drm/i915/bios: add proper documentation for the Video BIOS Table (VBT)

Add an overview and documentation for the VBT/BDB header structures.
Acked-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Signed-off-by: Jani Nikula <jani.nikula@intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/3d826d4600688ca3518713776ab5bd8a8fc9f20f.1450702954.git.jani.nikula@intel.com

Documentation/DocBook/gpu.tmpl

@@ -4108,6 +4108,12 @@ int num_ioctls;</synopsis>
 !Pdrivers/gpu/drm/i915/intel_csr.c csr support for dmc
 !Idrivers/gpu/drm/i915/intel_csr.c
       </sect2>
+      <sect2>
+	<title>Video BIOS Table (VBT)</title>
+!Pdrivers/gpu/drm/i915/intel_bios.c Video BIOS Table (VBT)
+!Idrivers/gpu/drm/i915/intel_bios.c
+!Idrivers/gpu/drm/i915/intel_bios.h
+      </sect2>
     </sect1>
 
     <sect1>

drivers/gpu/drm/i915/intel_bios.c

@@ -31,6 +31,28 @@
 #include "i915_drv.h"
 #include "intel_bios.h"
 
+/**
+ * DOC: Video BIOS Table (VBT)
+ *
+ * The Video BIOS Table, or VBT, provides platform and board specific
+ * configuration information to the driver that is not discoverable or available
+ * through other means. The configuration is mostly related to display
+ * hardware. The VBT is available via the ACPI OpRegion or, on older systems, in
+ * the PCI ROM.
+ *
+ * The VBT consists of a VBT Header (defined as &struct vbt_header), a BDB
+ * Header (&struct bdb_header), and a number of BIOS Data Blocks (BDB) that
+ * contain the actual configuration information. The VBT Header, and thus the
+ * VBT, begins with "$VBT" signature. The VBT Header contains the offset of the
+ * BDB Header. The data blocks are concatenated after the BDB Header. The data
+ * blocks have a 1-byte Block ID, 2-byte Block Size, and Block Size bytes of
+ * data. (Block 53, the MIPI Sequence Block is an exception.)
+ *
+ * The driver parses the VBT during load. The relevant information is stored in
+ * driver private data for ease of use, and the actual VBT is not read after
+ * that.
+ */
+
 #define	SLAVE_ADDR1	0x70
 #define	SLAVE_ADDR2	0x72
 
@@ -1285,7 +1307,7 @@ static const struct vbt_header *find_vbt(void __iomem *bios, size_t size)
 
 /**
  * intel_bios_init - find VBT and initialize settings from the BIOS
- * @dev: DRM device
+ * @dev_priv: i915 device instance
  *
  * Loads the Video BIOS and checks that the VBT exists.  Sets scratch registers
  * to appropriate values.

drivers/gpu/drm/i915/intel_bios.h

@@ -28,22 +28,40 @@
 #ifndef _I830_BIOS_H_
 #define _I830_BIOS_H_
 
+/**
+ * struct vbt_header - VBT Header structure
+ * @signature:		VBT signature, always starts with "$VBT"
+ * @version:		Version of this structure
+ * @header_size:	Size of this structure
+ * @vbt_size:		Size of VBT (VBT Header, BDB Header and data blocks)
+ * @vbt_checksum:	Checksum
+ * @reserved0:		Reserved
+ * @bdb_offset:		Offset of &struct bdb_header from beginning of VBT
+ * @aim_offset:		Offsets of add-in data blocks from beginning of VBT
+ */
 struct vbt_header {
-	u8 signature[20];		/**< Always starts with 'VBT$' */
-	u16 version;			/**< decimal */
-	u16 header_size;		/**< in bytes */
-	u16 vbt_size;			/**< in bytes */
+	u8 signature[20];
+	u16 version;
+	u16 header_size;
+	u16 vbt_size;
 	u8 vbt_checksum;
 	u8 reserved0;
-	u32 bdb_offset;			/**< from beginning of VBT */
-	u32 aim_offset[4];		/**< from beginning of VBT */
+	u32 bdb_offset;
+	u32 aim_offset[4];
 } __packed;
 
+/**
+ * struct bdb_header - BDB Header structure
+ * @signature:		BDB signature "BIOS_DATA_BLOCK"
+ * @version:		Version of the data block definitions
+ * @header_size:	Size of this structure
+ * @bdb_size:		Size of BDB (BDB Header and data blocks)
+ */
 struct bdb_header {
-	u8 signature[16];		/**< Always 'BIOS_DATA_BLOCK' */
-	u16 version;			/**< decimal */
-	u16 header_size;		/**< in bytes */
-	u16 bdb_size;			/**< in bytes */
+	u8 signature[16];
+	u16 version;
+	u16 header_size;
+	u16 bdb_size;
 } __packed;
 
 /* strictly speaking, this is a "skip" block, but it has interesting info */