307 lines
12 KiB
Plaintext
307 lines
12 KiB
Plaintext
The Frame Buffer Device API
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---------------------------
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Last revised: June 21, 2011
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0. Introduction
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---------------
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This document describes the frame buffer API used by applications to interact
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with frame buffer devices. In-kernel APIs between device drivers and the frame
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buffer core are not described.
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Due to a lack of documentation in the original frame buffer API, drivers
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behaviours differ in subtle (and not so subtle) ways. This document describes
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the recommended API implementation, but applications should be prepared to
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deal with different behaviours.
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1. Capabilities
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---------------
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Device and driver capabilities are reported in the fixed screen information
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capabilities field.
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struct fb_fix_screeninfo {
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...
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__u16 capabilities; /* see FB_CAP_* */
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...
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};
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Application should use those capabilities to find out what features they can
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expect from the device and driver.
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- FB_CAP_FOURCC
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The driver supports the four character code (FOURCC) based format setting API.
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When supported, formats are configured using a FOURCC instead of manually
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specifying color components layout.
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2. Types and visuals
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--------------------
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Pixels are stored in memory in hardware-dependent formats. Applications need
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to be aware of the pixel storage format in order to write image data to the
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frame buffer memory in the format expected by the hardware.
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Formats are described by frame buffer types and visuals. Some visuals require
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additional information, which are stored in the variable screen information
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bits_per_pixel, grayscale, red, green, blue and transp fields.
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Visuals describe how color information is encoded and assembled to create
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macropixels. Types describe how macropixels are stored in memory. The following
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types and visuals are supported.
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- FB_TYPE_PACKED_PIXELS
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Macropixels are stored contiguously in a single plane. If the number of bits
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per macropixel is not a multiple of 8, whether macropixels are padded to the
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next multiple of 8 bits or packed together into bytes depends on the visual.
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Padding at end of lines may be present and is then reported through the fixed
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screen information line_length field.
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- FB_TYPE_PLANES
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Macropixels are split across multiple planes. The number of planes is equal to
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the number of bits per macropixel, with plane i'th storing i'th bit from all
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macropixels.
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Planes are located contiguously in memory.
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- FB_TYPE_INTERLEAVED_PLANES
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Macropixels are split across multiple planes. The number of planes is equal to
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the number of bits per macropixel, with plane i'th storing i'th bit from all
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macropixels.
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Planes are interleaved in memory. The interleave factor, defined as the
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distance in bytes between the beginning of two consecutive interleaved blocks
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belonging to different planes, is stored in the fixed screen information
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type_aux field.
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- FB_TYPE_FOURCC
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Macropixels are stored in memory as described by the format FOURCC identifier
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stored in the variable screen information grayscale field.
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- FB_VISUAL_MONO01
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Pixels are black or white and stored on a number of bits (typically one)
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specified by the variable screen information bpp field.
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Black pixels are represented by all bits set to 1 and white pixels by all bits
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set to 0. When the number of bits per pixel is smaller than 8, several pixels
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are packed together in a byte.
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FB_VISUAL_MONO01 is currently used with FB_TYPE_PACKED_PIXELS only.
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- FB_VISUAL_MONO10
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Pixels are black or white and stored on a number of bits (typically one)
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specified by the variable screen information bpp field.
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Black pixels are represented by all bits set to 0 and white pixels by all bits
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set to 1. When the number of bits per pixel is smaller than 8, several pixels
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are packed together in a byte.
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FB_VISUAL_MONO01 is currently used with FB_TYPE_PACKED_PIXELS only.
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- FB_VISUAL_TRUECOLOR
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Pixels are broken into red, green and blue components, and each component
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indexes a read-only lookup table for the corresponding value. Lookup tables
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are device-dependent, and provide linear or non-linear ramps.
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Each component is stored in a macropixel according to the variable screen
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information red, green, blue and transp fields.
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- FB_VISUAL_PSEUDOCOLOR and FB_VISUAL_STATIC_PSEUDOCOLOR
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Pixel values are encoded as indices into a colormap that stores red, green and
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blue components. The colormap is read-only for FB_VISUAL_STATIC_PSEUDOCOLOR
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and read-write for FB_VISUAL_PSEUDOCOLOR.
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Each pixel value is stored in the number of bits reported by the variable
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screen information bits_per_pixel field.
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- FB_VISUAL_DIRECTCOLOR
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Pixels are broken into red, green and blue components, and each component
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indexes a programmable lookup table for the corresponding value.
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Each component is stored in a macropixel according to the variable screen
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information red, green, blue and transp fields.
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- FB_VISUAL_FOURCC
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Pixels are encoded and interpreted as described by the format FOURCC
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identifier stored in the variable screen information grayscale field.
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3. Screen information
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---------------------
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Screen information are queried by applications using the FBIOGET_FSCREENINFO
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and FBIOGET_VSCREENINFO ioctls. Those ioctls take a pointer to a
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fb_fix_screeninfo and fb_var_screeninfo structure respectively.
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struct fb_fix_screeninfo stores device independent unchangeable information
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about the frame buffer device and the current format. Those information can't
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be directly modified by applications, but can be changed by the driver when an
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application modifies the format.
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struct fb_fix_screeninfo {
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char id[16]; /* identification string eg "TT Builtin" */
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unsigned long smem_start; /* Start of frame buffer mem */
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/* (physical address) */
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__u32 smem_len; /* Length of frame buffer mem */
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__u32 type; /* see FB_TYPE_* */
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__u32 type_aux; /* Interleave for interleaved Planes */
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__u32 visual; /* see FB_VISUAL_* */
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__u16 xpanstep; /* zero if no hardware panning */
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__u16 ypanstep; /* zero if no hardware panning */
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__u16 ywrapstep; /* zero if no hardware ywrap */
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__u32 line_length; /* length of a line in bytes */
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unsigned long mmio_start; /* Start of Memory Mapped I/O */
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/* (physical address) */
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__u32 mmio_len; /* Length of Memory Mapped I/O */
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__u32 accel; /* Indicate to driver which */
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/* specific chip/card we have */
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__u16 capabilities; /* see FB_CAP_* */
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__u16 reserved[2]; /* Reserved for future compatibility */
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};
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struct fb_var_screeninfo stores device independent changeable information
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about a frame buffer device, its current format and video mode, as well as
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other miscellaneous parameters.
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struct fb_var_screeninfo {
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__u32 xres; /* visible resolution */
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__u32 yres;
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__u32 xres_virtual; /* virtual resolution */
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__u32 yres_virtual;
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__u32 xoffset; /* offset from virtual to visible */
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__u32 yoffset; /* resolution */
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__u32 bits_per_pixel; /* guess what */
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__u32 grayscale; /* 0 = color, 1 = grayscale, */
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/* >1 = FOURCC */
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struct fb_bitfield red; /* bitfield in fb mem if true color, */
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struct fb_bitfield green; /* else only length is significant */
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struct fb_bitfield blue;
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struct fb_bitfield transp; /* transparency */
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__u32 nonstd; /* != 0 Non standard pixel format */
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__u32 activate; /* see FB_ACTIVATE_* */
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__u32 height; /* height of picture in mm */
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__u32 width; /* width of picture in mm */
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__u32 accel_flags; /* (OBSOLETE) see fb_info.flags */
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/* Timing: All values in pixclocks, except pixclock (of course) */
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__u32 pixclock; /* pixel clock in ps (pico seconds) */
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__u32 left_margin; /* time from sync to picture */
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__u32 right_margin; /* time from picture to sync */
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__u32 upper_margin; /* time from sync to picture */
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__u32 lower_margin;
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__u32 hsync_len; /* length of horizontal sync */
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__u32 vsync_len; /* length of vertical sync */
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__u32 sync; /* see FB_SYNC_* */
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__u32 vmode; /* see FB_VMODE_* */
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__u32 rotate; /* angle we rotate counter clockwise */
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__u32 colorspace; /* colorspace for FOURCC-based modes */
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__u32 reserved[4]; /* Reserved for future compatibility */
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};
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To modify variable information, applications call the FBIOPUT_VSCREENINFO
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ioctl with a pointer to a fb_var_screeninfo structure. If the call is
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successful, the driver will update the fixed screen information accordingly.
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Instead of filling the complete fb_var_screeninfo structure manually,
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applications should call the FBIOGET_VSCREENINFO ioctl and modify only the
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fields they care about.
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4. Format configuration
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-----------------------
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Frame buffer devices offer two ways to configure the frame buffer format: the
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legacy API and the FOURCC-based API.
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The legacy API has been the only frame buffer format configuration API for a
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long time and is thus widely used by application. It is the recommended API
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for applications when using RGB and grayscale formats, as well as legacy
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non-standard formats.
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To select a format, applications set the fb_var_screeninfo bits_per_pixel field
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to the desired frame buffer depth. Values up to 8 will usually map to
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monochrome, grayscale or pseudocolor visuals, although this is not required.
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- For grayscale formats, applications set the grayscale field to one. The red,
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blue, green and transp fields must be set to 0 by applications and ignored by
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drivers. Drivers must fill the red, blue and green offsets to 0 and lengths
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to the bits_per_pixel value.
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- For pseudocolor formats, applications set the grayscale field to zero. The
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red, blue, green and transp fields must be set to 0 by applications and
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ignored by drivers. Drivers must fill the red, blue and green offsets to 0
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and lengths to the bits_per_pixel value.
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- For truecolor and directcolor formats, applications set the grayscale field
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to zero, and the red, blue, green and transp fields to describe the layout of
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color components in memory.
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struct fb_bitfield {
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__u32 offset; /* beginning of bitfield */
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__u32 length; /* length of bitfield */
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__u32 msb_right; /* != 0 : Most significant bit is */
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/* right */
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};
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Pixel values are bits_per_pixel wide and are split in non-overlapping red,
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green, blue and alpha (transparency) components. Location and size of each
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component in the pixel value are described by the fb_bitfield offset and
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length fields. Offset are computed from the right.
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Pixels are always stored in an integer number of bytes. If the number of
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bits per pixel is not a multiple of 8, pixel values are padded to the next
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multiple of 8 bits.
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Upon successful format configuration, drivers update the fb_fix_screeninfo
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type, visual and line_length fields depending on the selected format.
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The FOURCC-based API replaces format descriptions by four character codes
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(FOURCC). FOURCCs are abstract identifiers that uniquely define a format
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without explicitly describing it. This is the only API that supports YUV
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formats. Drivers are also encouraged to implement the FOURCC-based API for RGB
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and grayscale formats.
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Drivers that support the FOURCC-based API report this capability by setting
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the FB_CAP_FOURCC bit in the fb_fix_screeninfo capabilities field.
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FOURCC definitions are located in the linux/videodev2.h header. However, and
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despite starting with the V4L2_PIX_FMT_prefix, they are not restricted to V4L2
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and don't require usage of the V4L2 subsystem. FOURCC documentation is
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available in Documentation/media/uapi/v4l/pixfmt.rst.
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To select a format, applications set the grayscale field to the desired FOURCC.
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For YUV formats, they should also select the appropriate colorspace by setting
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the colorspace field to one of the colorspaces listed in linux/videodev2.h and
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documented in Documentation/media/uapi/v4l/colorspaces.rst.
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The red, green, blue and transp fields are not used with the FOURCC-based API.
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For forward compatibility reasons applications must zero those fields, and
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drivers must ignore them. Values other than 0 may get a meaning in future
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extensions.
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Upon successful format configuration, drivers update the fb_fix_screeninfo
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type, visual and line_length fields depending on the selected format. The type
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and visual fields are set to FB_TYPE_FOURCC and FB_VISUAL_FOURCC respectively.
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