docs/system: ppc: Add documentation for ppce500 machine

This adds detailed documentation for PowerPC `ppce500` machine,
including the following information:

- Supported devices
- Hardware configuration information
- Boot options
- Running Linux kernel
- Running U-Boot

Signed-off-by: Bin Meng <bmeng.cn@gmail.com>
Signed-off-by: David Gibson <david@gibson.dropbear.id.au>

docs/system/ppc/ppce500.rst

+ppce500 generic platform (``ppce500``)
+======================================
+
+QEMU for PPC supports a special ``ppce500`` machine designed for emulation and
+virtualization purposes.
+
+Supported devices
+-----------------
+
+The ``ppce500`` machine supports the following devices:
+
+* PowerPC e500 series core (e500v2/e500mc/e5500/e6500)
+* Configuration, Control, and Status Register (CCSR)
+* Multicore Programmable Interrupt Controller (MPIC) with MSI support
+* 1 16550A UART device
+* 1 Freescale MPC8xxx I2C controller
+* 1 Pericom pt7c4338 RTC via I2C
+* 1 Freescale MPC8xxx GPIO controller
+* Power-off functionality via one GPIO pin
+* 1 Freescale MPC8xxx PCI host controller
+* VirtIO devices via PCI bus
+
+Hardware configuration information
+----------------------------------
+
+The ``ppce500`` machine automatically generates a device tree blob ("dtb")
+which it passes to the guest, if there is no ``-dtb`` option. This provides
+information about the addresses, interrupt lines and other configuration of
+the various devices in the system.
+
+If users want to provide their own DTB, they can use the ``-dtb`` option.
+These DTBs should have the following requirements:
+
+* The number of subnodes under /cpus node should match QEMU's ``-smp`` option
+* The /memory reg size should match QEMU’s selected ram_size via ``-m``
+
+Both ``qemu-system-ppc`` and ``qemu-system-ppc64`` provide emulation for the
+following 32-bit PowerPC CPUs:
+
+* e500v2
+* e500mc
+
+Additionally ``qemu-system-ppc64`` provides support for the following 64-bit
+PowerPC CPUs:
+
+* e5500
+* e6500
+
+The CPU type can be specified via the ``-cpu`` command line. If not specified,
+it creates a machine with e500v2 core. The following example shows an e6500
+based machine creation:
+
+.. code-block:: bash
+
+  $ qemu-system-ppc64 -nographic -M ppce500 -cpu e6500
+
+Boot options
+------------
+
+The ``ppce500`` machine can start using the standard -kernel functionality
+for loading a payload like an OS kernel (e.g.: Linux), or U-Boot firmware.
+
+When -bios is omitted, the default pc-bios/u-boot.e500 firmware image is used
+as the BIOS. QEMU follows below truth table to select which payload to execute:
+
+===== ========== =======
+-bios    -kernel payload
+===== ========== =======
+    N          N  u-boot
+    N          Y  kernel
+    Y don't care  u-boot
+===== ========== =======
+
+When both -bios and -kernel are present, QEMU loads U-Boot and U-Boot in turns
+automatically loads the kernel image specified by the -kernel parameter via
+U-Boot's built-in "bootm" command, hence a legacy uImage format is required in
+such senario.
+
+Running Linux kernel
+--------------------
+
+Linux mainline v5.11 release is tested at the time of writing. To build a
+Linux mainline kernel that can be booted by the ``ppce500`` machine in
+64-bit mode, simply configure the kernel using the defconfig configuration:
+
+.. code-block:: bash
+
+  $ export ARCH=powerpc
+  $ export CROSS_COMPILE=powerpc-linux-
+  $ make corenet64_smp_defconfig
+  $ make menuconfig
+
+then manually select the following configuration:
+
+  Platform support > Freescale Book-E Machine Type > QEMU generic e500 platform
+
+To boot the newly built Linux kernel in QEMU with the ``ppce500`` machine:
+
+.. code-block:: bash
+
+  $ qemu-system-ppc64 -M ppce500 -cpu e5500 -smp 4 -m 2G \
+      -display none -serial stdio \
+      -kernel vmlinux \
+      -initrd /path/to/rootfs.cpio \
+      -append "root=/dev/ram"
+
+To build a Linux mainline kernel that can be booted by the ``ppce500`` machine
+in 32-bit mode, use the same 64-bit configuration steps except the defconfig
+file should use corenet32_smp_defconfig.
+
+To boot the 32-bit Linux kernel:
+
+.. code-block:: bash
+
+  $ qemu-system-ppc{64|32} -M ppce500 -cpu e500mc -smp 4 -m 2G \
+      -display none -serial stdio \
+      -kernel vmlinux \
+      -initrd /path/to/rootfs.cpio \
+      -append "root=/dev/ram"
+
+Running U-Boot
+--------------
+
+U-Boot mainline v2021.04 release is tested at the time of writing. To build a
+U-Boot mainline bootloader that can be booted by the ``ppce500`` machine, use
+the qemu-ppce500_defconfig with similar commands as described above for Linux:
+
+.. code-block:: bash
+
+  $ export CROSS_COMPILE=powerpc-linux-
+  $ make qemu-ppce500_defconfig
+
+You will get u-boot file in the build tree.
+
+When U-Boot boots, you will notice the following if using with ``-cpu e6500``:
+
+.. code-block:: none
+
+  CPU:   Unknown, Version: 0.0, (0x00000000)
+  Core:  e6500, Version: 2.0, (0x80400020)
+
+This is because we only specified a core name to QEMU and it does not have a
+meaningful SVR value which represents an actual SoC that integrates such core.
+You can specify a real world SoC device that QEMU has built-in support but all
+these SoCs are e500v2 based MPC85xx series, hence you cannot test anything
+built for P4080 (e500mc), P5020 (e5500) and T2080 (e6500).
+
+By default a VirtIO standard PCI networking device is connected as an ethernet
+interface at PCI address 0.1.0, but we can switch that to an e1000 NIC by:
+
+.. code-block:: bash
+
+  $ qemu-system-ppc -M ppce500 -smp 4 -m 2G \
+                    -display none -serial stdio \
+                    -bios u-boot \
+                    -nic tap,ifname=tap0,script=no,downscript=no,model=e1000

docs/system/target-ppc.rst

@@ -20,5 +20,6 @@ help``.
    ppc/embedded
    ppc/powermac
    ppc/powernv
+   ppc/ppce500
    ppc/prep
    ppc/pseries