soc/cores/led/WS2812: Add Bus Mastering capability.

Useful on small FPGAs to reduce resource usage: When enabling bus mastering,
the core is able to automatically read led values from the bus and can then
avoid the internal memory. This is particularly useful when reading values
from SPI Flash with a small "Player" core just updating the base address.

litex/soc/cores/led.py

@@ -117,7 +117,10 @@ class WS2812(Module):
      sys_clk_freq: int, in
          System Clk Frequency.
     """
-    def __init__(self, pad, nleds, sys_clk_freq):
+    def __init__(self, pad, nleds, sys_clk_freq, bus_mastering=False, bus_base=None):
+        if bus_mastering:
+            self.bus  = bus = wishbone.Interface(data_width=32)
+        else:
             # Memory.
             mem = Memory(32, nleds)
             port = mem.get_port()
@@ -131,6 +134,7 @@ class WS2812(Module):
             )
             self.bus = self.wb_mem.bus
 
+
         # Internal Signals.
         led_data  = Signal(24)
         bit_count = Signal(8)
@@ -164,6 +168,25 @@ class WS2812(Module):
                 NextState("LED-SHIFT")
             )
         )
+        if bus_mastering:
+             fsm.act("LED-SHIFT",
+                bus.stb.eq(1),
+                bus.cyc.eq(1),
+                bus.we.eq(0),
+                bus.sel.eq(2**(bus.data_width//8)-1),
+                bus.adr.eq(bus_base[2:] + led_count),
+                If(bus.ack,
+                    NextValue(bit_count, 24-1),
+                    NextValue(led_data,  bus.dat_r),
+                    NextValue(led_count, led_count + 1),
+                    If(led_count == (nleds-1),
+                        NextState("RST")
+                    ).Else(
+                        NextState("BIT-TEST")
+                    )
+                )
+            )
+        else:
             self.comb += port.adr.eq(led_count)
             fsm.act("LED-SHIFT",
                 NextValue(bit_count, 24-1),
@@ -175,6 +198,7 @@ class WS2812(Module):
                     NextState("BIT-TEST")
                 )
             )
+
         fsm.act("BIT-TEST",
             If(led_data[-1] == 0,
                 NextState("ZERO-SEND"),