[PATCH] QEMU fw_cfg DMA interface documentation

["Marc Marí" <markmb@redhat.com>]

Add fw_cfg DMA interface specfication in the fw_cfg documentation.

Signed-off-by: Marc Marí <markmb@redhat.com>
---
 Documentation/devicetree/bindings/arm/fw-cfg.txt | 36 ++++++++++++++++++++++++
 1 file changed, 36 insertions(+)

diff --git a/Documentation/devicetree/bindings/arm/fw-cfg.txt b/Documentation/devicetree/bindings/arm/fw-cfg.txt
index 953fb64..c880eec 100644
--- a/Documentation/devicetree/bindings/arm/fw-cfg.txt
+++ b/Documentation/devicetree/bindings/arm/fw-cfg.txt
@@ -49,6 +49,41 @@ 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.
 
+Starting from revision 2, a DMA interface has also been added. This can be used
+through a write-only, 64-bit wide address register.
+
+In this register, a pointer to a FWCfgDmaAccess structure can be written, in
+big endian mode. This is the format of the FWCfgDmaAccess structure:
+
+typedef struct FWCfgDmaAccess {
+    uint64_t address;
+    uint32_t length;
+    uint32_t control;
+} FWCfgDmaAccess;
+
+Once the address to this structure has been written, an DMA operation is
+started. If the "control" field has value 2, a read operation will be performed.
+"length" bytes for the current selector and offset will be mapped into the
+address specified by the "address" field.
+
+If the field "address" has value 0, the read is considered a skip, and
+the data is not copied anywhere, but the offset is still incremented.
+
+To check result, read the control register:
+   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).
+
+Target address goes up and transfer length goes down as the transfer
+happens, so after a successful transfer the length register is zero
+and the address register points right after the memory block written.
+
+If a partial transfer happened before an error occured the address and
+length registers indicate how much data has been transfered
+successfully.
+
 Required properties:
 
 - compatible: "qemu,fw-cfg-mmio".
@@ -56,6 +91,7 @@ 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.
+  * From revision 2: Bytes 0xa to 0x11 cover the DMA address register.
   * Further registers may be appended to the region in case of future interface
     revisions / feature bits.
 
-- 
2.4.3