Clarify documentation for alignment parameters to mention it is log2.

Fixes #6.

Author: Jean-François Nguyen

nmigen_soc/csr/bus.py

@@ -192,28 +192,29 @@ class Multiplexer(Elaboratable):
     are possible for connecting the CSR bus to the CPU:
         * The CPU could access the CSR bus directly (with no intervening logic other than simple
           translation of control signals). In this case, the register alignment should be set
-          to 1, and each *w*-bit register would occupy *ceil(w/n)* addresses from the CPU
-          perspective, requiring the same amount of memory instructions to access.
+          to 1 (i.e. `alignment` should be set to 0), and each *w*-bit register would occupy
+          *ceil(w/n)* addresses from the CPU perspective, requiring the same amount of memory
+          instructions to access.
         * The CPU could also access the CSR bus through a width down-converter, which would issue
           *k/n* CSR accesses for each CPU access. In this case, the register alignment should be
           set to *k/n*, and each *w*-bit register would occupy *ceil(w/k)* addresses from the CPU
           perspective, requiring the same amount of memory instructions to access.
 
-    If alignment is greater than 1, it affects which CSR bus write is considered a write to
-    the last register chunk. For example, if a 24-bit register is used with a 8-bit CSR bus and
-    a CPU with a 32-bit datapath, a write to this register requires 4 CSR bus writes to complete
-    and the 4th write is the one that actually writes the value to the register. This allows
-    determining write latency solely from the amount of addresses the register occupies in
-    the CPU address space, and the width of the CSR bus.
+    If the register alignment (i.e. `2 ** alignment`) is greater than 1, it affects which CSR bus
+    write is considered a write to the last register chunk. For example, if a 24-bit register is
+    used with a 8-bit CSR bus and a CPU with a 32-bit datapath, a write to this register requires
+    4 CSR bus writes to complete and the 4th write is the one that actually writes the value to
+    the register. This allows determining write latency solely from the amount of addresses the
+    register occupies in the CPU address space, and the width of the CSR bus.
 
     Parameters
     ----------
     addr_width : int
         Address width. See :class:`Interface`.
     data_width : int
         Data width. See :class:`Interface`.
-    alignment : int
-        Register alignment. See :class:`Interface`.
+    alignment : log2 of int
+        Register alignment. See :class:`..memory.MemoryMap`.
 
     Attributes
     ----------
@@ -327,8 +328,8 @@ class Decoder(Elaboratable):
         Address width. See :class:`Interface`.
     data_width : int
         Data width. See :class:`Interface`.
-    alignment : int
-        Window alignment. See :class:`Interface`.
+    alignment : log2 of int
+        Window alignment. See :class:`..memory.MemoryMap`.
 
     Attributes
     ----------

nmigen_soc/memory.py

@@ -70,7 +70,7 @@ class MemoryMap:
         Address width.
     data_width : int
         Data width.
-    alignment : int
+    alignment : log2 of int
         Range alignment. Each added resource and window will be placed at an address that is
         a multiple of ``2 ** alignment``, and its size will be rounded up to be a multiple of
         ``2 ** alignment``.
@@ -142,7 +142,7 @@ def align_to(self, alignment):
 
         Arguments
         ---------
-        alignment : int
+        alignment : log2 of int
             Address alignment. The start of the implicit next address will be a multiple of
             ``2 ** max(alignment, self.alignment)``.
 
@@ -216,7 +216,7 @@ def add_resource(self, resource, *, size, addr=None, alignment=None, extend=Fals
         size : int
             Size of the resource, in minimal addressable units. Rounded up to a multiple of
             ``2 ** max(alignment, self.alignment)``.
-        alignment : int or None
+        alignment : log2 of int or None
             Alignment of the resource. If not specified, the memory map alignment is used.
         extend: bool
             Allow memory map extension. If ``True``, the upper bound of the address space is

nmigen_soc/wishbone/bus.py

@@ -184,8 +184,8 @@ class Decoder(Elaboratable):
         Granularity. See :class:`Interface`
     features : iter(str)
         Optional signal set. See :class:`Interface`.
-    alignment : int
-        Window alignment. See :class:`Interface`.
+    alignment : log2 of int
+        Window alignment. See :class:`..memory.MemoryMap`
 
     Attributes
     ----------