Add fw_cfg DMA interface specfication in the fw_cfg documentation.
Signed-off-by: Marc Marí <markmb@xxxxxxxxxx>
---
Documentation/devicetree/bindings/arm/fw-cfg.txt | 52 +++++++++++++++++++++++-
1 file changed, 51 insertions(+), 1 deletion(-)
diff --git a/Documentation/devicetree/bindings/arm/fw-cfg.txt b/Documentation/devicetree/bindings/arm/fw-cfg.txt
index 953fb64..0633aad 100644
--- a/Documentation/devicetree/bindings/arm/fw-cfg.txt
+++ b/Documentation/devicetree/bindings/arm/fw-cfg.txt
@@ -38,6 +38,9 @@ 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".
+If the DMA interface is available, then reading the DMA Address Register
+returns 0x51454d5520434647 ("QEMU CFG" in big-endian format).
+
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
@@ -45,6 +48,51 @@ 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.
+If bit 1 of the feature bitmap is set, the DMA interface is present. This
+can be used through the 64-bit wide address register.
+
+The address register is in big-endian format. The value for the register is 0
+at startup and after an operation. A write to the lower half triggers an
+operation. This means, that operations with 32-bit addresses can be triggered
+with just one write, whereas operations with 64-bit addresses can be triggered
+with one 64-bit write or two 32-bit writes, starting with the higher part.
+
+In this register, the physical address of a FWCfgDmaAccess structure in RAM
+should be written. This is the format of the FWCfgDmaAccess structure:
+
+typedef struct FWCfgDmaAccess {
+ uint32_t control;
+ uint32_t length;
+ uint64_t address;
+} FWCfgDmaAccess;
+
+The fields of the structure are in big endian mode, and the field at the lowest
+address is the "control" field.
+
+The "control" field has the following bits:
+ - Bit 0: Error
+ - Bit 1: Read
+ - Bit 2: Skip
+ - Bit 3: Select. The upper 16 bits are the selected index.
+
+When an operation is triggered, if the "control" field has bit 3 set, the
+upper 16 bits are interpreted as an index of a firmware configuration item.
+This has the same effect as writing the selector register.
+
+If the "control" field has bit 1 set, a read operation will be performed.
+"length" bytes for the current selector and offset will be copied into the
+physical RAM address specified by the "address" field.
+
+If the "control" field has bit 2 set (and not bit 1), a skip operation will be
+performed. The offset for the current selector will be advanced "length" bytes.
+
+To check the result, read the "control" field:
+ error bit set -> something went wrong.
+ all bits cleared -> transfer finished successfully.
+ otherwise -> transfer still in progress (doesn't happen
+ today due to implementation not being async,
+ but may in the future).
+
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.
@@ -56,6 +104,8 @@ Required properties:
- reg: the MMIO region used by the device.
* Bytes 0x0 to 0x7 cover the data register.
* Bytes 0x8 to 0x9 cover the selector register.
+ * With DMA interface enabled: Bytes 0x10 to 0x17 cover the DMA address
+ register.
* Further registers may be appended to the region in case of future interface
revisions / feature bits.
@@ -66,7 +116,7 @@ Example:
#address-cells = <0x2>;
fw-cfg@9020000 {
+ reg = <0x0 0x9020000 0x0 0x18>;
compatible = "qemu,fw-cfg-mmio";
- reg = <0x0 0x9020000 0x0 0xa>;
};
};
--
2.4.3
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