1 ===========================
2 The Frame Buffer Device API
3 ===========================
4
5 Last revised: June 21, 2011
6
7
8 0. Introduction
9 ---------------
10
11 This document describes the frame buffer API used by applications to interact
12 with frame buffer devices. In-kernel APIs between device drivers and the frame
13 buffer core are not described.
14
15 Due to a lack of documentation in the original frame buffer API, drivers
16 behaviours differ in subtle (and not so subtle) ways. This document describes
17 the recommended API implementation, but applications should be prepared to
18 deal with different behaviours.
19
20
21 1. Capabilities
22 ---------------
23
24 Device and driver capabilities are reported in the fixed screen information
25 capabilities field::
26
27 struct fb_fix_screeninfo {
28 ...
29 __u16 capabilities; /* see FB_CAP_* */
30 ...
31 };
32
33 Application should use those capabilities to find out what features they can
34 expect from the device and driver.
35
36 - FB_CAP_FOURCC
37
38 The driver supports the four character code (FOURCC) based format setting API.
39 When supported, formats are configured using a FOURCC instead of manually
40 specifying color components layout.
41
42
43 2. Types and visuals
44 --------------------
45
46 Pixels are stored in memory in hardware-dependent formats. Applications need
47 to be aware of the pixel storage format in order to write image data to the
48 frame buffer memory in the format expected by the hardware.
49
50 Formats are described by frame buffer types and visuals. Some visuals require
51 additional information, which are stored in the variable screen information
52 bits_per_pixel, grayscale, red, green, blue and transp fields.
53
54 Visuals describe how color information is encoded and assembled to create
55 macropixels. Types describe how macropixels are stored in memory. The following
56 types and visuals are supported.
57
58 - FB_TYPE_PACKED_PIXELS
59
60 Macropixels are stored contiguously in a single plane. If the number of bits
61 per macropixel is not a multiple of 8, whether macropixels are padded to the
62 next multiple of 8 bits or packed together into bytes depends on the visual.
63
64 Padding at end of lines may be present and is then reported through the fixed
65 screen information line_length field.
66
67 - FB_TYPE_PLANES
68
69 Macropixels are split across multiple planes. The number of planes is equal to
70 the number of bits per macropixel, with plane i'th storing i'th bit from all
71 macropixels.
72
73 Planes are located contiguously in memory.
74
75 - FB_TYPE_INTERLEAVED_PLANES
76
77 Macropixels are split across multiple planes. The number of planes is equal to
78 the number of bits per macropixel, with plane i'th storing i'th bit from all
79 macropixels.
80
81 Planes are interleaved in memory. The interleave factor, defined as the
82 distance in bytes between the beginning of two consecutive interleaved blocks
83 belonging to different planes, is stored in the fixed screen information
84 type_aux field.
85
86 - FB_TYPE_FOURCC
87
88 Macropixels are stored in memory as described by the format FOURCC identifier
89 stored in the variable screen information grayscale field.
90
91 - FB_VISUAL_MONO01
92
93 Pixels are black or white and stored on a number of bits (typically one)
94 specified by the variable screen information bpp field.
95
96 Black pixels are represented by all bits set to 1 and white pixels by all bits
97 set to 0. When the number of bits per pixel is smaller than 8, several pixels
98 are packed together in a byte.
99
100 FB_VISUAL_MONO01 is currently used with FB_TYPE_PACKED_PIXELS only.
101
102 - FB_VISUAL_MONO10
103
104 Pixels are black or white and stored on a number of bits (typically one)
105 specified by the variable screen information bpp field.
106
107 Black pixels are represented by all bits set to 0 and white pixels by all bits
108 set to 1. When the number of bits per pixel is smaller than 8, several pixels
109 are packed together in a byte.
110
111 FB_VISUAL_MONO01 is currently used with FB_TYPE_PACKED_PIXELS only.
112
113 - FB_VISUAL_TRUECOLOR
114
115 Pixels are broken into red, green and blue components, and each component
116 indexes a read-only lookup table for the corresponding value. Lookup tables
117 are device-dependent, and provide linear or non-linear ramps.
118
119 Each component is stored in a macropixel according to the variable screen
120 information red, green, blue and transp fields.
121
122 - FB_VISUAL_PSEUDOCOLOR and FB_VISUAL_STATIC_PSEUDOCOLOR
123
124 Pixel values are encoded as indices into a colormap that stores red, green and
125 blue components. The colormap is read-only for FB_VISUAL_STATIC_PSEUDOCOLOR
126 and read-write for FB_VISUAL_PSEUDOCOLOR.
127
128 Each pixel value is stored in the number of bits reported by the variable
129 screen information bits_per_pixel field.
130
131 - FB_VISUAL_DIRECTCOLOR
132
133 Pixels are broken into red, green and blue components, and each component
134 indexes a programmable lookup table for the corresponding value.
135
136 Each component is stored in a macropixel according to the variable screen
137 information red, green, blue and transp fields.
138
139 - FB_VISUAL_FOURCC
140
141 Pixels are encoded and interpreted as described by the format FOURCC
142 identifier stored in the variable screen information grayscale field.
143
144
145 3. Screen information
146 ---------------------
147
148 Screen information are queried by applications using the FBIOGET_FSCREENINFO
149 and FBIOGET_VSCREENINFO ioctls. Those ioctls take a pointer to a
150 fb_fix_screeninfo and fb_var_screeninfo structure respectively.
151
152 struct fb_fix_screeninfo stores device independent unchangeable information
153 about the frame buffer device and the current format. Those information can't
154 be directly modified by applications, but can be changed by the driver when an
155 application 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
178 struct fb_var_screeninfo stores device independent changeable information
179 about a frame buffer device, its current format and video mode, as well as
180 other 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
222 To modify variable information, applications call the FBIOPUT_VSCREENINFO
223 ioctl with a pointer to a fb_var_screeninfo structure. If the call is
224 successful, the driver will update the fixed screen information accordingly.
225
226 Instead of filling the complete fb_var_screeninfo structure manually,
227 applications should call the FBIOGET_VSCREENINFO ioctl and modify only the
228 fields they care about.
229
230
231 4. Format configuration
232 -----------------------
233
234 Frame buffer devices offer two ways to configure the frame buffer format: the
235 legacy API and the FOURCC-based API.
236
237
238 The legacy API has been the only frame buffer format configuration API for a
239 long time and is thus widely used by application. It is the recommended API
240 for applications when using RGB and grayscale formats, as well as legacy
241 non-standard formats.
242
243 To select a format, applications set the fb_var_screeninfo bits_per_pixel field
244 to the desired frame buffer depth. Values up to 8 will usually map to
245 monochrome, 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 by
249 drivers. Drivers must fill the red, blue and green offsets to 0 and lengths
250 to the bits_per_pixel value.
251
252 - For pseudocolor formats, applications set the grayscale field to zero. The
253 red, blue, green and transp fields must be set to 0 by applications and
254 ignored by drivers. Drivers must fill the red, blue and green offsets to 0
255 and lengths to the bits_per_pixel value.
256
257 - For truecolor and directcolor formats, applications set the grayscale field
258 to zero, and the red, blue, green and transp fields to describe the layout of
259 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 each
270 component in the pixel value are described by the fb_bitfield offset and
271 length fields. Offset are computed from the right.
272
273 Pixels are always stored in an integer number of bytes. If the number of
274 bits per pixel is not a multiple of 8, pixel values are padded to the next
275 multiple of 8 bits.
276
277 Upon successful format configuration, drivers update the fb_fix_screeninfo
278 type, visual and line_length fields depending on the selected format.
279
280
281 The FOURCC-based API replaces format descriptions by four character codes
282 (FOURCC). FOURCCs are abstract identifiers that uniquely define a format
283 without explicitly describing it. This is the only API that supports YUV
284 formats. Drivers are also encouraged to implement the FOURCC-based API for RGB
285 and grayscale formats.
286
287 Drivers that support the FOURCC-based API report this capability by setting
288 the FB_CAP_FOURCC bit in the fb_fix_screeninfo capabilities field.
289
290 FOURCC definitions are located in the linux/videodev2.h header. However, and
291 despite starting with the V4L2_PIX_FMT_prefix, they are not restricted to V4L2
292 and don't require usage of the V4L2 subsystem. FOURCC documentation is
293 available in Documentation/userspace-api/media/v4l/pixfmt.rst.
294
295 To select a format, applications set the grayscale field to the desired FOURCC.
296 For YUV formats, they should also select the appropriate colorspace by setting
297 the colorspace field to one of the colorspaces listed in linux/videodev2.h and
298 documented in Documentation/userspace-api/media/v4l/colorspaces.rst.
299
300 The red, green, blue and transp fields are not used with the FOURCC-based API.
301 For forward compatibility reasons applications must zero those fields, and
302 drivers must ignore them. Values other than 0 may get a meaning in future
303 extensions.
304
305 Upon successful format configuration, drivers update the fb_fix_screeninfo
306 type, visual and line_length fields depending on the selected format. The type
307 and visual fields are set to FB_TYPE_FOURCC and FB_VISUAL_FOURCC respectively.
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