[PATCH v4 4/4] devicetree: update documentation for fw_cfg ARM bindings

Gabriel L. Somlo somlo at cmu.edu
Fri Nov 13 22:03:55 EST 2015

From: Gabriel Somlo <somlo at cmu.edu>

Remove redundant details from
and replace them with a pointer to the more comprehensive
fw_cfg documentation privided by
leaving the specific ARM DTB node description in place.

Signed-off-by: Gabriel Somlo <somlo at cmu.edu>
Cc: Laszlo Ersek <lersek at redhat.com>
 Documentation/devicetree/bindings/arm/fw-cfg.txt | 37 ++----------------------
 1 file changed, 2 insertions(+), 35 deletions(-)

diff --git a/Documentation/devicetree/bindings/arm/fw-cfg.txt b/Documentation/devicetree/bindings/arm/fw-cfg.txt
index 953fb64..7aeb48a 100644
--- a/Documentation/devicetree/bindings/arm/fw-cfg.txt
+++ b/Documentation/devicetree/bindings/arm/fw-cfg.txt
@@ -11,43 +11,10 @@ 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
-The selector register takes keys in big endian byte order.
+For a comprehensive description of the behavior of fw_cfg, please see
-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.
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