""" nmigen implementation of buffered pipeline stage, based on zipcpu:
https://zipcpu.com/blog/2017/08/14/strategies-for-pipelining.html
+
+ this module requires quite a bit of thought to understand how it works
+ (and why it is needed in the first place). reading the above is
+ *strongly* recommended.
+
+ unlike john dawson's IEEE754 FPU STB/ACK signalling, which requires
+ the STB / ACK signals to raise and lower (on separate clocks) before
+ data may proceeed (thus only allowing one piece of data to proceed
+ on *ALTERNATE* cycles), the signalling here is a true pipeline
+ where data will flow on *every* clock when the conditions are right.
+
+ input acceptance conditions are when:
+ * incoming previous-stage strobe (i_p_stb) is HIGH
+ * outgoing previous-stage busy (o_p_busy) is LOW
+
+ output transmission conditions are when:
+ * outgoing next-stage strobe (o_n_stb) is HIGH
+ * outgoing next-stage busy (i_n_busy) is LOW
+
+ the tricky bit is when the input has valid data and the output is not
+ ready to accept it. if it wasn't for the clock synchronisation, it
+ would be possible to tell the input "hey don't send that data, we're
+ not ready". unfortunately, it's not possible to "change the past":
+ the previous stage *has no choice* but to pass on its data.
+
+ therefore, the incoming data *must* be accepted - and stored.
+ on the same clock, it's possible to tell the input that it must
+ not send any more data. this is the "stall" condition.
+
+ we now effectively have *two* possible pieces of data to "choose" from:
+ the buffered data, and the incoming data. the decision as to which
+ to process and output is based on whether we are in "stall" or not.
+ i.e. when the next stage is no longer busy, the output comes from
+ the buffer if a stall had previously occurred, otherwise it comes
+ direct from processing the input.
+
+ it's quite a complex state machine!
"""
+
from nmigen import Signal, Cat, Const, Mux, Module
-from nmigen.compat.sim import run_simulation
from nmigen.cli import verilog, rtlil
class BufPipe:
+ """ buffered pipeline stage
+
+ stage-1 i_p_stb >>in stage o_n_stb out>> stage+1
+ stage-1 o_p_busy <<out stage i_n_busy <<in stage+1
+ stage-1 i_data >>in stage o_data out>> stage+1
+ | |
+ +-------> process
+ | |
+ +-- r_data ---+
+ """
def __init__(self):
# input
+ #self.i_p_rst = Signal() # >>in - comes in from PREVIOUS stage
self.i_p_stb = Signal() # >>in - comes in from PREVIOUS stage
self.i_n_busy = Signal() # in<< - comes in from the NEXT stage
- self.i_data = Signal(32) # >>in - comes in from the PREVIOUS stage
+ self.i_data = Signal(16) # >>in - comes in from the PREVIOUS stage
#self.i_rst = Signal()
# buffered
- self.r_data = Signal(32)
+ self.r_data = Signal(16)
# output
self.o_n_stb = Signal() # out>> - goes out to the NEXT stage
self.o_p_busy = Signal() # <<out - goes out to the PREVIOUS stage
- self.o_data = Signal(32) # out>> - goes out to the NEXT stage
+ self.o_data = Signal(16) # out>> - goes out to the NEXT stage
def pre_process(self, d_in):
- return d_in
+ return d_in | 0xf0000
def process(self, d_in):
return d_in + 1
def elaborate(self, platform):
m = Module()
+ # establish some combinatorial temporaries
+ o_p_busyn = Signal(reset_less=True)
+ o_n_stbn = Signal(reset_less=True)
+ i_n_busyn = Signal(reset_less=True)
i_p_stb_o_p_busyn = Signal(reset_less=True)
- m.d.comb += i_p_stb_o_p_busyn.eq(self.i_p_stb & (~self.o_p_busy))
-
- with m.If(~self.i_n_busy): # previous stage is not busy
- with m.If(~self.o_p_busy): # not stalled
+ m.d.comb += [i_n_busyn.eq(~self.i_n_busy),
+ o_n_stbn.eq(~self.o_n_stb),
+ o_p_busyn.eq(~self.o_p_busy),
+ i_p_stb_o_p_busyn.eq(self.i_p_stb & o_p_busyn),
+ ]
+
+ # store result of processing in combinatorial temporary
+ result = Signal(16)
+ with m.If(self.i_p_stb): # input is valid: process it
+ m.d.comb += result.eq(self.process(self.i_data))
+ with m.If(o_p_busyn): # not stalled
+ m.d.sync += self.r_data.eq(result)
+
+ #with m.If(self.i_p_rst): # reset
+ # m.d.sync += self.o_n_stb.eq(0)
+ # m.d.sync += self.o_p_busy.eq(0)
+ with m.If(i_n_busyn): # next stage is not busy
+ with m.If(o_p_busyn): # not stalled
# nothing in buffer: send input direct to output
- m.d.sync += self.o_n_stb.eq(self.i_p_stb)
- m.d.sync += self.o_data.eq(self.process(self.i_data))
+ m.d.sync += [self.o_n_stb.eq(self.i_p_stb),
+ self.o_data.eq(result),
+ ]
with m.Else(): # o_p_busy is true, and something is in our buffer.
- # Flush the buffer to the output port.
- m.d.sync += self.o_n_stb.eq(1)
- m.d.sync += self.o_data.eq(self.r_data)
+ # Flush the [already processed] buffer to the output port.
+ m.d.sync += [self.o_n_stb.eq(1),
+ self.o_data.eq(self.r_data),
+ # clear stall condition, declare register empty.
+ self.o_p_busy.eq(0),
+ ]
# ignore input, since o_p_busy is also true.
- # also clear stall condition, declare register to be empty.
- m.d.sync += self.o_p_busy.eq(0)
-
- # (i_n_busy) is true here: previous stage is busy
- with m.Elif(~self.o_n_stb): # next stage being told "not busy"
- m.d.sync += self.o_n_stb.eq(self.i_p_stb)
- m.d.sync += self.o_p_busy.eq(0) # Keep the buffer empty
- # Apply the logic to the input data, and set the output data
- m.d.sync += self.o_data.eq(self.process(self.i_data))
+ # (i_n_busy) is true here: next stage is busy
+ with m.Elif(o_n_stbn): # next stage being told "not busy"
+ m.d.sync += [self.o_n_stb.eq(self.i_p_stb),
+ self.o_p_busy.eq(0), # Keep the buffer empty
+ # set the output data (from comb result)
+ self.o_data.eq(result),
+ ]
# (i_n_busy) and (o_n_stb) both true:
with m.Elif(i_p_stb_o_p_busyn):
- # If next stage *is* busy, and not stalled yet, accept requested
- # input and store in temporary
+ # If next stage *is* busy, and not stalled yet, accept input
m.d.sync += self.o_p_busy.eq(self.i_p_stb & self.o_n_stb)
- with m.If(~self.o_n_stb):
- m.d.sync += self.r_data.eq(self.i_data)
- with m.If(~self.o_p_busy): # not stalled
- m.d.sync += self.r_data.eq(self.pre_process(self.i_data))
+ with m.If(o_p_busyn): # not stalled
+ # turns out that from all of the above conditions, just
+ # always put result into buffer if not busy
+ m.d.sync += self.r_data.eq(result)
return m
]
-def testbench(dut):
- yield dut.i_data.eq(5)
- yield dut.i_p_stb.eq(1)
- yield
- yield dut.i_data.eq(7)
- yield
- yield dut.i_data.eq(2)
- yield
- yield dut.i_n_busy.eq(1)
- yield dut.i_data.eq(9)
- yield
- yield dut.i_data.eq(12)
- yield
- yield dut.i_n_busy.eq(0)
- yield
- yield
- yield
-
-
if __name__ == '__main__':
dut = BufPipe()
vl = rtlil.convert(dut, ports=dut.ports())
with open("test_bufpipe.il", "w") as f:
f.write(vl)
- run_simulation(dut, testbench(dut), vcd_name="test_bufpipe.vcd")
projects / ieee754fpu.git / blobdiff