doc: allwinner: Reorder sections, document memory mapping

Update the Allwinner platform documentation.
Reorder the section, to have the build instructions first, followed by
hints about the installation.

Add some ASCII art about the layout of our virtual memory map, which
uses a non-trivial condensed virtual address space.

Change-Id: Iaaa79b4366012394e15e4c1b26c212b5efb6ed6a
Signed-off-by: Andre Przywara <andre.przywara@arm.com>

docs/plat/allwinner.rst

@@ -5,22 +5,8 @@ Trusted Firmware-A (TF-A) implements the EL3 firmware layer for Allwinner
 SoCs with ARMv8 cores. Only BL31 is used to provide proper EL3 setup and
 PSCI runtime services.
 
-U-Boot's SPL acts as a loader, loading both BL31 and BL33 (typically U-Boot).
-Loading is done from SD card, eMMC or SPI flash, also via an USB debug
-interface (FEL).
-
-BL31 lives in SRAM A2, which is documented to be accessible from secure
-world only.
-
-Current limitations:
-
--  Missing PMIC support
-
-After building bl31.bin, the binary must be fed to the U-Boot build system
-to include it in the FIT image that the SPL loader will process.
-bl31.bin can be either copied (or sym-linked) into U-Boot's root directory,
-or the environment variable BL31 must contain the binary's path.
-See the respective `U-Boot documentation`_ for more details.
+Building TF-A
+-------------
 
 To build for machines with an A64 or H5 SoC:
 
@@ -34,8 +20,58 @@ To build for machines with an H6 SoC:
 
     make CROSS_COMPILE=aarch64-linux-gnu- PLAT=sun50i_h6 DEBUG=1 bl31
 
+
+Installation
+------------
+
+U-Boot's SPL acts as a loader, loading both BL31 and BL33 (typically U-Boot).
+Loading is done from SD card, eMMC or SPI flash, also via an USB debug
+interface (FEL).
+
+After building bl31.bin, the binary must be fed to the U-Boot build system
+to include it in the FIT image that the SPL loader will process.
+bl31.bin can be either copied (or sym-linked) into U-Boot's root directory,
+or the environment variable BL31 must contain the binary's path.
+See the respective `U-Boot documentation`_ for more details.
+
 .. _U-Boot documentation: https://gitlab.denx.de/u-boot/u-boot/-/blob/master/board/sunxi/README.sunxi64
 
+Memory layout
+-------------
+
+A64, H5 and H6 SoCs
+~~~~~~~~~~~~~~~~~~~
+
+BL31 lives in SRAM A2, which is documented to be accessible from secure
+world only. Since this SRAM region is very limited (48 KB), we take
+several measures to reduce memory consumption. One of them is to confine
+BL31 to only 28 bits of virtual address space, which reduces the number
+of required page tables (each occupying 4KB of memory).
+The mapping we use on those SoCs is as follows:
+
+::
+
+   0 64K         16M             1GB         1G+160M     physical address
+   +-+------+-+---+------+--...---+-------+----+------+----------
+   |B|      |S|///|      |//...///|       |////|      |
+   |R| SRAM |C|///| dev  |//...///| (sec) |////| BL33 |  DRAM ...
+   |O|      |P|///| MMIO |//...///| DRAM  |////|      |
+   |M|      | |///|      |//...///| (32M) |////|      |
+   +-+------+-+---+------+--...---+-------+----+------+----------
+   | |      | |   |      |       /       /   /      /
+   | |      | |   |      |      /       /  /      /
+   | |      | |   |      |     /       / /      /
+   | |      | |   |      |    /       //      /
+   | |      | |   |      |   /       /      /
+   +-+------+-+---+------+--+-------+------+
+   |B|      |S|///|      |//|       |      |
+   |R| SRAM |C|///| dev  |//|  sec  | BL33 |
+   |O|      |P|///| MMIO |//| DRAM  |      |
+   |M|      | |///|      |//|       |      |
+   +-+------+-+---+------+--+-------+------+
+   0 64K         16M       160M    192M  256M             virtual address
+
+
 Trusted OS dispatcher
 ---------------------