docs/system: Document Arm Versatile Express boards

Provide a minimal documentation of the Versatile Express boards
(vexpress-a9, vexpress-a15).

Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
Reviewed-by: Philippe Mathieu-Daudé <philmd@redhat.com>
Reviewed-by: Edgar E. Iglesias <edgar.iglesias@xilinx.com>
Reviewed-by: Alex Bennée <alex.bennee@linaro.org>
Message-id: 20200507151819.28444-4-peter.maydell@linaro.org

MAINTAINERS

@@ -828,6 +828,7 @@ M: Peter Maydell <peter.maydell@linaro.org>
 L: qemu-arm@nongnu.org
 S: Maintained
 F: hw/arm/vexpress.c
+F: docs/system/arm/vexpress.rst
 
 Versatile PB
 M: Peter Maydell <peter.maydell@linaro.org>

docs/system/arm/vexpress.rst

+Arm Versatile Express boards (``vexpress-a9``, ``vexpress-a15``)
+================================================================
+
+QEMU models two variants of the Arm Versatile Express development
+board family:
+
+- ``vexpress-a9`` models the combination of the Versatile Express
+  motherboard and the CoreTile Express A9x4 daughterboard
+- ``vexpress-a15`` models the combination of the Versatile Express
+  motherboard and the CoreTile Express A15x2 daughterboard
+
+Note that as this hardware does not have PCI, IDE or SCSI,
+the only available storage option is emulated SD card.
+
+Implemented devices:
+
+- PL041 audio
+- PL181 SD controller
+- PL050 keyboard and mouse
+- PL011 UARTs
+- SP804 timers
+- I2C controller
+- PL031 RTC
+- PL111 LCD display controller
+- Flash memory
+- LAN9118 ethernet
+
+Unimplemented devices:
+
+- SP810 system control block
+- PCI-express
+- USB controller (Philips ISP1761)
+- Local DAP ROM
+- CoreSight interfaces
+- PL301 AXI interconnect
+- SCC
+- System counter
+- HDLCD controller (``vexpress-a15``)
+- SP805 watchdog
+- PL341 dynamic memory controller
+- DMA330 DMA controller
+- PL354 static memory controller
+- BP147 TrustZone Protection Controller
+- TrustZone Address Space Controller
+
+Other differences between the hardware and the QEMU model:
+
+- QEMU will default to creating one CPU unless you pass a different
+  ``-smp`` argument
+- QEMU allows the amount of RAM provided to be specified with the
+  ``-m`` argument
+- QEMU defaults to providing a CPU which does not provide either
+  TrustZone or the Virtualization Extensions: if you want these you
+  must enable them with ``-machine secure=on`` and ``-machine
+  virtualization=on``
+- QEMU provides 4 virtio-mmio virtio transports; these start at
+  address ``0x10013000`` for ``vexpress-a9`` and at ``0x1c130000`` for
+  ``vexpress-a15``, and have IRQs from 40 upwards. If a dtb is
+  provided on the command line then QEMU will edit it to include
+  suitable entries describing these transports for the guest.

docs/system/target-arm.rst

@@ -78,6 +78,7 @@ undocumented; you can get a complete list by running
    arm/integratorcp
    arm/realview
    arm/versatile
+   arm/vexpress
    arm/musicpal
    arm/nseries
    arm/orangepi