
             &send->s_wr, send->s_wr.num_sge, send->s_wr.next);

        sent += len;
        off += len;
        if (off == ib_sg_dma_len(dev, scat)) {
            scat++;
            off = 0;
        }

add_header:
        /* Tack on the header after the data. The header SGE should already
         * have been set up to point to the right header buffer. */
        memcpy(&ic->i_send_hdrs[pos], &rm->m_inc.i_hdr, sizeof(struct rds_header));

        if (0) {
            struct rds_header *hdr = &ic->i_send_hdrs[pos];

            printk(KERN_NOTICE "send WR dport=%u flags=0x%x len=%d\n",
                be16_to_cpu(hdr->h_dport),
                hdr->h_flags,
                be32_to_cpu(hdr->h_len));
        }
        if (adv_credits) {
            struct rds_header *hdr = &ic->i_send_hdrs[pos];

            /* add credit and redo the header checksum */
            hdr->h_credit = adv_credits;
            rds_message_make_checksum(hdr);
            adv_credits = 0;
            rds_iw_stats_inc(s_iw_tx_credit_updates);
        }

        if (prev)
            prev->s_wr.next = &send->s_wr;
        prev = send;

        pos = (pos + 1) % ic->i_send_ring.w_nr;
    }

    /* Account the RDS header in the number of bytes we sent, but just once.
     * The caller has no concept of fragmentation. */
    if (hdr_off == 0)
        sent += sizeof(struct rds_header);

    /* if we finished the message then send completion owns it */
    if (scat == &rm->data.op_sg[rm->data.op_count]) {
        prev->s_rm = ic->i_rm;
        prev->s_wr.send_flags |= IB_SEND_SIGNALED | IB_SEND_SOLICITED;
        ic->i_rm = NULL;
    }

    if (i < work_alloc) {
        rds_iw_ring_unalloc(&ic->i_send_ring, work_alloc - i);
        work_alloc = i;
    }
    if (ic->i_flowctl && i < credit_alloc)
        rds_iw_send_add_credits(conn, credit_alloc - i);

    /* XXX need to worry about failed_wr and partial sends. */
    failed_wr = &first->s_wr;
    ret = ib_post_send(ic->i_cm_id->qp, &first->s_wr, &failed_wr);
    rdsdebug("ic %p first %p (wr %p) ret %d wr %p\n", ic,
         first, &first->s_wr, ret, failed_wr);
    BUG_ON(failed_wr != &first->s_wr);
    if (ret) {
        printk(KERN_WARNING "RDS/IW: ib_post_send to %pI4 "
               "returned %d\n", &conn->c_faddr, ret);
        rds_iw_ring_unalloc(&ic->i_send_ring, work_alloc);
        if (prev->s_rm) {
            ic->i_rm = prev->s_rm;
            prev->s_rm = NULL;
        }
        goto out;
    }

    ret = sent;
out:
    BUG_ON(adv_credits);
    return ret;
}

static void rds_iw_build_send_fastreg(struct rds_iw_device *rds_iwdev, struct rds_iw_connection *ic, struct rds_iw_send_work *send, int nent, int len, u64 sg_addr)
{
    BUG_ON(nent > send->s_page_list->max_page_list_len);
    /*
     * Perform a WR for the fast_reg_mr. Each individual page
     * in the sg list is added to the fast reg page list and placed
     * inside the fast_reg_mr WR.
     */
    send->s_wr.opcode = IB_WR_FAST_REG_MR;
    send->s_wr.wr.fast_reg.length = len;
    send->s_wr.wr.fast_reg.rkey = send->s_mr->rkey;
    send->s_wr.wr.fast_reg.page_list = send->s_page_list;
    send->s_wr.wr.fast_reg.page_list_len = nent;
    send->s_wr.wr.fast_reg.page_shift = PAGE_SHIFT;
    send->s_wr.wr.fast_reg.access_flags = IB_ACCESS_REMOTE_WRITE;
    send->s_wr.wr.fast_reg.iova_start = sg_addr;

    ib_update_fast_reg_key(send->s_mr, send->s_remap_count++);
}

int rds_iw_xmit_rdma(struct rds_connection *conn, struct rm_rdma_op *op)
{
    struct rds_iw_connection *ic = conn->c_transport_data;
    struct rds_iw_send_work *send = NULL;
    struct rds_iw_send_work *first;
    struct rds_iw_send_work *prev;
    struct ib_send_wr *failed_wr;
    struct rds_iw_device *rds_iwdev;
    struct scatterlist *scat;
    unsigned long len;
    u64 remote_addr = op->op_remote_addr;
    u32 pos, fr_pos;
    u32 work_alloc;
    u32 i;
    u32 j;
    int sent;
    int ret;
    int num_sge;

    rds_iwdev = ib_get_client_data(ic->i_cm_id->device, &rds_iw_client);

    /* map the message the first time we see it */
    if (!op->op_mapped) {
        op->op_count = ib_dma_map_sg(ic->i_cm_id->device,
                         op->op_sg, op->op_nents, (op->op_write) ?
                         DMA_TO_DEVICE : DMA_FROM_DEVICE);
        rdsdebug("ic %p mapping op %p: %d\n", ic, op, op->op_count);
        if (op->op_count == 0) {
            rds_iw_stats_inc(s_iw_tx_sg_mapping_failure);
            ret = -ENOMEM; /* XXX ? */
            goto out;
        }

        op->op_mapped = 1;
    }

    if (!op->op_write) {
        /* Alloc space on the send queue for the fastreg */
        work_alloc = rds_iw_ring_alloc(&ic->i_send_ring, 1, &fr_pos);
        if (work_alloc != 1) {
            rds_iw_ring_unalloc(&ic->i_send_ring, work_alloc);
            rds_iw_stats_inc(s_iw_tx_ring_full);
            ret = -ENOMEM;
            goto out;
        }
    }

    /*
     * Instead of knowing how to return a partial rdma read/write we insist that there
     * be enough work requests to send the entire message.
     */
    i = ceil(op->op_count, rds_iwdev->max_sge);

    work_alloc = rds_iw_ring_alloc(&ic->i_send_ring, i, &pos);
    if (work_alloc != i) {
        rds_iw_ring_unalloc(&ic->i_send_ring, work_alloc);
        rds_iw_stats_inc(s_iw_tx_ring_full);
        ret = -ENOMEM;
        goto out;
    }

    send = &ic->i_sends[pos];
    if (!op->op_write) {
        first = prev = &ic->i_sends[fr_pos];
    } else {
        first = send;
        prev = NULL;
    }
    scat = &op->op_sg[0];
    sent = 0;
    num_sge = op->op_count;

    for (i = 0; i < work_alloc && scat != &op->op_sg[op->op_count]; i++) {
        send->s_wr.send_flags = 0;
        send->s_queued = jiffies;

        /*
         * We want to delay signaling completions just enough to get
         * the batching benefits but not so much that we create dead time on the wire.
         */
        if (ic->i_unsignaled_wrs-- == 0) {
            ic->i_unsignaled_wrs = rds_iw_sysctl_max_unsig_wrs;
            send->s_wr.send_flags = IB_SEND_SIGNALED;
        }

        /* To avoid the need to have the plumbing to invalidate the fastreg_mr used
         * for local access after RDS is finished with it, using
         * IB_WR_RDMA_READ_WITH_INV will invalidate it after the read has completed.
         */
        if (op->op_write)
            send->s_wr.opcode = IB_WR_RDMA_WRITE;
        else
            send->s_wr.opcode = IB_WR_RDMA_READ_WITH_INV;

        send->s_wr.wr.rdma.remote_addr = remote_addr;
        send->s_wr.wr.rdma.rkey = op->op_rkey;
        send->s_op = op;

        if (num_sge > rds_iwdev->max_sge) {
            send->s_wr.num_sge = rds_iwdev->max_sge;
            num_sge -= rds_iwdev->max_sge;
        } else
            send->s_wr.num_sge = num_sge;

        send->s_wr.next = NULL;

        if (prev)
            prev->s_wr.next = &send->s_wr;

        for (j = 0; j < send->s_wr.num_sge && scat != &op->op_sg[op->op_count]; j++) {
            len = ib_sg_dma_len(ic->i_cm_id->device, scat);

            if (send->s_wr.opcode == IB_WR_RDMA_READ_WITH_INV)
                send->s_page_list->page_list[j] = ib_sg_dma_address(ic->i_cm_id->device, scat);
            else {
                send->s_sge[j].addr = ib_sg_dma_address(ic->i_cm_id->device, scat);
                send->s_sge[j].length = len;
                send->s_sge[j].lkey = rds_iw_local_dma_lkey(ic);
            }

            sent += len;
            rdsdebug("ic %p sent %d remote_addr %llu\n", ic, sent, remote_addr);
            remote_addr += len;

            scat++;
        }

        if (send->s_wr.opcode == IB_WR_RDMA_READ_WITH_INV) {
            send->s_wr.num_sge = 1;
            send->s_sge[0].addr = conn->c_xmit_rm->m_rs->rs_user_addr;
            send->s_sge[0].length = conn->c_xmit_rm->m_rs->rs_user_bytes;
            send->s_sge[0].lkey = ic->i_sends[fr_pos].s_mr->lkey;
        }

        rdsdebug("send %p wr %p num_sge %u next %p\n", send,
            &send->s_wr, send->s_wr.num_sge, send->s_wr.next);

        prev = send;
        if (++send == &ic->i_sends[ic->i_send_ring.w_nr])
            send = ic->i_sends;
    }

    /* if we finished the message then send completion owns it */
    if (scat == &op->op_sg[op->op_count])
        first->s_wr.send_flags = IB_SEND_SIGNALED;

    if (i < work_alloc) {
        rds_iw_ring_unalloc(&ic->i_send_ring, work_alloc - i);
        work_alloc = i;
    }

    /* On iWARP, local memory access by a remote system (ie, RDMA Read) is not
     * recommended.  Putting the lkey on the wire is a security hole, as it can
     * allow for memory access to all of memory on the remote system.  Some
     * adapters do not allow using the lkey for this at all.  To bypass this use a
     * fastreg_mr (or possibly a dma_mr)
     */
    if (!op->op_write) {
        rds_iw_build_send_fastreg(rds_iwdev, ic, &ic->i_sends[fr_pos],
            op->op_count, sent, conn->c_xmit_rm->m_rs->rs_user_addr);
        work_alloc++;
    }

    failed_wr = &first->s_wr;
    ret = ib_post_send(ic->i_cm_id->qp, &first->s_wr, &failed_wr);
    rdsdebug("ic %p first %p (wr %p) ret %d wr %p\n", ic,
         first, &first->s_wr, ret, failed_wr);
    BUG_ON(failed_wr != &first->s_wr);
    if (ret) {
        printk(KERN_WARNING "RDS/IW: rdma ib_post_send to %pI4 "
               "returned %d\n", &conn->c_faddr, ret);
        rds_iw_ring_unalloc(&ic->i_send_ring, work_alloc);
        goto out;
    }

out:
    return ret;
}

void rds_iw_xmit_complete(struct rds_connection *conn)
{
    struct rds_iw_connection *ic = conn->c_transport_data;

    /* We may have a pending ACK or window update we were unable
     * to send previously (due to flow control). Try again. */
    rds_iw_attempt_ack(ic);
}