From: David Shah <dave@ds0.me>
Date: Tue, 16 Jul 2019 15:46:41 +0000 (+0100)
Subject: xilinx: Treat DSP48E1 as 24x17 unsigned for now (actual behaviour is 25x18 signed)
X-Git-Tag: working-ls180~1039^2~357^2~1
X-Git-Url: https://git.libre-soc.org/?a=commitdiff_plain;h=95c8d27b0bfdea330a62a18825dea3691b4affe2;p=yosys.git

xilinx: Treat DSP48E1 as 24x17 unsigned for now (actual behaviour is 25x18 signed)

Signed-off-by: David Shah <dave@ds0.me>
---

diff --git a/techlibs/xilinx/cells_map.v b/techlibs/xilinx/cells_map.v
index 6ebca0d54..8302e0b3a 100644
--- a/techlibs/xilinx/cells_map.v
+++ b/techlibs/xilinx/cells_map.v
@@ -366,7 +366,7 @@ module \$__XILINX_MUXF78 (O, I0, I1, I2, I3, S0, S1);
 endmodule
 `endif
 
-module \$__MUL25X18 (input [24:0] A, input [17:0] B, output [42:0] OUT);
+module \$__MUL25X18 (input [23:0] A, input [16:0] B, output [40:0] OUT);
 	wire [47:0] P_48;
 	DSP48E1 #(
 		// Disable all registers
@@ -388,8 +388,8 @@ module \$__MUL25X18 (input [24:0] A, input [17:0] B, output [42:0] OUT);
 		.PREG(0)
 	) _TECHMAP_REPLACE_ (
 		//Data path
-		.A({5'b0, A}),
-		.B(B),
+		.A({6'b0, A}),
+		.B({1'b0, B}),
 		.C(48'b0),
 		.D(24'b0),
 		.P(P_48),
diff --git a/techlibs/xilinx/synth_xilinx.cc b/techlibs/xilinx/synth_xilinx.cc
index 01e75b50e..5bfbd1583 100644
--- a/techlibs/xilinx/synth_xilinx.cc
+++ b/techlibs/xilinx/synth_xilinx.cc
@@ -284,8 +284,12 @@ struct SynthXilinxPass : public ScriptPass
 
 			run("techmap -map +/cmp2lut.v -D LUT_WIDTH=6");
 
+			// The actual behaviour of the Xilinx DSP is a signed 25x18 multiply
+			// Due to current limitations of mul2dsp, we are actually mapping as a 24x17
+			// unsigned multiply with MSBs set to 1'b0
+
 			if (!nodsp || help_mode)
-				run("techmap -map +/mul2dsp.v -D DSP_A_MAXWIDTH=25 -D DSP_B_MAXWIDTH=18 -D DSP_NAME=$__MUL25X18");
+				run("techmap -map +/mul2dsp.v -D DSP_A_MAXWIDTH=24 -D DSP_B_MAXWIDTH=17 -D DSP_NAME=$__MUL25X18");
 
 			run("alumacc");
 			run("share");