vexriscv: reset wishbone bus on CPU reset

If the CPU is resetting during a Wishbone transfer, assert the ERR line.

Because the resetOut line is likely multiple cycles long, this should
give Wishbone enough time to finish its transfer, which will cause d.stb
and i.stb to go to 0, which will return d_err and i_err to 0.

Signed-off-by: Sean Cross <sean@xobs.io>

litex/soc/cores/cpu/vexriscv/core.py

@@ -10,6 +10,8 @@ class VexRiscv(Module, AutoCSR):
         assert variant in (None, "debug"), "Unsupported variant %s" % variant
         self.ibus = i = wishbone.Interface()
         self.dbus = d = wishbone.Interface()
+        i_err = Signal()
+        d_err = Signal()
 
         self.interrupt = Signal(32)
 
@@ -34,6 +36,8 @@ class VexRiscv(Module, AutoCSR):
             self.o_rsp_data = Signal(32)
             self.o_resetOut = Signal()
 
+            reset_debug_logic = Signal()
+
             self.transfer_complete = Signal()
             self.transfer_in_progress = Signal()
             self.transfer_wait_for_ack = Signal()
@@ -42,6 +46,7 @@ class VexRiscv(Module, AutoCSR):
 
             self.sync += [
                 self.debug_bus.dat_r.eq(self.o_rsp_data),
+                cpu_reset.eq(reset_debug_logic | ResetSignal()),
             ]
 
             self.sync += [
@@ -74,12 +79,21 @@ class VexRiscv(Module, AutoCSR):
                 ).Elif(self.transfer_wait_for_ack & ~(self.debug_bus.stb & self.debug_bus.cyc),
                     self.transfer_wait_for_ack.eq(0),
                     self.debug_bus.ack.eq(0)
+                ),
+                # Force a Wishbone error if transferring during a reset sequence.
+                # Because o_resetOut is multiple cycles and i.stb/d.stb should
+                # deassert one cycle after i_err/i_ack/d_err/d_ack are asserted,
+                # this will give i_err and o_err enough time to be reset to 0
+                # once the reset cycle finishes.
+                If(self.o_resetOut,
+                    If(i.cyc & i.stb, i_err.eq(1)).Else(i_err.eq(0)),
+                    If(d.cyc & d.stb, d_err.eq(1)).Else(d_err.eq(0)),
+                    reset_debug_logic.eq(1))
+                .Else(
+                    reset_debug_logic.eq(0)
                 )
             ]
 
-            cpu_reset.eq((~i.cyc & ~d.cyc & ~d.stb & ~i.stb &
-                            self.o_resetOut) | ResetSignal()),
-
             cpu_args.update({
                 "i_debugReset": ResetSignal(),
                 "i_debug_bus_cmd_valid": self.i_cmd_valid,
@@ -91,6 +105,10 @@ class VexRiscv(Module, AutoCSR):
                 "o_debug_resetOut": self.o_resetOut
             })
 
+        self.comb += [
+            i.err.eq(i_err),
+            d.err.eq(d_err),
+        ]
         self.specials += Instance("VexRiscv",
                 **cpu_args,
 
@@ -111,7 +129,7 @@ class VexRiscv(Module, AutoCSR):
                 o_iBusWishbone_BTE=i.bte,
                 i_iBusWishbone_DAT_MISO=i.dat_r,
                 i_iBusWishbone_ACK=i.ack,
-                i_iBusWishbone_ERR=i.err,
+                i_iBusWishbone_ERR=i_err,
 
                 o_dBusWishbone_ADR=d.adr,
                 o_dBusWishbone_DAT_MOSI=d.dat_w,
@@ -123,7 +141,7 @@ class VexRiscv(Module, AutoCSR):
                 o_dBusWishbone_BTE=d.bte,
                 i_dBusWishbone_DAT_MISO=d.dat_r,
                 i_dBusWishbone_ACK=d.ack,
-                i_dBusWishbone_ERR=d.err)
+                i_dBusWishbone_ERR=d_err)
 
         # add verilog sources
         self.add_sources(platform, cpu_filename)