bram: integrate into SoC using Wishbone bus, and note alignment

gateware/rtl/bram/bram.v.m4

@@ -6,12 +6,9 @@ m4_changecom(⟨/*⟩, ⟨*/⟩)
  * For license terms, refer to the files in `doc/copying` in the Upsilon
  * source distribution.
  *
- * BRAM to Wishbone interface.
+ * This BRAM can only handle aligned accesses.
  */
-module bram_interface #(
-	/* This is the last INDEX of the word array, which is indexed in
-	 * words, not octets. */
-	parameter [WORD_AMNT_WID-1:0] WORD_AMNT = 2047,
+module bram #(
 	/* Width of the memory bus */
 	parameter BUS_WID = 32,
 	/* Width of a request. */
@@ -26,37 +23,45 @@ module bram_interface #(
 	input wb_we,
 	input [4-1:0] wb_sel,
 	input [BUS_WID-1:0] wb_addr,
-	input [BUS_WID-1:0] wb_data_i,
+	input [BUS_WID-1:0] wb_dat_w,
 	output reg wb_ack,
-	output wb_stall,
-	output reg [BUS_WID-1:0] wb_data_o,
+	output reg [BUS_WID-1:0] wb_dat_r,
 );
 
-assign wb_stall = wb_ack;
+/* When the size of the memory is a power of 2, the mask is the
+ * last addressable index in the array.
+ *
+ * Since this buffer stores words, this is divided by 4 (32 bits).
+ * When accessing a single byte, the address
+ * 0b......Xab
+ * is shifted to the right by two bits, throwing away "ab". This indexes
+ * the 32 bit word that contains the address. This applies to halfwords
+ * and words as long as the accesses are aligned.
+ */
+reg [WORD_WID-1:0] buffer [(ADDR_MASK >> 2):0];
 
-reg [BUS_WID-1:0] buffer [WORD_AMNT:0];
+/* Current index into the buffer. */
+wire [13-1:0] ind = (wb_addr & ADDR_MASK) >> 2;
 
 m4_define(⟨bufwrite⟩,  ⟨begin
-	buffer[mem_addr & ADDR_MASK] <=
-		(buffer[wb_addr & ADDR_MASK] & $1)
-		| wb_data_i[$2];
+	buffer[ind] <= (buffer[ind] & $1) | wb_dat_w[$2];
 end⟩)
 
 always @ (posedge clk) if (wb_cyc && wb_stb && !wb_ack)
 	if (!wb_we) begin
-		wb_data_o <= buffer[wb_addr & ADDR_MASK];
+		wb_dat_r <= buffer[ind];
 		wb_ack <= 1;
 	end else begin
 		wb_ack <= 1;
 		case (wb_sel)
-		4'b1111: buffer[wb_addr & ADDR_MASK] <= wb_data_o;
+		4'b1111: buffer[ind] <= wb_dat_w;
+		4'b0011: bufwrite(32'hFFFF0000, 15:0)
+		4'b1100: bufwrite(32'h0000FFFF, 31:16)
 		4'b0001: bufwrite(32'hFFFFFF00, 7:0)
 		4'b0010: bufwrite(32'hFFFF00FF, 15:8)
-		4'b0011: bufwrite(32'hFFFF0000, 15:0)
 		4'b0100: bufwrite(32'hFF00FFFF, 23:16)
 		4'b1000: bufwrite(32'h00FFFFFF, 31:24)
-		4'b1100: bufwrite(32'h0000FFFF, 31:16)
-		default: mem_ready <= 1;
+		default: ;
 		endcase
 	end
 else if (!wb_stb) begin

gateware/soc.py

@@ -51,6 +51,7 @@ from litex.soc.integration.soc_core import SoCCore
 from litex.soc.integration.soc import SoCRegion
 from litex.soc.cores.clock import S7PLL, S7IDELAYCTRL
 from litex.soc.interconnect.csr import AutoCSR, Module, CSRStorage, CSRStatus
+from litex.soc.interconnect.wishbone import Interface
 
 from litedram.phy import s7ddrphy
 from litedram.modules import MT41K128M16
@@ -183,6 +184,25 @@ class Base(Module, AutoCSR):
 
         self.specials += Instance("base", **self.kwargs)
 
+class BRAM(Module):
+    def __init__(self, clk):
+        self.bus = Interface(data_width=32, address_width=32, addressing="byte")
+        self.comb += [
+                self.bus.cti.eq(0),
+                self.bus.bte.eq(0),
+        ]
+        self.specials += Instance("bram",
+                i_clk = clk,
+                i_wb_cyc = self.bus.cyc,
+                i_wb_stb = self.bus.stb,
+                i_wb_we = self.bus.we,
+                i_wb_sel = self.bus.sel,
+                i_wb_addr = self.bus.adr,
+                i_wb_dat_w = self.bus.dat_w,
+                o_wb_ack = self.bus.ack,
+                o_wb_dat_r = self.bus.dat_r,
+        )
+
 # Clock and Reset Generator
 # I don't know how this works, I only know that it does.
 class _CRG(Module):
@@ -222,6 +242,8 @@ class UpsilonSoC(SoCCore):
             self.add_constant(f"{ip_name}{seg_num}", int(ip_byte))
     def add_bram(self, region_name):
         self.bus.add_region(region_name, SoCRegion(size=0x2000, cached=False))
+        # TODO: special name
+        self.submodules.bram0 = BRAM(ClockSignal())
 
     def __init__(self,
                  variant="a7-100",
@@ -259,7 +281,8 @@ class UpsilonSoC(SoCCore):
         platform.add_source("rtl/control_loop/control_loop.v")
 #       platform.add_source("rtl/waveform/bram_interface_preprocessed.v")
 #       platform.add_source("rtl/waveform/waveform_preprocessed.v")
-        platform.add_source("rtl/bram/bram_preprocessed.v")
+#       when SoC cannot find a source file, it will fail with a confusing error message
+        platform.add_source("rtl/bram/bram.v")
         platform.add_source("rtl/base/base.v")
 
         # SoCCore does not have sane defaults (no integrated rom)
@@ -307,7 +330,7 @@ class UpsilonSoC(SoCCore):
         self.add_ip(remote_ip, "REMOTEIP")
         self.add_constant("TFTP_SERVER_PORT", tftp_port)
 
-        self.add_bram("BRAM0")
+        self.add_bram("bram0")
 
         # Add pins
         platform.add_extension(io)