/*************************************************************************
 * Copyright (c) 2018-2021, NVIDIA CORPORATION. All rights reserved.
 * Modifications Copyright (c) 2019-2021 Advanced Micro Devices, Inc. All rights reserved.
 *
 * See LICENSE.txt for license information
 ************************************************************************/

#include "core.h"
#include "graph.h"
#include "topo.h"
#include "comm.h"
#include "net.h"
#include "channel.h"

// Pre-compute GPU->NIC, GPU->GPU and NIC->GPU paths

struct ncclTopoNodeList {
  struct ncclTopoNode* list[NCCL_TOPO_MAX_NODES];
  int count;
};

static ncclResult_t getPath(struct ncclTopoSystem* system, struct ncclTopoNode* node, int t, int64_t id, struct ncclTopoLinkList** path) {
  for (int i=0; i<system->nodes[t].count; i++) {
    if (system->nodes[t].nodes[i].id == id) {
      *path = node->paths[t]+i;
      return ncclSuccess;
    }
  }
  WARN("Could not find node of type %d id %lx", t, id);
  return ncclInternalError;
}

NCCL_PARAM(NvbDisable, "NVB_DISABLE", 0);

static ncclResult_t ncclTopoSetPaths(struct ncclTopoNode* baseNode, struct ncclTopoSystem* system) {
  if (baseNode->paths[baseNode->type] == NULL) {
    NCCLCHECK(ncclCalloc(baseNode->paths+baseNode->type, system->nodes[baseNode->type].count));
  }

  // breadth-first search to set all paths to that node in the system
  struct ncclTopoNodeList nodeList;
  struct ncclTopoNodeList nextNodeList;
  nodeList.count = 1; nodeList.list[0] = baseNode;
  nextNodeList.count = 0;
  struct ncclTopoLinkList* basePath;
  NCCLCHECK(getPath(system, baseNode, baseNode->type, baseNode->id, &basePath));
  basePath->count = 0;
  basePath->width = LOC_WIDTH;
  basePath->type = PATH_LOC;

  while (nodeList.count) {
    nextNodeList.count = 0;
    for (int n=0; n<nodeList.count; n++) {
      struct ncclTopoNode* node = nodeList.list[n];
      struct ncclTopoLinkList* path;
      NCCLCHECK(getPath(system, node, baseNode->type, baseNode->id, &path));
      for (int l=0; l<node->nlinks; l++) {
        struct ncclTopoLink* link = node->links+l;
        struct ncclTopoNode* remNode = link->remNode;
        if (remNode->paths[baseNode->type] == NULL) {
          NCCLCHECK(ncclCalloc(remNode->paths+baseNode->type, system->nodes[baseNode->type].count));
        }
        struct ncclTopoLinkList* remPath;
        NCCLCHECK(getPath(system, remNode, baseNode->type, baseNode->id, &remPath));
        float width = std::min(path->width, link->width);
        if (remPath->width < width) {
          // Find reverse link
          for (int l=0; l<remNode->nlinks; l++) {
            if (remNode->links[l].remNode == node) {
              remPath->list[0] = remNode->links+l;
              break;
            }
          }
          if (remPath->list[0] == NULL) {
            WARN("Failed to find reverse path from remNode %d/%lx nlinks %d to node %d/%lx",
                 remNode->type, remNode->id, remNode->nlinks, node->type, node->id);
            return ncclInternalError;
          }
          // Copy the rest of the path
          for (int i=0; i<path->count; i++) remPath->list[i+1] = path->list[i];
          remPath->count = path->count + 1;
          remPath->width = width;

          // Start with path type = link type. PATH and LINK types are supposed to match.
          // Don't consider LINK_NET as we only care about the NIC->GPU path.
          int type = link->type == LINK_NET ? LINK_LOC : link->type;
          // Differentiate between one and multiple PCI switches
          if (node->type == PCI && remNode->type == PCI) type = PATH_PXB;
          // Consider a path going through the CPU as PATH_PHB
          if (link->type == LINK_PCI && (node->type == CPU || link->remNode->type == CPU)) type = PATH_PHB;
          // Set 1 hop NVLink as NVB
          //if (node->type == GPU && path->type == PATH_NVL && type == PATH_NVL && remPath->count > 1) type = PATH_NVB;

          remPath->type = std::max(path->type, type);

          // Add to the list for the next iteration if not already in the list
          // Disallow GPUs as intermediate steps for now
          if (remNode->type != GPU) {
            int i;
            for (i=0; i<nextNodeList.count; i++) if (nextNodeList.list[i] == remNode) break;
            if (i == nextNodeList.count) nextNodeList.list[nextNodeList.count++] = remNode;
          }
        }
      }
    }
    memcpy(&nodeList, &nextNodeList, sizeof(nodeList));
  }
  return ncclSuccess;
}

static void printNodePaths(struct ncclTopoSystem* system, struct ncclTopoNode* node) {
  char line[1024];
#ifdef ENABLE_TRACE
  INFO(NCCL_GRAPH, "Paths from %s/%lX :", topoNodeTypeStr[node->type], node->id);
#else
  sprintf(line, "%s/%lX :", topoNodeTypeStr[node->type], node->id);
  int offset = strlen(line);
#endif
  for (int t=0; t<NCCL_TOPO_NODE_TYPES; t++) {
    if (node->paths[t] == NULL) continue;
    for (int n = 0; n<system->nodes[t].count; n++) {
#ifdef ENABLE_TRACE
      line[0] = 0;
      int offset = 0;
      for (int i=0; i<node->paths[t][n].count; i++) {
        struct ncclTopoLink* link = node->paths[t][n].list[i];
        struct ncclTopoNode* remNode = link->remNode;
        sprintf(line+offset, "--%s->%s/%lX", topoLinkTypeStr[link->type], topoNodeTypeStr[remNode->type], remNode->id);
        offset = strlen(line);
      }
      INFO(NCCL_GRAPH, "%s (%f)", line, node->paths[t][n].width);
#else
      sprintf(line+offset, "%s/%lX (%d/%f/%s) ", topoNodeTypeStr[t], system->nodes[t].nodes[n].id, node->paths[t][n].count, node->paths[t][n].width, topoPathTypeStr[node->paths[t][n].type]);
      offset = strlen(line);
#endif
    }
  }
#ifndef ENABLE_TRACE
  INFO(NCCL_GRAPH, "%s", line);
#endif
}

ncclResult_t ncclTopoPrintPaths(struct ncclTopoSystem* system) {
  for (int i=0; i<system->nodes[GPU].count; i++) {
    printNodePaths(system, system->nodes[GPU].nodes+i);
  }
  for (int i=0; i<system->nodes[NET].count; i++) {
    printNodePaths(system, system->nodes[NET].nodes+i);
  }
  return ncclSuccess;
}

static ncclResult_t getLocalCpu(struct ncclTopoSystem* system, int gpu, int* retCpu) {
  // Find the closest CPU to a GPU
  int minHops = 0;
  int localCpu = -1;
  struct ncclTopoLinkList* paths = system->nodes[GPU].nodes[gpu].paths[CPU];
  for (int c=0; c<system->nodes[CPU].count; c++) {
    int hops = paths[c].count;
    if (minHops == 0 || hops < minHops) {
      localCpu = c;
      minHops = hops;
    }
  }
  if (localCpu == -1) {
    WARN("Error : could not find CPU close to GPU %d", gpu);
    return ncclInternalError;
  }
  *retCpu = localCpu;
  return ncclSuccess;
}

static ncclResult_t addCpuStep(struct ncclTopoSystem* system, int c, int t1, int i1, int t2, int i2) {
  struct ncclTopoNode* cpuNode = system->nodes[CPU].nodes+c;
  struct ncclTopoNode* srcNode = system->nodes[t1].nodes+i1;

  int l=0;
  // Node 1 -> CPU
  for (int i=0; i<srcNode->paths[CPU][c].count; i++) srcNode->paths[t2][i2].list[l++] = srcNode->paths[CPU][c].list[i];
  // CPU -> Node 2
  for (int i=0; i<cpuNode->paths[t2][i2].count; i++) srcNode->paths[t2][i2].list[l++] = cpuNode->paths[t2][i2].list[i];

  // Update path characteristics
  srcNode->paths[t2][i2].count = l;
  srcNode->paths[t2][i2].type = std::max(srcNode->paths[CPU][c].type, cpuNode->paths[t2][i2].type);
  srcNode->paths[t2][i2].width = std::min(srcNode->paths[CPU][c].width, cpuNode->paths[t2][i2].width);
  return ncclSuccess;
}

// Remove/free paths for a given type
static void ncclTopoRemovePathType(struct ncclTopoSystem* system, int nodeType) {
  for (int t=0; t<NCCL_TOPO_NODE_TYPES; t++) {
    // Remove links _to_ the given type
    for (int n=0; n<system->nodes[t].count; n++) {
      struct ncclTopoNode* node = system->nodes[t].nodes+n;
      free(node->paths[nodeType]);
      node->paths[nodeType] = NULL;
    }
    // Remove links _from_ the given type
    for (int n=0; n<system->nodes[nodeType].count; n++) {
      struct ncclTopoNode* node = system->nodes[nodeType].nodes+n;
      free(node->paths[t]);
      node->paths[t] = NULL;
    }
  }
}

static const int levelsOldToNew[] = { PATH_LOC, PATH_PIX, PATH_PXB, PATH_PHB, PATH_SYS, PATH_SYS };
ncclResult_t ncclGetLevel(int* level, const char* disableEnv, const char* levelEnv) {
  if (*level == -1) {
    int l = -1;
    if (disableEnv) {
      char* str = getenv(disableEnv);
      if (str) {
        int disable = strtol(str, NULL, 0);
        if (disable == 1) l = 0;
      }
    }
    if (l == -1) {
      char* str = getenv(levelEnv);
      if (str) {
        for (int i=0; i<=PATH_SYS; i++) {
          if (strcmp(str, topoPathTypeStr[i]) == 0) {
            l = i;
            break;
          }
        }
        // Old style numbering
        if (l == -1 && str[0] >= '0' && str[0] <= '9') {
          int oldLevel = strtol(str, NULL, 0);
          const int maxOldLevel = sizeof(levelsOldToNew)/sizeof(int) - 1;
          if (oldLevel > maxOldLevel) oldLevel = maxOldLevel;
          l = levelsOldToNew[oldLevel];
        }
      }
    }
    if (l >= 0) INFO(NCCL_ALL, "%s set by environment to %s", levelEnv, topoPathTypeStr[l]);
    *level = l >= 0 ? l : -2;
  }
  return ncclSuccess;
}

int ncclTopoUserP2pLevel = -1;
ncclResult_t ncclTopoCheckP2p(struct ncclTopoSystem* system, int64_t id1, int64_t id2, int* p2p, int *read, int* intermediateRank) {
  *p2p = 0;
  if (read) *read = 0;
  if (intermediateRank) *intermediateRank = -1;

  // Get GPUs from topology
  int g1, g2;
  NCCLCHECK(ncclTopoIdToIndex(system, GPU, id1, &g1));
  struct ncclTopoNode* gpu1 = system->nodes[GPU].nodes+g1;
  if (ncclTopoIdToIndex(system, GPU, id2, &g2) == ncclInternalError) {
    // GPU not found, we can't use p2p.
    return ncclSuccess;
  }


  // Set intermediate GPU rank, if routing through an intermediate GPU.
  struct ncclTopoLinkList* path = gpu1->paths[GPU]+g2;
  if (path->count == 2) {
    struct ncclTopoNode* intermediateNode = path->list[0]->remNode;
    if (intermediateNode->type == GPU && intermediateRank) {
      *intermediateRank = intermediateNode->gpu.rank;
    }
  }

  // In general, use P2P whenever we can.
  int p2pLevel = PATH_SYS;

  // User override
  if (ncclTopoUserP2pLevel == -1)
    NCCLCHECK(ncclGetLevel(&ncclTopoUserP2pLevel, "NCCL_P2P_DISABLE", "NCCL_P2P_LEVEL"));
  if (ncclTopoUserP2pLevel != -2) {
    p2pLevel = ncclTopoUserP2pLevel;
    goto compare;
  }

  // Don't use P2P through ARM CPUs
  int arch, vendor, model;
  NCCLCHECK(ncclTopoCpuType(system, &arch, &vendor, &model));
  if (arch == NCCL_TOPO_CPU_ARCH_ARM) p2pLevel = PATH_PXB;
  if (arch == NCCL_TOPO_CPU_ARCH_X86 && vendor == NCCL_TOPO_CPU_VENDOR_INTEL) {
    p2pLevel = PATH_PXB;
  }
  if (arch == NCCL_TOPO_CPU_ARCH_X86 && vendor == NCCL_TOPO_CPU_VENDOR_ZHAOXIN) {
    p2pLevel = PATH_PXB;
  }

compare:
  // Compute the PCI distance and compare with the p2pLevel.
  if (path->type <= p2pLevel) *p2p = 1;

  if (path->type == PATH_NVL) {
    struct ncclTopoNode* gpu2 = system->nodes[GPU].nodes+g2;
    // Enable P2P Read for Ampere/NVLink only
    if (read && (gpu1->gpu.cudaCompCap == gpu2->gpu.cudaCompCap) && (gpu1->gpu.cudaCompCap == 80)) *read = 1;
  }

  return ncclSuccess;
}

NCCL_PARAM(NetGdrRead, "NET_GDR_READ", -2);
int ncclTopoUserGdrLevel = -1;

ncclResult_t ncclTopoCheckGdr(struct ncclTopoSystem* system, int64_t busId, int netDev, int read, int* useGdr) {
  *useGdr = 0;

  // Get GPU and NET
  int n, g;
  NCCLCHECK(ncclTopoIdToIndex(system, NET, netDev, &n));
  struct ncclTopoNode* net = system->nodes[NET].nodes+n;
  NCCLCHECK(ncclTopoIdToIndex(system, GPU, busId, &g));
  struct ncclTopoNode* gpu = system->nodes[GPU].nodes+g;

  // Check that both the NIC and GPUs support it
  if (net->net.gdrSupport == 0) return ncclSuccess;
  if (gpu->gpu.gdrSupport == 0) return ncclSuccess;

  if (read) { // For reads (sends) only enable under certain conditions
    int gdrReadParam = ncclParamNetGdrRead();
    if (gdrReadParam == 0) return ncclSuccess;
    if (gdrReadParam < 0) {
      int nvlink = 0;
      // Since we don't know whether there are other communicators,
      // it's better to keep things local if we have a single GPU.
      if (system->nodes[GPU].count == 1) nvlink = 1;
      for (int i=0; i<system->nodes[GPU].count; i++) {
        if (i == g) continue;
        if (gpu->paths[GPU][i].type == PATH_NVL) {
          nvlink = 1;
          break;
        }
      }
      if (!nvlink) return ncclSuccess;
    }
  }

  // Check if we are close enough that it makes sense to enable GDR
  int netGdrLevel = system->netGdrLevel == -2 ? PATH_PXB : system->netGdrLevel;
  NCCLCHECK(ncclGetLevel(&ncclTopoUserGdrLevel, NULL, "NCCL_NET_GDR_LEVEL"));
  if (ncclTopoUserGdrLevel != -2) netGdrLevel = ncclTopoUserGdrLevel;
  else {
    int arch, vendor, model;
    NCCLCHECK(ncclTopoCpuType(system, &arch, &vendor, &model));
    if (arch == NCCL_TOPO_CPU_ARCH_X86 && vendor == NCCL_TOPO_CPU_VENDOR_AMD && model == NCCL_TOPO_CPU_TYPE_ROME) {
      int i, d1 = -1, d2 = -1;
      for (i = 0; i < system->nodes[CPU].count; i++)
        if (system->nodes[GPU].nodes[g].paths[CPU][i].count == 2) break;
      if (i <system->nodes[CPU].count) d1 = system->nodes[CPU].nodes[i].id;
      for (i = 0; i < system->nodes[CPU].count; i++)
        if (system->nodes[NET].nodes[n].paths[CPU][i].count == 2) break;
      if (i <system->nodes[CPU].count) d2 = system->nodes[CPU].nodes[i].id;
      if (d1 != -1 && d2 != -1 && d1 == d2 &&
        (system->nodes[GPU].nodes[g].id & 0xf0000) == (system->nodes[NET].nodes[n].net.busId & 0xf0000)) {
        netGdrLevel = PATH_PHB;
      }
    }
  }

  int distance = gpu->paths[NET][n].type;
  if (distance > netGdrLevel) {
    INFO(NCCL_NET,"GPU Direct RDMA Disabled for GPU %lx / HCA %d (distance %d > %d)", busId, netDev, distance, netGdrLevel);
    return ncclSuccess;
  }

  *useGdr = 1;
  INFO(NCCL_NET,"GPU Direct RDMA Enabled for GPU %lx / HCA %d (distance %d <= %d), read %d", busId, netDev, distance, netGdrLevel, read);
  return ncclSuccess;
}

ncclResult_t ncclTopoComputePaths(struct ncclTopoSystem* system, struct ncclPeerInfo* peerInfos) {
  // Precompute paths between GPUs/NICs.

  // Remove everything in case we're re-computing
  for (int t=0; t<NCCL_TOPO_NODE_TYPES; t++) ncclTopoRemovePathType(system, t);

  // Set direct paths from/to CPUs. We need them in many cases.
  for (int c=0; c<system->nodes[CPU].count; c++) {
    NCCLCHECK(ncclTopoSetPaths(system->nodes[CPU].nodes+c, system));
  }

  // Set direct paths from/to GPUs.
  for (int g=0; g<system->nodes[GPU].count; g++) {
    // Compute paths to GPU g
    NCCLCHECK(ncclTopoSetPaths(system->nodes[GPU].nodes+g, system));

    // Update path when we don't want to / can't use GPU Direct P2P
    for (int p=0; p<system->nodes[GPU].count; p++) {
      int p2p;
      NCCLCHECK(ncclTopoCheckP2p(system, system->nodes[GPU].nodes[p].id, system->nodes[GPU].nodes[g].id, &p2p, NULL, NULL));
      if (p2p == 0) {
        // Divert all traffic through the CPU
        int cpu;
        NCCLCHECK(getLocalCpu(system, g, &cpu));
        NCCLCHECK(addCpuStep(system, cpu, GPU, p, GPU, g));
      }
    }

    if (peerInfos == NULL) continue;
    // Remove GPUs we can't talk to because of containers.
    struct ncclPeerInfo* dstInfo = peerInfos+system->nodes[GPU].nodes[g].gpu.rank;
    for (int p=0; p<system->nodes[GPU].count; p++) {
      if (p == g) continue;
      struct ncclPeerInfo* srcInfo = peerInfos+system->nodes[GPU].nodes[p].gpu.rank;
      int shm;
      NCCLCHECK(ncclTransports[TRANSPORT_SHM].canConnect(&shm, system, NULL, srcInfo, dstInfo));
      int p2p;
      NCCLCHECK(ncclTransports[TRANSPORT_P2P].canConnect(&p2p, system, NULL, srcInfo, dstInfo));
      if (shm == 0 && p2p == 0) {
        // Mark this peer as inaccessible. We'll trim it later.
        system->nodes[GPU].nodes[p].paths[GPU][g].count = 0;
      }
    }
  }

  // Set direct paths from/to NICs.
  for (int n=0; n<system->nodes[NET].count; n++) {
    struct ncclTopoNode* netNode = system->nodes[NET].nodes+n;
    NCCLCHECK(ncclTopoSetPaths(netNode, system));

    for (int g=0; g<system->nodes[GPU].count; g++) {
      // Update path when we dont want to / can't use GPU Direct RDMA.
      int gdr;
      NCCLCHECK(ncclTopoCheckGdr(system, system->nodes[GPU].nodes[g].id, netNode->id, 0, &gdr));
      if (gdr == 0) {
        // We cannot use GPU Direct RDMA, divert all traffic through the CPU local to the GPU
        int localCpu;
        NCCLCHECK(getLocalCpu(system, g, &localCpu));
        NCCLCHECK(addCpuStep(system, localCpu, NET, n, GPU, g));
        NCCLCHECK(addCpuStep(system, localCpu, GPU, g, NET, n));
      }
    }
  }
  return ncclSuccess;
}

ncclResult_t ncclTopoTrimSystem(struct ncclTopoSystem* system, struct ncclComm* comm) {
  int *domains;
  int64_t *ids;
  NCCLCHECK(ncclCalloc(&domains, system->nodes[GPU].count));
  NCCLCHECK(ncclCalloc(&ids, system->nodes[GPU].count));
  int myDomain = 0;
  for (int g=0; g<system->nodes[GPU].count; g++) {
    struct ncclTopoNode* gpu = system->nodes[GPU].nodes+g;
    domains[g] = g;
    ids[g] = gpu->id;
    for (int p=0; p<g; p++) {
      if (gpu->paths[GPU][p].count > 0) {
        domains[g] = std::min(domains[g], domains[p]);
      }
    }
    if (gpu->gpu.rank == comm->rank) myDomain = domains[g];
  }

  int ngpus = system->nodes[GPU].count;
  for (int i=0; i<ngpus; i++) {
    if (domains[i] == myDomain) continue;
    struct ncclTopoNode* gpu = NULL;
    int g;
    for (g=0; g<system->nodes[GPU].count /* This one varies over the loops */; g++) {
      gpu = system->nodes[GPU].nodes+g;
      if (gpu->id == ids[i]) break; else gpu=NULL;
    }
    if (gpu == NULL) {
      WARN("Could not find id %lx", ids[i]);
      free(domains);
      free(ids);
      return ncclInternalError;
    }
    NCCLCHECK(ncclTopoRemoveNode(system, GPU, g));
  }

  // trim low speed port on same NIC
  for (int i = 0; i < system->nodes[NET].count; i ++) {
    for (int j = 0; j < system->nodes[NET].count; j ++) {
      if (i == j) continue;
      if (system->nodes[NET].nodes[i].net.asic == system->nodes[NET].nodes[j].net.asic) {
        if (system->nodes[NET].nodes[i].net.width > system->nodes[NET].nodes[j].net.width)
          system->nodes[NET].nodes[j].net.width = 0;
      }
    }
  }
  do {
    int n;
    for (n=0; n<system->nodes[NET].count; n++) {
      if (system->nodes[NET].nodes[n].net.width == 0) break;
    }
    if (n<system->nodes[NET].count) {
      NCCLCHECK(ncclTopoRemoveNode(system, NET, n));
    }
    else
      break;
  } while (system->nodes[NET].count);

  int remove = 1;
  int arch, vendor, model;
  NCCLCHECK(ncclTopoCpuType(system, &arch, &vendor, &model));
  if (arch == NCCL_TOPO_CPU_ARCH_X86 && vendor == NCCL_TOPO_CPU_VENDOR_AMD
    && model == NCCL_TOPO_CPU_TYPE_ROME) {
    int gdr, ret = 1;
    int64_t net;
    for (int g = 0; g < system->nodes[GPU].count; g++) {
      NCCLCHECK(ncclTopoGetLocalNet(system, system->nodes[GPU].nodes[g].gpu.rank, &net, 0));
      NCCLCHECK(ncclTopoCheckGdr(system, system->nodes[GPU].nodes[g].id, net, 1, &gdr));
      if (!gdr) {
        ret = 0;
        break;
      }
    }
    if (ret) {
      bool allXgmi = true;
      // don't trim NICs unless all GPUs are connected by XGMI
      for (int i = 0; i < system->nodes[GPU].count && allXgmi; i++) {
        int cudaDev1 = system->nodes[GPU].nodes[i].gpu.dev;
        for (int j = 0; j < system->nodes[GPU].count && allXgmi; j++) {
          if (i == j) continue;
          int cudaDev2 = system->nodes[GPU].nodes[j].gpu.dev;
          bool isXGMI;
          NCCLCHECK(ncclTopoGetLinkType(comm->topo, cudaDev1, cudaDev2, &isXGMI));
          allXgmi &= isXGMI;
        }
      }
      if (!allXgmi) {
        remove = 0;
        system->type |= RCCL_TOPO_GDR_ALL;
        INFO(NCCL_GRAPH, "GDR is available on all GPUs");
      }
    }
  }
  comm->localRanks = system->nodes[GPU].count;
  if (system->nodes[GPU].count == comm->nRanks && remove) {
    for (int n=system->nodes[NET].count-1; n>=0; n--)
      NCCLCHECK(ncclTopoRemoveNode(system, NET, n));
  }

  free(domains);
  free(ids);
  return ncclSuccess;
}

void ncclTopoFree(struct ncclTopoSystem* system) {
  for (int t=0; t<NCCL_TOPO_NODE_TYPES; t++) ncclTopoRemovePathType(system, t);
  free(system);
}

static ncclResult_t ncclTopoGetNchannels(struct ncclTopoSystem* system, int g /*local gpu index*/, int peerRank, int* nChannels) {
  int peer;
  struct ncclTopoLinkList* path = NULL;
  if (ncclTopoRankToIndex(system, peerRank, &peer) == ncclSuccess) {
    // Same rank
    if (g == peer) {
      *nChannels = -1;
      return ncclSuccess;
    }
    // Local rank
    path = system->nodes[GPU].nodes[peer].paths[GPU]+g;
    if (path->type == PATH_NVL) {
      float nvlWidth = ncclTopoNVLinkSpeed(system->nodes[GPU].nodes[g].gpu.cudaCompCap);
      *nChannels = 2*std::max(1, (int)(path->width / nvlWidth));
    } else {
      *nChannels = 2;
    }
  } else {
    // Remote rank, use network
    *nChannels = 1;
  }
  return ncclSuccess;
}

NCCL_PARAM(MinP2pNChannels, "MIN_P2P_NCHANNELS", 1);
NCCL_PARAM(MaxP2pNChannels, "MAX_P2P_NCHANNELS", MAXCHANNELS);

static int nextPow2(int v) {
  int pow2 = 1;
  while (pow2 < v) pow2 <<= 1;
  return pow2;
}

ncclResult_t ncclTopoComputeP2pChannels(struct ncclComm* comm) {
  comm->p2pnChannels = std::min(comm->nChannels, (int)ncclParamMaxP2pNChannels());
  comm->p2pnChannels = std::max(comm->p2pnChannels, (int)ncclParamMinP2pNChannels());
  int minChannels = comm->p2pnChannels;
  // We need to loop through all local GPUs to have a global picture
  for (int g=0; g<comm->topo->nodes[GPU].count; g++) {
    for (int r=0; r<comm->nRanks; r++) {
      int nChannels;
      NCCLCHECK(ncclTopoGetNchannels(comm->topo, g, r, &nChannels));
      if (nChannels >= 0) minChannels = std::min(minChannels, nChannels);
    }
  }

  if (comm->topo->nodes[GPU].count == comm->topo->nRanks && (comm->topo->type & RCCL_TOPO_4P2H_ROME) && !(comm->topo->type & RCCL_TOPO_GDR_ALL) && (comm->topo->nodes[GPU].nodes[0].gpu.gcn != 910)) {
    // Adjust P2P channels on Rome
    comm->p2pnChannelsPerPeer = 2;
    comm->p2pnChannels = 2;
  }
  else {
    // Round to next pow2 nChannelsPerPeer and nChannels
    comm->p2pnChannelsPerPeer = nextPow2(minChannels);
    comm->p2pnChannels = nextPow2(comm->p2pnChannels);
  }

  // Init channels that weren't used so far
  for (int c=comm->nChannels; c<std::max(comm->nChannels, comm->p2pnChannels); c++) NCCLCHECK(initChannel(comm, c));

  // We want to spread channels used when there aren't many and progressively
  // fill the whole space of nChannels. To do so we mirror the bits in the
  // nChannels space.
  for (int c=0; c<comm->p2pnChannelsPerPeer; c++) {
    int mirror = 0;
    for (int b=1, mb=(comm->p2pnChannels>>1); b<comm->p2pnChannels; b<<=1, mb>>=1) if (c & b) mirror |= mb;
    comm->p2pChannels[c] = mirror;
  }
  INFO(NCCL_INIT, "%d coll channels, %d p2p channels, %d p2p channels per peer", comm->nChannels, comm->p2pnChannels, comm->p2pnChannelsPerPeer);
  return ncclSuccess;
}

ncclResult_t ncclTopoGetNvbGpus(struct ncclTopoSystem* system, int rank, int* nranks, int** ranks) {
  int ngpus = system->nodes[GPU].count;
  NCCLCHECK(ncclCalloc(ranks, ngpus));
  int nvbGpus = 0;
  for (int g=0; g<ngpus; g++) {
    struct ncclTopoNode* gpu = system->nodes[GPU].nodes+g;
    if (gpu->gpu.rank != rank) continue;
    for (int p=0; p<ngpus; p++) {
      if (gpu->paths[GPU][p].type == PATH_NVB) {
        (*ranks)[nvbGpus++] = system->nodes[GPU].nodes[p].gpu.rank;
      }
    }
  }
  *nranks = nvbGpus;
  return ncclSuccess;
}