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1===========================2The Frame Buffer Device API3===========================4 5Last revised: June 21, 20116 7 80. Introduction9---------------10 11This document describes the frame buffer API used by applications to interact12with frame buffer devices. In-kernel APIs between device drivers and the frame13buffer core are not described.14 15Due to a lack of documentation in the original frame buffer API, drivers16behaviours differ in subtle (and not so subtle) ways. This document describes17the recommended API implementation, but applications should be prepared to18deal with different behaviours.19 20 211. Capabilities22---------------23 24Device and driver capabilities are reported in the fixed screen information25capabilities field::26 27 struct fb_fix_screeninfo {28 ...29 __u16 capabilities; /* see FB_CAP_* */30 ...31 };32 33Application should use those capabilities to find out what features they can34expect from the device and driver.35 36- FB_CAP_FOURCC37 38The driver supports the four character code (FOURCC) based format setting API.39When supported, formats are configured using a FOURCC instead of manually40specifying color components layout.41 42 432. Types and visuals44--------------------45 46Pixels are stored in memory in hardware-dependent formats. Applications need47to be aware of the pixel storage format in order to write image data to the48frame buffer memory in the format expected by the hardware.49 50Formats are described by frame buffer types and visuals. Some visuals require51additional information, which are stored in the variable screen information52bits_per_pixel, grayscale, red, green, blue and transp fields.53 54Visuals describe how color information is encoded and assembled to create55macropixels. Types describe how macropixels are stored in memory. The following56types and visuals are supported.57 58- FB_TYPE_PACKED_PIXELS59 60Macropixels are stored contiguously in a single plane. If the number of bits61per macropixel is not a multiple of 8, whether macropixels are padded to the62next multiple of 8 bits or packed together into bytes depends on the visual.63 64Padding at end of lines may be present and is then reported through the fixed65screen information line_length field.66 67- FB_TYPE_PLANES68 69Macropixels are split across multiple planes. The number of planes is equal to70the number of bits per macropixel, with plane i'th storing i'th bit from all71macropixels.72 73Planes are located contiguously in memory.74 75- FB_TYPE_INTERLEAVED_PLANES76 77Macropixels are split across multiple planes. The number of planes is equal to78the number of bits per macropixel, with plane i'th storing i'th bit from all79macropixels.80 81Planes are interleaved in memory. The interleave factor, defined as the82distance in bytes between the beginning of two consecutive interleaved blocks83belonging to different planes, is stored in the fixed screen information84type_aux field.85 86- FB_TYPE_FOURCC87 88Macropixels are stored in memory as described by the format FOURCC identifier89stored in the variable screen information grayscale field.90 91- FB_VISUAL_MONO0192 93Pixels are black or white and stored on a number of bits (typically one)94specified by the variable screen information bpp field.95 96Black pixels are represented by all bits set to 1 and white pixels by all bits97set to 0. When the number of bits per pixel is smaller than 8, several pixels98are packed together in a byte.99 100FB_VISUAL_MONO01 is currently used with FB_TYPE_PACKED_PIXELS only.101 102- FB_VISUAL_MONO10103 104Pixels are black or white and stored on a number of bits (typically one)105specified by the variable screen information bpp field.106 107Black pixels are represented by all bits set to 0 and white pixels by all bits108set to 1. When the number of bits per pixel is smaller than 8, several pixels109are packed together in a byte.110 111FB_VISUAL_MONO01 is currently used with FB_TYPE_PACKED_PIXELS only.112 113- FB_VISUAL_TRUECOLOR114 115Pixels are broken into red, green and blue components, and each component116indexes a read-only lookup table for the corresponding value. Lookup tables117are device-dependent, and provide linear or non-linear ramps.118 119Each component is stored in a macropixel according to the variable screen120information red, green, blue and transp fields.121 122- FB_VISUAL_PSEUDOCOLOR and FB_VISUAL_STATIC_PSEUDOCOLOR123 124Pixel values are encoded as indices into a colormap that stores red, green and125blue components. The colormap is read-only for FB_VISUAL_STATIC_PSEUDOCOLOR126and read-write for FB_VISUAL_PSEUDOCOLOR.127 128Each pixel value is stored in the number of bits reported by the variable129screen information bits_per_pixel field.130 131- FB_VISUAL_DIRECTCOLOR132 133Pixels are broken into red, green and blue components, and each component134indexes a programmable lookup table for the corresponding value.135 136Each component is stored in a macropixel according to the variable screen137information red, green, blue and transp fields.138 139- FB_VISUAL_FOURCC140 141Pixels are encoded and interpreted as described by the format FOURCC142identifier stored in the variable screen information grayscale field.143 144 1453. Screen information146---------------------147 148Screen information are queried by applications using the FBIOGET_FSCREENINFO149and FBIOGET_VSCREENINFO ioctls. Those ioctls take a pointer to a150fb_fix_screeninfo and fb_var_screeninfo structure respectively.151 152struct fb_fix_screeninfo stores device independent unchangeable information153about the frame buffer device and the current format. Those information can't154be directly modified by applications, but can be changed by the driver when an155application modifies the format::156 157 struct fb_fix_screeninfo {158 char id[16]; /* identification string eg "TT Builtin" */159 unsigned long smem_start; /* Start of frame buffer mem */160 /* (physical address) */161 __u32 smem_len; /* Length of frame buffer mem */162 __u32 type; /* see FB_TYPE_* */163 __u32 type_aux; /* Interleave for interleaved Planes */164 __u32 visual; /* see FB_VISUAL_* */165 __u16 xpanstep; /* zero if no hardware panning */166 __u16 ypanstep; /* zero if no hardware panning */167 __u16 ywrapstep; /* zero if no hardware ywrap */168 __u32 line_length; /* length of a line in bytes */169 unsigned long mmio_start; /* Start of Memory Mapped I/O */170 /* (physical address) */171 __u32 mmio_len; /* Length of Memory Mapped I/O */172 __u32 accel; /* Indicate to driver which */173 /* specific chip/card we have */174 __u16 capabilities; /* see FB_CAP_* */175 __u16 reserved[2]; /* Reserved for future compatibility */176 };177 178struct fb_var_screeninfo stores device independent changeable information179about a frame buffer device, its current format and video mode, as well as180other miscellaneous parameters::181 182 struct fb_var_screeninfo {183 __u32 xres; /* visible resolution */184 __u32 yres;185 __u32 xres_virtual; /* virtual resolution */186 __u32 yres_virtual;187 __u32 xoffset; /* offset from virtual to visible */188 __u32 yoffset; /* resolution */189 190 __u32 bits_per_pixel; /* guess what */191 __u32 grayscale; /* 0 = color, 1 = grayscale, */192 /* >1 = FOURCC */193 struct fb_bitfield red; /* bitfield in fb mem if true color, */194 struct fb_bitfield green; /* else only length is significant */195 struct fb_bitfield blue;196 struct fb_bitfield transp; /* transparency */197 198 __u32 nonstd; /* != 0 Non standard pixel format */199 200 __u32 activate; /* see FB_ACTIVATE_* */201 202 __u32 height; /* height of picture in mm */203 __u32 width; /* width of picture in mm */204 205 __u32 accel_flags; /* (OBSOLETE) see fb_info.flags */206 207 /* Timing: All values in pixclocks, except pixclock (of course) */208 __u32 pixclock; /* pixel clock in ps (pico seconds) */209 __u32 left_margin; /* time from sync to picture */210 __u32 right_margin; /* time from picture to sync */211 __u32 upper_margin; /* time from sync to picture */212 __u32 lower_margin;213 __u32 hsync_len; /* length of horizontal sync */214 __u32 vsync_len; /* length of vertical sync */215 __u32 sync; /* see FB_SYNC_* */216 __u32 vmode; /* see FB_VMODE_* */217 __u32 rotate; /* angle we rotate counter clockwise */218 __u32 colorspace; /* colorspace for FOURCC-based modes */219 __u32 reserved[4]; /* Reserved for future compatibility */220 };221 222To modify variable information, applications call the FBIOPUT_VSCREENINFO223ioctl with a pointer to a fb_var_screeninfo structure. If the call is224successful, the driver will update the fixed screen information accordingly.225 226Instead of filling the complete fb_var_screeninfo structure manually,227applications should call the FBIOGET_VSCREENINFO ioctl and modify only the228fields they care about.229 230 2314. Format configuration232-----------------------233 234Frame buffer devices offer two ways to configure the frame buffer format: the235legacy API and the FOURCC-based API.236 237 238The legacy API has been the only frame buffer format configuration API for a239long time and is thus widely used by application. It is the recommended API240for applications when using RGB and grayscale formats, as well as legacy241non-standard formats.242 243To select a format, applications set the fb_var_screeninfo bits_per_pixel field244to the desired frame buffer depth. Values up to 8 will usually map to245monochrome, grayscale or pseudocolor visuals, although this is not required.246 247- For grayscale formats, applications set the grayscale field to one. The red,248 blue, green and transp fields must be set to 0 by applications and ignored by249 drivers. Drivers must fill the red, blue and green offsets to 0 and lengths250 to the bits_per_pixel value.251 252- For pseudocolor formats, applications set the grayscale field to zero. The253 red, blue, green and transp fields must be set to 0 by applications and254 ignored by drivers. Drivers must fill the red, blue and green offsets to 0255 and lengths to the bits_per_pixel value.256 257- For truecolor and directcolor formats, applications set the grayscale field258 to zero, and the red, blue, green and transp fields to describe the layout of259 color components in memory::260 261 struct fb_bitfield {262 __u32 offset; /* beginning of bitfield */263 __u32 length; /* length of bitfield */264 __u32 msb_right; /* != 0 : Most significant bit is */265 /* right */266 };267 268 Pixel values are bits_per_pixel wide and are split in non-overlapping red,269 green, blue and alpha (transparency) components. Location and size of each270 component in the pixel value are described by the fb_bitfield offset and271 length fields. Offset are computed from the right.272 273 Pixels are always stored in an integer number of bytes. If the number of274 bits per pixel is not a multiple of 8, pixel values are padded to the next275 multiple of 8 bits.276 277Upon successful format configuration, drivers update the fb_fix_screeninfo278type, visual and line_length fields depending on the selected format.279 280 281The FOURCC-based API replaces format descriptions by four character codes282(FOURCC). FOURCCs are abstract identifiers that uniquely define a format283without explicitly describing it. This is the only API that supports YUV284formats. Drivers are also encouraged to implement the FOURCC-based API for RGB285and grayscale formats.286 287Drivers that support the FOURCC-based API report this capability by setting288the FB_CAP_FOURCC bit in the fb_fix_screeninfo capabilities field.289 290FOURCC definitions are located in the linux/videodev2.h header. However, and291despite starting with the V4L2_PIX_FMT_prefix, they are not restricted to V4L2292and don't require usage of the V4L2 subsystem. FOURCC documentation is293available in Documentation/userspace-api/media/v4l/pixfmt.rst.294 295To select a format, applications set the grayscale field to the desired FOURCC.296For YUV formats, they should also select the appropriate colorspace by setting297the colorspace field to one of the colorspaces listed in linux/videodev2.h and298documented in Documentation/userspace-api/media/v4l/colorspaces.rst.299 300The red, green, blue and transp fields are not used with the FOURCC-based API.301For forward compatibility reasons applications must zero those fields, and302drivers must ignore them. Values other than 0 may get a meaning in future303extensions.304 305Upon successful format configuration, drivers update the fb_fix_screeninfo306type, visual and line_length fields depending on the selected format. The type307and visual fields are set to FB_TYPE_FOURCC and FB_VISUAL_FOURCC respectively.308