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 <peter.maydell@xxxxxxxxxx> Signed-off-by: Laszlo Ersek <lersek@xxxxxxxxxx> --- Documentation/devicetree/bindings/arm/fw-cfg.txt | 47 ++++++++++++++++++++++++ 1 file changed, 47 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 0000000..d131453 --- /dev/null +++ b/Documentation/devicetree/bindings/arm/fw-cfg.txt @@ -0,0 +1,47 @@ +* 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, 8-bit wide data register. + +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 port. If the +selected entry is writable, the guest can rewrite it through the data port. The +authoritative registry of the valid selector values and their meanings is the +QEMU source code. + +QEMU exposes the control and data register to x86 guests at fixed IO ports. ARM +guests can access them as memory mapped registers, and 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 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. + +The addresses and sizes of the Firmware Configuration registers are given by +the /fw-cfg node. The "virt" board invariably uses <2> as #size-cells and +#address-cells in the context of this node. + +The "reg" property is therefore an array of four uint64_t elements (eight +uint32_t cells in total), the first uint64_t pair describing the address and +size of the control register, the second uint64_t pair describing the address +and size of the data register. (See +<http://devicetree.org/Device_Tree_Usage#How_Addressing_Works>.) + +The first string in the "compatible" property is "fw-cfg,mmio". + +Example: + +/ { + #size-cells = <0x2>; + #address-cells = <0x2>; + + fw-cfg@9020000 { + reg = <0x0 0x9020000 0x0 0x2 0x0 0x9020002 0x0 0x1>; + compatible = "fw-cfg,mmio"; + }; +}; -- 1.8.3.1 -- To unsubscribe from this list: send the line "unsubscribe devicetree" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
