[Qemu-devel] [PATCH v2 2/5] fw_cfg DMA interface documentation

["Marc Marí" <markmb@redhat.com>]

Add fw_cfg DMA interface specification in the documentation.

Based on Gerd Hoffman's initial implementation.

Signed-off-by: Marc Marí <markmb@redhat.com>
---
 docs/specs/fw_cfg.txt | 68 ++++++++++++++++++++++++++++++++++++++++++++++++---
 1 file changed, 64 insertions(+), 4 deletions(-)

diff --git a/docs/specs/fw_cfg.txt b/docs/specs/fw_cfg.txt
index 5bc7b96..06302f6 100644
--- a/docs/specs/fw_cfg.txt
+++ b/docs/specs/fw_cfg.txt
@@ -76,6 +76,13 @@ increasing address order, similar to memcpy().
 
 Selector Register IOport: 0x510
 Data Register IOport:     0x511
+DMA Address IOport:       0x514
+
+=== ARM Register Locations ===
+
+Selector Register address: 0x09020000
+Data Register address:     0x09020008
+DMA Address address:       0x0902000c
 
 == Firmware Configuration Items ==
 
@@ -86,11 +93,12 @@ by selecting the "signature" item using key 0x0000 (FW_CFG_SIGNATURE),
 and reading four bytes from the data register. If the fw_cfg device is
 present, the four bytes read will contain the characters "QEMU".
 
-=== Revision (Key 0x0001, FW_CFG_ID) ===
+=== Revision / feature bitmap (Key 0x0001, FW_CFG_ID) ===
 
-A 32-bit little-endian unsigned int, this item is used as an interface
-revision number, and is currently set to 1 by QEMU when fw_cfg is
-initialized.
+A 32-bit little-endian unsigned int, this item is used to check for enabled
+features.
+ - Bit 0: traditional interface. Always set.
+ - Bit 1: DMA interface.
 
 === File Directory (Key 0x0019, FW_CFG_FILE_DIR) ===
 
@@ -132,6 +140,58 @@ Selector Reg.    Range Usage
 In practice, the number of allowed firmware configuration items is given
 by the value of FW_CFG_MAX_ENTRY (see fw_cfg.h).
 
+= Guest-side DMA Interface =
+
+If bit 1 of the feature bitmap is set, the DMA interface is present. This does
+not replace the existing fw_cfg interface, it is an add-on. This interface
+can be used through the 64-bit wide address register.
+
+The address register, as the selector register, is in little-endian format
+when using IOports, and in big-endian format when using MMIO. 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, a physical RAM address to a FWCfgDmaAccess structure 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
+
+When an operation is triggered, 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 address specified by the "address" field.
+
+If the control field has only bit 2 set, a skip operation will be perfomed.
+The offset for the current selector will be advanced "length" bytes.
+
+To check 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).
+
+Target address goes up and transfer length goes down as the transfer happens,
+so after a successful transfer the length field is zero and the address field
+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.
+
 = Host-side API =
 
 The following functions are available to the QEMU programmer for adding
-- 
2.4.3