ice40: Introduce the concept of forPadIn SB_GB

Those are cells that are created mainly to handle the various sources a
global network can be driven from other than a user net.

When the flag is set, this means the global network usually driven by
this BEL is in fact driven by something else and so that SB_GB BEL and
matching global network can't be used.

This is also what gets used to set the extra bits during bitstream
generation.

Signed-off-by: Sylvain Munaut <tnt@246tNt.com>

ice40/arch.cc

@@ -1044,6 +1044,8 @@ void Arch::assignCellInfo(CellInfo *cell)
             cell->lcInfo.inputCount++;
     } else if (cell->type == id_SB_IO) {
         cell->ioInfo.lvds = str_or_default(cell->params, id_IO_STANDARD, "SB_LVCMOS") == "SB_LVDS_INPUT";
+    } else if (cell->type == id_SB_GB) {
+        cell->gbInfo.forPadIn = bool_or_default(cell->attrs, this->id("FOR_PAD_IN"));
     }
 }
 

ice40/arch_place.cc

@@ -161,6 +161,8 @@ bool Arch::isValidBelForCell(CellInfo *cell, BelId bel) const
 
         return getBelPackagePin(bel) != "";
     } else if (cell->type == id_SB_GB) {
+        if (cell->gbInfo.forPadIn)
+            return true;
         NPNR_ASSERT(cell->ports.at(id_GLOBAL_BUFFER_OUTPUT).net != nullptr);
         const NetInfo *net = cell->ports.at(id_GLOBAL_BUFFER_OUTPUT).net;
         IdString glb_net = getWireName(getBelPinWire(bel, id_GLOBAL_BUFFER_OUTPUT));

ice40/archdefs.h

@@ -152,6 +152,10 @@ struct ArchCellInfo
             bool lvds;
             // TODO: clk packing checks...
         } ioInfo;
+        struct
+        {
+            bool forPadIn;
+        } gbInfo;
     };
 };
 

ice40/bitstream.cc

@@ -269,6 +269,9 @@ void write_asc(const Context *ctx, std::ostream &out)
             config.at(y).at(x).resize(rows, std::vector<int8_t>(cols));
         }
     }
+
+    std::vector<std::tuple<int, int, int>> extra_bits;
+
     out << ".comment from next-pnr" << std::endl;
 
     switch (ctx->args.type) {
@@ -513,7 +516,16 @@ void write_asc(const Context *ctx, std::ostream &out)
                 }
             }
         } else if (cell.second->type == ctx->id("SB_GB")) {
-            // no cell config bits
+            if (cell.second->gbInfo.forPadIn) {
+                Loc gb_loc = ctx->getBelLocation(bel);
+                for (int i = 0; i < ci.num_global_networks; i++) {
+                    if ((gb_loc.x == ci.global_network_info[i].gb_x) && (gb_loc.y == ci.global_network_info[i].gb_y)) {
+                        extra_bits.push_back(std::make_tuple(ci.global_network_info[i].pi_eb_bank,
+                                                             ci.global_network_info[i].pi_eb_x,
+                                                             ci.global_network_info[i].pi_eb_y));
+                    }
+                }
+            }
         } else if (cell.second->type == ctx->id("ICESTORM_RAM")) {
             const BelInfoPOD &beli = ci.bel_data[bel.index];
             int x = beli.x, y = beli.y;
@@ -795,6 +807,10 @@ void write_asc(const Context *ctx, std::ostream &out)
         }
     }
 
+    // Write extra-bits
+    for (auto eb : extra_bits)
+        out << ".extra_bit " << std::get<0>(eb) << " " << std::get<1>(eb) << " " << std::get<2>(eb) << std::endl;
+
     // Write symbols
     // const bool write_symbols = 1;
     for (auto wire : ctx->getWires()) {

ice40/pack.cc

@@ -381,6 +381,33 @@ static void pack_constants(Context *ctx)
     }
 }
 
+static std::unique_ptr<CellInfo> create_padin_gbuf(Context *ctx, CellInfo *cell, IdString port_name,
+                                                   std::string gbuf_name)
+{
+    // Find the matching SB_GB BEL connected to the same global network
+    BelId gb_bel;
+    BelId bel = ctx->getBelByName(ctx->id(cell->attrs[ctx->id("BEL")]));
+    auto wire = ctx->getBelPinWire(bel, port_name);
+    for (auto src_bel : ctx->getWireBelPins(wire)) {
+        if (ctx->getBelType(src_bel.bel) == id_SB_GB && src_bel.pin == id_GLOBAL_BUFFER_OUTPUT) {
+            gb_bel = src_bel.bel;
+            break;
+        }
+    }
+
+    NPNR_ASSERT(gb_bel != BelId());
+
+    // Create a SB_GB Cell and lock it there
+    std::unique_ptr<CellInfo> gb = create_ice_cell(ctx, ctx->id("SB_GB"), gbuf_name);
+    gb->attrs[ctx->id("FOR_PAD_IN")] = "1";
+    gb->attrs[ctx->id("BEL")] = ctx->getBelName(gb_bel).str(ctx);
+
+    // Reconnect the net to that port for easier identification it's a global net
+    replace_port(cell, port_name, gb.get(), id_GLOBAL_BUFFER_OUTPUT);
+
+    return gb;
+}
+
 static bool is_nextpnr_iob(Context *ctx, CellInfo *cell)
 {
     return cell->type == ctx->id("$nextpnr_ibuf") || cell->type == ctx->id("$nextpnr_obuf") ||
@@ -1003,13 +1030,13 @@ bool Arch::pack()
     try {
         log_break();
         pack_constants(ctx);
-        promote_globals(ctx);
         pack_io(ctx);
         pack_lut_lutffs(ctx);
         pack_nonlut_ffs(ctx);
         pack_carries(ctx);
         pack_ram(ctx);
         pack_special(ctx);
+        promote_globals(ctx);
         ctx->assignArchInfo();
         constrain_chains(ctx);
         ctx->assignArchInfo();