fbdev: Add FOURCC-based format configuration API

			The Frame Buffer Device API

			---------------------------

Last revised: June 21, 2011

0. Introduction

---------------

This document describes the frame buffer API used by applications to interact

with frame buffer devices. In-kernel APIs between device drivers and the frame

buffer core are not described.

Due to a lack of documentation in the original frame buffer API, drivers

behaviours differ in subtle (and not so subtle) ways. This document describes

the recommended API implementation, but applications should be prepared to

deal with different behaviours.

1. Capabilities

---------------

Device and driver capabilities are reported in the fixed screen information

capabilities field.

struct fb_fix_screeninfo {

	...

	__u16 capabilities;		/* see FB_CAP_*			*/

	...

};

Application should use those capabilities to find out what features they can

expect from the device and driver.

- FB_CAP_FOURCC

The driver supports the four character code (FOURCC) based format setting API.

When supported, formats are configured using a FOURCC instead of manually

specifying color components layout.

2. Types and visuals

--------------------

Pixels are stored in memory in hardware-dependent formats. Applications need

to be aware of the pixel storage format in order to write image data to the

frame buffer memory in the format expected by the hardware.

Formats are described by frame buffer types and visuals. Some visuals require

additional information, which are stored in the variable screen information

bits_per_pixel, grayscale, red, green, blue and transp fields.

Visuals describe how color information is encoded and assembled to create

macropixels. Types describe how macropixels are stored in memory. The following

types and visuals are supported.

- FB_TYPE_PACKED_PIXELS

Macropixels are stored contiguously in a single plane. If the number of bits

per macropixel is not a multiple of 8, whether macropixels are padded to the

next multiple of 8 bits or packed together into bytes depends on the visual.

Padding at end of lines may be present and is then reported through the fixed

screen information line_length field.

- FB_TYPE_PLANES

Macropixels are split across multiple planes. The number of planes is equal to

the number of bits per macropixel, with plane i'th storing i'th bit from all

macropixels.

Planes are located contiguously in memory.

- FB_TYPE_INTERLEAVED_PLANES

Macropixels are split across multiple planes. The number of planes is equal to

the number of bits per macropixel, with plane i'th storing i'th bit from all

macropixels.

Planes are interleaved in memory. The interleave factor, defined as the

distance in bytes between the beginning of two consecutive interleaved blocks

belonging to different planes, is stored in the fixed screen information

type_aux field.

- FB_TYPE_FOURCC

Macropixels are stored in memory as described by the format FOURCC identifier

stored in the variable screen information grayscale field.

- FB_VISUAL_MONO01

Pixels are black or white and stored on a number of bits (typically one)

specified by the variable screen information bpp field.

Black pixels are represented by all bits set to 1 and white pixels by all bits

set to 0. When the number of bits per pixel is smaller than 8, several pixels

are packed together in a byte.

FB_VISUAL_MONO01 is currently used with FB_TYPE_PACKED_PIXELS only.

- FB_VISUAL_MONO10

Pixels are black or white and stored on a number of bits (typically one)

specified by the variable screen information bpp field.

Black pixels are represented by all bits set to 0 and white pixels by all bits

set to 1. When the number of bits per pixel is smaller than 8, several pixels

are packed together in a byte.

FB_VISUAL_MONO01 is currently used with FB_TYPE_PACKED_PIXELS only.

- FB_VISUAL_TRUECOLOR

Pixels are broken into red, green and blue components, and each component

indexes a read-only lookup table for the corresponding value. Lookup tables

are device-dependent, and provide linear or non-linear ramps.

Each component is stored in a macropixel according to the variable screen

information red, green, blue and transp fields.

- FB_VISUAL_PSEUDOCOLOR and FB_VISUAL_STATIC_PSEUDOCOLOR

Pixel values are encoded as indices into a colormap that stores red, green and

blue components. The colormap is read-only for FB_VISUAL_STATIC_PSEUDOCOLOR

and read-write for FB_VISUAL_PSEUDOCOLOR.

Each pixel value is stored in the number of bits reported by the variable

screen information bits_per_pixel field.

- FB_VISUAL_DIRECTCOLOR

Pixels are broken into red, green and blue components, and each component

indexes a programmable lookup table for the corresponding value.

Each component is stored in a macropixel according to the variable screen

information red, green, blue and transp fields.

- FB_VISUAL_FOURCC

Pixels are encoded and  interpreted as described by the format FOURCC

identifier stored in the variable screen information grayscale field.

3. Screen information

---------------------

Screen information are queried by applications using the FBIOGET_FSCREENINFO

and FBIOGET_VSCREENINFO ioctls. Those ioctls take a pointer to a

fb_fix_screeninfo and fb_var_screeninfo structure respectively.

struct fb_fix_screeninfo stores device independent unchangeable information

about the frame buffer device and the current format. Those information can't

be directly modified by applications, but can be changed by the driver when an

application modifies the format.

struct fb_fix_screeninfo {

	char id[16];			/* identification string eg "TT Builtin" */

	unsigned long smem_start;	/* Start of frame buffer mem */

					/* (physical address) */

	__u32 smem_len;			/* Length of frame buffer mem */

	__u32 type;			/* see FB_TYPE_*		*/

	__u32 type_aux;			/* Interleave for interleaved Planes */

	__u32 visual;			/* see FB_VISUAL_*		*/

	__u16 xpanstep;			/* zero if no hardware panning  */

	__u16 ypanstep;			/* zero if no hardware panning  */

	__u16 ywrapstep;		/* zero if no hardware ywrap    */

	__u32 line_length;		/* length of a line in bytes    */

	unsigned long mmio_start;	/* Start of Memory Mapped I/O   */

					/* (physical address) */

	__u32 mmio_len;			/* Length of Memory Mapped I/O  */

	__u32 accel;			/* Indicate to driver which	*/

					/*  specific chip/card we have	*/

	__u16 capabilities;		/* see FB_CAP_*			*/

	__u16 reserved[2];		/* Reserved for future compatibility */

};

struct fb_var_screeninfo stores device independent changeable information

about a frame buffer device, its current format and video mode, as well as

other miscellaneous parameters.

struct fb_var_screeninfo {

	__u32 xres;			/* visible resolution		*/

	__u32 yres;

	__u32 xres_virtual;		/* virtual resolution		*/

	__u32 yres_virtual;

	__u32 xoffset;			/* offset from virtual to visible */

	__u32 yoffset;			/* resolution			*/

	__u32 bits_per_pixel;		/* guess what			*/

	__u32 grayscale;		/* 0 = color, 1 = grayscale,	*/

					/* >1 = FOURCC			*/

	struct fb_bitfield red;		/* bitfield in fb mem if true color, */

	struct fb_bitfield green;	/* else only length is significant */

	struct fb_bitfield blue;

	struct fb_bitfield transp;	/* transparency			*/

	__u32 nonstd;			/* != 0 Non standard pixel format */

	__u32 activate;			/* see FB_ACTIVATE_*		*/

	__u32 height;			/* height of picture in mm    */

	__u32 width;			/* width of picture in mm     */

	__u32 accel_flags;		/* (OBSOLETE) see fb_info.flags */

	/* Timing: All values in pixclocks, except pixclock (of course) */

	__u32 pixclock;			/* pixel clock in ps (pico seconds) */

	__u32 left_margin;		/* time from sync to picture	*/

	__u32 right_margin;		/* time from picture to sync	*/

	__u32 upper_margin;		/* time from sync to picture	*/

	__u32 lower_margin;

	__u32 hsync_len;		/* length of horizontal sync	*/

	__u32 vsync_len;		/* length of vertical sync	*/

	__u32 sync;			/* see FB_SYNC_*		*/

	__u32 vmode;			/* see FB_VMODE_*		*/

	__u32 rotate;			/* angle we rotate counter clockwise */

	__u32 colorspace;		/* colorspace for FOURCC-based modes */

	__u32 reserved[4];		/* Reserved for future compatibility */

};

To modify variable information, applications call the FBIOPUT_VSCREENINFO

ioctl with a pointer to a fb_var_screeninfo structure. If the call is

successful, the driver will update the fixed screen information accordingly.

Instead of filling the complete fb_var_screeninfo structure manually,

applications should call the FBIOGET_VSCREENINFO ioctl and modify only the

fields they care about.

4. Format configuration

-----------------------

Frame buffer devices offer two ways to configure the frame buffer format: the

legacy API and the FOURCC-based API.

The legacy API has been the only frame buffer format configuration API for a

long time and is thus widely used by application. It is the recommended API

for applications when using RGB and grayscale formats, as well as legacy

non-standard formats.

To select a format, applications set the fb_var_screeninfo bits_per_pixel field

to the desired frame buffer depth. Values up to 8 will usually map to

monochrome, grayscale or pseudocolor visuals, although this is not required.

- For grayscale formats, applications set the grayscale field to one. The red,

  blue, green and transp fields must be set to 0 by applications and ignored by

  drivers. Drivers must fill the red, blue and green offsets to 0 and lengths

  to the bits_per_pixel value.

- For pseudocolor formats, applications set the grayscale field to zero. The

  red, blue, green and transp fields must be set to 0 by applications and

  ignored by drivers. Drivers must fill the red, blue and green offsets to 0

  and lengths to the bits_per_pixel value.

- For truecolor and directcolor formats, applications set the grayscale field

  to zero, and the red, blue, green and transp fields to describe the layout of

  color components in memory.

struct fb_bitfield {

	__u32 offset;			/* beginning of bitfield	*/

	__u32 length;			/* length of bitfield		*/

	__u32 msb_right;		/* != 0 : Most significant bit is */

					/* right */

};

  Pixel values are bits_per_pixel wide and are split in non-overlapping red,

  green, blue and alpha (transparency) components. Location and size of each

  component in the pixel value are described by the fb_bitfield offset and

  length fields. Offset are computed from the right.

  Pixels are always stored in an integer number of bytes. If the number of

  bits per pixel is not a multiple of 8, pixel values are padded to the next

  multiple of 8 bits.

Upon successful format configuration, drivers update the fb_fix_screeninfo

type, visual and line_length fields depending on the selected format.

The FOURCC-based API replaces format descriptions by four character codes

(FOURCC). FOURCCs are abstract identifiers that uniquely define a format

without explicitly describing it. This is the only API that supports YUV

formats. Drivers are also encouraged to implement the FOURCC-based API for RGB

and grayscale formats.

Drivers that support the FOURCC-based API report this capability by setting

the FB_CAP_FOURCC bit in the fb_fix_screeninfo capabilities field.

FOURCC definitions are located in the linux/videodev2.h header. However, and

despite starting with the V4L2_PIX_FMT_prefix, they are not restricted to V4L2

and don't require usage of the V4L2 subsystem. FOURCC documentation is

available in Documentation/DocBook/v4l/pixfmt.xml.

To select a format, applications set the grayscale field to the desired FOURCC.

For YUV formats, they should also select the appropriate colorspace by setting

the colorspace field to one of the colorspaces listed in linux/videodev2.h and

documented in Documentation/DocBook/v4l/colorspaces.xml.

The red, green, blue and transp fields are not used with the FOURCC-based API.

For forward compatibility reasons applications must zero those fields, and

drivers must ignore them. Values other than 0 may get a meaning in future

extensions.

Upon successful format configuration, drivers update the fb_fix_screeninfo

type, visual and line_length fields depending on the selected format. The type

and visual fields are set to FB_TYPE_FOURCC and FB_VISUAL_FOURCC respectively.

	    memcmp(&info->var, var, sizeof(struct fb_var_screeninfo))) {

		u32 activate = var->activate;

		/* When using FOURCC mode, make sure the red, green, blue and

		 * transp fields are set to 0.

		 */

		if ((info->fix.capabilities & FB_CAP_FOURCC) &&

		    var->grayscale > 1) {

			if (var->red.offset     || var->green.offset    ||

			    var->blue.offset    || var->transp.offset   ||

			    var->red.length     || var->green.length    ||

			    var->blue.length    || var->transp.length   ||

			    var->red.msb_right  || var->green.msb_right ||

			    var->blue.msb_right || var->transp.msb_right)

				return -EINVAL;

		}

		if (!info->fbops->fb_check_var) {

			*var = info->var;

			goto done;

#define FB_TYPE_INTERLEAVED_PLANES	2	/* Interleaved planes	*/

#define FB_TYPE_TEXT			3	/* Text/attributes	*/

#define FB_TYPE_VGA_PLANES		4	/* EGA/VGA planes	*/

#define FB_TYPE_FOURCC			5	/* Type identified by a V4L2 FOURCC */

#define FB_AUX_TEXT_MDA		0	/* Monochrome text */

#define FB_AUX_TEXT_CGA		1	/* CGA/EGA/VGA Color text */

#define FB_VISUAL_PSEUDOCOLOR		3	/* Pseudo color (like atari) */

#define FB_VISUAL_DIRECTCOLOR		4	/* Direct color */

#define FB_VISUAL_STATIC_PSEUDOCOLOR	5	/* Pseudo color readonly */

#define FB_VISUAL_FOURCC		6	/* Visual identified by a V4L2 FOURCC */

#define FB_ACCEL_NONE		0	/* no hardware accelerator	*/

#define FB_ACCEL_ATARIBLITT	1	/* Atari Blitter		*/

#define FB_ACCEL_PUV3_UNIGFX	0xa0	/* PKUnity-v3 Unigfx		*/

#define FB_CAP_FOURCC		1	/* Device supports FOURCC-based formats */

struct fb_fix_screeninfo {

	char id[16];			/* identification string eg "TT Builtin" */

	unsigned long smem_start;	/* Start of frame buffer mem */

	__u32 mmio_len;			/* Length of Memory Mapped I/O  */

	__u32 accel;			/* Indicate to driver which	*/

					/*  specific chip/card we have	*/

	__u16 reserved[3];		/* Reserved for future compatibility */

	__u16 capabilities;		/* see FB_CAP_*			*/

	__u16 reserved[2];		/* Reserved for future compatibility */

};

/* Interpretation of offset for color fields: All offsets are from the right,

	__u32 yoffset;			/* resolution			*/

	__u32 bits_per_pixel;		/* guess what			*/

	__u32 grayscale;		/* != 0 Graylevels instead of colors */

	__u32 grayscale;		/* 0 = color, 1 = grayscale,	*/

					/* >1 = FOURCC			*/

	struct fb_bitfield red;		/* bitfield in fb mem if true color, */

	struct fb_bitfield green;	/* else only length is significant */

	struct fb_bitfield blue;

	__u32 sync;			/* see FB_SYNC_*		*/

	__u32 vmode;			/* see FB_VMODE_*		*/

	__u32 rotate;			/* angle we rotate counter clockwise */

	__u32 reserved[5];		/* Reserved for future compatibility */

	__u32 colorspace;		/* colorspace for FOURCC-based modes */

	__u32 reserved[4];		/* Reserved for future compatibility */

};

struct fb_cmap {