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hdl.mem: remove WritePort(priority=) argument.

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The write port priority in Yosys is derived directly from the order
in which the ports are declared in the Verilog frontend. It is being
removed for several reasons:
  1. It is not clear if it works correctly for all cases (FFRAM,
     LUTRAM, BRAM).
  2. Although it is roundtripped via Verilog with correct simulation
     semantics, the resulting code has a high chance of being
     interpreted incorrectly by Xilinx tools.
  3. It cannot be roundtripped via FIRRTL, which is an alternative
     backend that is an interesting future option. (FIRRTL leaves
     write collision completely undefined.)
  3. It is a niche feature that, if it is needed, can be completely
     replaced using an explicit comparator, priority encoder, and
     write enable gating circuit. (This is what Xilinx recommends
     for handling this case.)

In the future we should extend nMigen's formal verification to assert
that a write collision does not happen.
This commit is contained in:
whitequark 2019-09-28 01:29:56 +00:00
parent e3a1d05f23
commit a02e3750bf
2 changed files with 2 additions and 5 deletions
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5
nmigen/hdl/mem.py
View file

@ -132,7 +132,7 @@ class ReadPort(Elaboratable):
class WritePort(Elaboratable):
def __init__(self, memory, *, domain="sync", priority=0, granularity=None):
def __init__(self, memory, *, domain="sync", granularity=None):
if granularity is None:
granularity = memory.width
if not isinstance(granularity, int) or granularity < 0:
@ -147,7 +147,6 @@ class WritePort(Elaboratable):
self.memory = memory
self.domain = domain
self.priority = priority
self.granularity = granularity
self.addr = Signal.range(memory.depth,
@ -164,7 +163,7 @@ class WritePort(Elaboratable):
p_WIDTH=self.data.width,
p_CLK_ENABLE=1,
p_CLK_POLARITY=1,
p_PRIORITY=self.priority,
p_PRIORITY=0,
i_CLK=ClockSignal(self.domain),
i_EN=Cat(Repl(en_bit, self.granularity) for en_bit in self.en),
i_ADDR=self.addr,

2
nmigen/test/test_hdl_mem.py
View file

@ -83,7 +83,6 @@ class MemoryTestCase(FHDLTestCase):
wrport = mem.write_port()
self.assertEqual(wrport.memory, mem)
self.assertEqual(wrport.domain, "sync")
self.assertEqual(wrport.priority, 0)
self.assertEqual(wrport.granularity, 8)
self.assertEqual(len(wrport.addr), 2)
self.assertEqual(len(wrport.data), 8)
@ -94,7 +93,6 @@ class MemoryTestCase(FHDLTestCase):
wrport = mem.write_port(granularity=2)
self.assertEqual(wrport.memory, mem)
self.assertEqual(wrport.domain, "sync")
self.assertEqual(wrport.priority, 0)
self.assertEqual(wrport.granularity, 2)
self.assertEqual(len(wrport.addr), 2)
self.assertEqual(len(wrport.data), 8)