nmigen_soc/scheduler.py

   1 from nmigen import *
   2
   3
   4 __all__ = ["RoundRobin"]
   5
   6
   7 class RoundRobin(Elaboratable):
   8     """A round-robin scheduler.
   9
  10     Parameters
  11     ----------
  12     n : int
  13         Maximum number of requests to handle.
  14
  15     Attributes
  16     ----------
  17     request : Signal(n)
  18         Signal where a '1' on the i-th bit represents an incoming request
  19         from the i-th device.
  20     grant : Signal(range(n))
  21         Signal that equals to the index of the device which is currently
  22         granted access.
  23     stb : Signal()
  24         Strobe signal to enable granting access to the next device
  25         requesting. Externally driven.
  26     """
  27     def __init__(self, n):
  28         self.n = n
  29         self.request = Signal(n)
  30         self.grant = Signal(range(n))
  31         self.stb = Signal()
  32
  33     def elaborate(self, platform):
  34         m = Module()
  35
  36         with m.If(self.stb):
  37             with m.Switch(self.grant):
  38                 for i in range(self.n):
  39                     with m.Case(i):
  40                         for j in reversed(range(i+1, i+self.n)):
  41                             # If i+1 <= j < n, then t == j;     (after i)
  42                             # If n <= j < i+n, then t == j - n  (before i)
  43                             t = j % self.n
  44                             with m.If(self.request[t]):
  45                                 m.d.sync += self.grant.eq(t)
  46
  47         return m