bank/description/CSRStorage: support alignment bits

diff --git a/migen/bank/description.py b/migen/bank/description.py
index 1eb5c2c11cd4e0ffc44ff82a45a5e8231b2e7085..7cc2caa1e06855044628542e80a610eb4ccb6578 100644 (file)
--- a/migen/bank/description.py
+++ b/migen/bank/description.py
@@ -47,14 +47,17 @@ class CSRStatus(_CompoundCSR):
                        self.simple_csrs.append(sc)
 
 class CSRStorage(_CompoundCSR):
-       def __init__(self, size=1, reset=0, atomic_write=False, write_from_dev=False, name=None):
+       def __init__(self, size=1, reset=0, atomic_write=False, write_from_dev=False, alignment_bits=0, name=None):
                _CompoundCSR.__init__(self, size, name)
-               self.storage = Signal(self.size, reset=reset)
+               self.alignment_bits = alignment_bits
+               self.storage_full = Signal(self.size, reset=reset)
+               self.storage = Signal(self.size - self.alignment_bits)
+               self.comb += self.storage.eq(self.storage_full[self.alignment_bits:])
                self.atomic_write = atomic_write
                if write_from_dev:
                        self.we = Signal()
-                       self.dat_w = Signal(self.size)
-                       self.sync += If(self.we, self.storage.eq(self.dat_w))
+                       self.dat_w = Signal(self.size - self.alignment_bits)
+                       self.sync += If(self.we, self.storage_full.eq(self.dat_w << self.alignment_bits))
 
        def do_finalize(self, busword):
                nwords = (self.size + busword - 1)//busword
@@ -63,20 +66,25 @@ class CSRStorage(_CompoundCSR):
                for i in reversed(range(nwords)):
                        nbits = min(self.size - i*busword, busword)
                        sc = CSR(nbits, self.name + str(i) if nwords else self.name)
+                       self.simple_csrs.append(sc)
                        lo = i*busword
                        hi = lo+nbits
                        # read
-                       self.comb += sc.w.eq(self.storage[lo:hi])
+                       if lo >= self.alignment_bits:
+                               self.comb += sc.w.eq(self.storage_full[lo:hi])
+                       elif hi > self.alignment_bits:
+                               self.comb += sc.w.eq(Cat(Replicate(0, hi - self.alignment_bits),
+                                       self.storage_full[self.alignment_bits:hi]))
+                       else:
+                               self.comb += sc.w.eq(0)
                        # write
                        if nwords > 1 and self.atomic_write:
                                if i:
                                        self.sync += If(sc.re, backstore[lo-busword:hi-busword].eq(sc.r))
                                else:
-                                       self.sync += If(sc.re, self.storage.eq(Cat(sc.r, backstore)))
+                                       self.sync += If(sc.re, self.storage_full.eq(Cat(sc.r, backstore)))
                        else:
-                               self.sync += If(sc.re, self.storage[lo:hi].eq(sc.r))
-
-                       self.simple_csrs.append(sc)
+                               self.sync += If(sc.re, self.storage_full[lo:hi].eq(sc.r))
 
 def csrprefix(prefix, csrs):
        for csr in csrs: