headergen_v2: add floating instances and nochild flag

Floating instances don't have an address and will only generate
child nodes and registers with offsets. The 'nochild' flag will
disable generating the children for a node but will generate the
node's own address, which can be used to generate base addresses.

Change-Id: Ib1014de94531436d5708db46aa684741e7740ace

utils/regtools/desc/spec-2.0.txt

@@ -146,7 +146,43 @@ are:
 - F[2] at 0x50+(2/2)*0x100+(2%2)*0x10 = 0x50+0x100 = 0x150
 - F[3] at 0x50+(3/2)*0x100+(3%2)*0x10 = 0x50+0x100+0x10 = 0x160
 
-1.3) Node description
+1.3) Floating and 'nochild' instances
+-------------------------------------
+
+Floating instances do not have an fixed address and instead "float" relative
+to a base address provided at runtime. Child nodes and registers are generated
+as usual, except that their addresses will not be defined, only their offsets
+relative to their floating parent node.
+
+<node>
+  <name>N</name>
+  <instance>
+    <name>F</name>
+    <floating>1</floating>
+  </instance>
+</node>
+
+Instances with the 'nochild' flag will emit their own base address, but won't
+generate any child nodes or registers themselves. They can be used alongside
+floating nodes to provide the base addresses for SoC peripheral instances:
+
+<node>
+  <name>N</name>
+  <instance>
+    <name>F</name>
+    <nochild>1</nochild>
+    <address>0x1234</address>
+  </instance>
+</node>
+
+Nochild instances can use both <address> and <range> for specifying their
+address.
+
+For generic code, storing the base address and computing offsets can generate
+more efficient code compared to indexed macros that calculate the base address,
+especially when the addresses don't follow a simple formula.
+
+1.4) Node description
 ---------------------
 
 For documentation purposes, node can of course carry some description, as well
@@ -177,7 +213,7 @@ The following example illustrates this:
   </instance>
 </node>
 
-1.4) Register description
+1.5) Register description
 --------------------------
 
 The goal of the register description is of course to describe registers!
@@ -247,7 +283,7 @@ In this example, the 8-bit registers has three fields:
 - PRIORITY(2:1): it has no named values
 - ARM_MODE(3): it has two named values IRQ(0) and FIQ(1)
 
-1.5) Register inheritance
+1.6) Register inheritance
 -------------------------
 
 The node hierarchy specifies instances, that is pairs of (path,address),
@@ -286,7 +322,7 @@ This example describes one register (let's call it blabla) and 9 instances:
 - DMAC.I2C_CHAN.CLR at 0x80000018, register blabla
 - DMAC.I2C_CHAN.TOG at 0x8000001C, register blabla
 
-1.6) Soc description
+1.7) Soc description
 --------------------
 
 The description file must also specify some information about the system-on-chip
@@ -352,8 +388,10 @@ It can contain at most one of each of the following tags:
 - desc: free form description of the instance
 - address: address for a single instance (non-negative number)
 - range: address range for multiple instances
-Note that address and range are mutually exclusive, and at least one of them
-must exists.
+- floating: 0 or 1, if 1 then the instance is floating and has no address
+- nochild: 0 or 1, if 1 then no child instances/registers are generated
+Note that address, range, and floating are mutually exclusive, and at least
+one of them must exist. An instance cannot be both floating and nochild.
 
 Element: range
 --------------