From 53275a61bc7a107dd75406931f8078c7d2c8b9db Mon Sep 17 00:00:00 2001 From: Laszlo Ersek Date: Wed, 7 Jan 2015 18:42:54 +0100 Subject: [PATCH] devicetree: document ARM bindings for QEMU's Firmware Config interface Peter Maydell suggested that we describe new devices / DTB nodes in the kernel Documentation tree that we expose to arm "virt" guests in QEMU. Although the kernel is not required to access the fw_cfg interface, "Documentation/devicetree/bindings/arm" is probably the best central spot to keep the fw_cfg description in. Suggested-by: Peter Maydell Signed-off-by: Laszlo Ersek Acked-by: Arnd Bergmann Acked-by: Mark Rutland Signed-off-by: Rob Herring --- .../devicetree/bindings/arm/fw-cfg.txt | 72 +++++++++++++++++++ 1 file changed, 72 insertions(+) create mode 100644 Documentation/devicetree/bindings/arm/fw-cfg.txt diff --git a/Documentation/devicetree/bindings/arm/fw-cfg.txt b/Documentation/devicetree/bindings/arm/fw-cfg.txt new file mode 100644 index 000000000000..953fb640d9c4 --- /dev/null +++ b/Documentation/devicetree/bindings/arm/fw-cfg.txt @@ -0,0 +1,72 @@ +* QEMU Firmware Configuration bindings for ARM + +QEMU's arm-softmmu and aarch64-softmmu emulation / virtualization targets +provide the following Firmware Configuration interface on the "virt" machine +type: + +- A write-only, 16-bit wide selector (or control) register, +- a read-write, 64-bit wide data register. + +QEMU exposes the control and data register to ARM guests as memory mapped +registers; their location is communicated to the guest's UEFI firmware in the +DTB that QEMU places at the bottom of the guest's DRAM. + +The guest writes a selector value (a key) to the selector register, and then +can read the corresponding data (produced by QEMU) via the data register. If +the selected entry is writable, the guest can rewrite it through the data +register. + +The selector register takes keys in big endian byte order. + +The data register allows accesses with 8, 16, 32 and 64-bit width (only at +offset 0 of the register). Accesses larger than a byte are interpreted as +arrays, bundled together only for better performance. The bytes constituting +such a word, in increasing address order, correspond to the bytes that would +have been transferred by byte-wide accesses in chronological order. + +The interface allows guest firmware to download various parameters and blobs +that affect how the firmware works and what tables it installs for the guest +OS. For example, boot order of devices, ACPI tables, SMBIOS tables, kernel and +initrd images for direct kernel booting, virtual machine UUID, SMP information, +virtual NUMA topology, and so on. + +The authoritative registry of the valid selector values and their meanings is +the QEMU source code; the structure of the data blobs corresponding to the +individual key values is also defined in the QEMU source code. + +The presence of the registers can be verified by selecting the "signature" blob +with key 0x0000, and reading four bytes from the data register. The returned +signature is "QEMU". + +The outermost protocol (involving the write / read sequences of the control and +data registers) is expected to be versioned, and/or described by feature bits. +The interface revision / feature bitmap can be retrieved with key 0x0001. The +blob to be read from the data register has size 4, and it is to be interpreted +as a uint32_t value in little endian byte order. The current value +(corresponding to the above outer protocol) is zero. + +The guest kernel is not expected to use these registers (although it is +certainly allowed to); the device tree bindings are documented here because +this is where device tree bindings reside in general. + +Required properties: + +- compatible: "qemu,fw-cfg-mmio". + +- reg: the MMIO region used by the device. + * Bytes 0x0 to 0x7 cover the data register. + * Bytes 0x8 to 0x9 cover the selector register. + * Further registers may be appended to the region in case of future interface + revisions / feature bits. + +Example: + +/ { + #size-cells = <0x2>; + #address-cells = <0x2>; + + fw-cfg@9020000 { + compatible = "qemu,fw-cfg-mmio"; + reg = <0x0 0x9020000 0x0 0xa>; + }; +}; -- 2.20.1