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fb21c2f4 LP |
1 | The Frame Buffer Device API |
2 | --------------------------- | |
3 | ||
4 | Last revised: June 21, 2011 | |
5 | ||
6 | ||
7 | 0. Introduction | |
8 | --------------- | |
9 | ||
10 | This document describes the frame buffer API used by applications to interact | |
11 | with frame buffer devices. In-kernel APIs between device drivers and the frame | |
12 | buffer core are not described. | |
13 | ||
14 | Due to a lack of documentation in the original frame buffer API, drivers | |
15 | behaviours differ in subtle (and not so subtle) ways. This document describes | |
16 | the recommended API implementation, but applications should be prepared to | |
17 | deal with different behaviours. | |
18 | ||
19 | ||
20 | 1. Capabilities | |
21 | --------------- | |
22 | ||
23 | Device and driver capabilities are reported in the fixed screen information | |
24 | capabilities field. | |
25 | ||
26 | struct fb_fix_screeninfo { | |
27 | ... | |
28 | __u16 capabilities; /* see FB_CAP_* */ | |
29 | ... | |
30 | }; | |
31 | ||
32 | Application should use those capabilities to find out what features they can | |
33 | expect from the device and driver. | |
34 | ||
35 | - FB_CAP_FOURCC | |
36 | ||
37 | The driver supports the four character code (FOURCC) based format setting API. | |
38 | When supported, formats are configured using a FOURCC instead of manually | |
39 | specifying color components layout. | |
40 | ||
41 | ||
42 | 2. Types and visuals | |
43 | -------------------- | |
44 | ||
45 | Pixels are stored in memory in hardware-dependent formats. Applications need | |
46 | to be aware of the pixel storage format in order to write image data to the | |
47 | frame buffer memory in the format expected by the hardware. | |
48 | ||
49 | Formats are described by frame buffer types and visuals. Some visuals require | |
50 | additional information, which are stored in the variable screen information | |
51 | bits_per_pixel, grayscale, red, green, blue and transp fields. | |
52 | ||
53 | Visuals describe how color information is encoded and assembled to create | |
54 | macropixels. Types describe how macropixels are stored in memory. The following | |
55 | types and visuals are supported. | |
56 | ||
57 | - FB_TYPE_PACKED_PIXELS | |
58 | ||
59 | Macropixels are stored contiguously in a single plane. If the number of bits | |
60 | per macropixel is not a multiple of 8, whether macropixels are padded to the | |
61 | next multiple of 8 bits or packed together into bytes depends on the visual. | |
62 | ||
63 | Padding at end of lines may be present and is then reported through the fixed | |
64 | screen information line_length field. | |
65 | ||
66 | - FB_TYPE_PLANES | |
67 | ||
68 | Macropixels are split across multiple planes. The number of planes is equal to | |
69 | the number of bits per macropixel, with plane i'th storing i'th bit from all | |
70 | macropixels. | |
71 | ||
72 | Planes are located contiguously in memory. | |
73 | ||
74 | - FB_TYPE_INTERLEAVED_PLANES | |
75 | ||
76 | Macropixels are split across multiple planes. The number of planes is equal to | |
77 | the number of bits per macropixel, with plane i'th storing i'th bit from all | |
78 | macropixels. | |
79 | ||
80 | Planes are interleaved in memory. The interleave factor, defined as the | |
81 | distance in bytes between the beginning of two consecutive interleaved blocks | |
82 | belonging to different planes, is stored in the fixed screen information | |
83 | type_aux field. | |
84 | ||
85 | - FB_TYPE_FOURCC | |
86 | ||
87 | Macropixels are stored in memory as described by the format FOURCC identifier | |
88 | stored in the variable screen information grayscale field. | |
89 | ||
90 | - FB_VISUAL_MONO01 | |
91 | ||
92 | Pixels are black or white and stored on a number of bits (typically one) | |
93 | specified by the variable screen information bpp field. | |
94 | ||
95 | Black pixels are represented by all bits set to 1 and white pixels by all bits | |
96 | set to 0. When the number of bits per pixel is smaller than 8, several pixels | |
97 | are packed together in a byte. | |
98 | ||
99 | FB_VISUAL_MONO01 is currently used with FB_TYPE_PACKED_PIXELS only. | |
100 | ||
101 | - FB_VISUAL_MONO10 | |
102 | ||
103 | Pixels are black or white and stored on a number of bits (typically one) | |
104 | specified by the variable screen information bpp field. | |
105 | ||
106 | Black pixels are represented by all bits set to 0 and white pixels by all bits | |
107 | set to 1. When the number of bits per pixel is smaller than 8, several pixels | |
108 | are packed together in a byte. | |
109 | ||
110 | FB_VISUAL_MONO01 is currently used with FB_TYPE_PACKED_PIXELS only. | |
111 | ||
112 | - FB_VISUAL_TRUECOLOR | |
113 | ||
114 | Pixels are broken into red, green and blue components, and each component | |
115 | indexes a read-only lookup table for the corresponding value. Lookup tables | |
116 | are device-dependent, and provide linear or non-linear ramps. | |
117 | ||
118 | Each component is stored in a macropixel according to the variable screen | |
119 | information red, green, blue and transp fields. | |
120 | ||
121 | - FB_VISUAL_PSEUDOCOLOR and FB_VISUAL_STATIC_PSEUDOCOLOR | |
122 | ||
123 | Pixel values are encoded as indices into a colormap that stores red, green and | |
124 | blue components. The colormap is read-only for FB_VISUAL_STATIC_PSEUDOCOLOR | |
125 | and read-write for FB_VISUAL_PSEUDOCOLOR. | |
126 | ||
127 | Each pixel value is stored in the number of bits reported by the variable | |
128 | screen information bits_per_pixel field. | |
129 | ||
130 | - FB_VISUAL_DIRECTCOLOR | |
131 | ||
132 | Pixels are broken into red, green and blue components, and each component | |
133 | indexes a programmable lookup table for the corresponding value. | |
134 | ||
135 | Each component is stored in a macropixel according to the variable screen | |
136 | information red, green, blue and transp fields. | |
137 | ||
138 | - FB_VISUAL_FOURCC | |
139 | ||
140 | Pixels are encoded and interpreted as described by the format FOURCC | |
141 | identifier stored in the variable screen information grayscale field. | |
142 | ||
143 | ||
144 | 3. Screen information | |
145 | --------------------- | |
146 | ||
147 | Screen information are queried by applications using the FBIOGET_FSCREENINFO | |
148 | and FBIOGET_VSCREENINFO ioctls. Those ioctls take a pointer to a | |
149 | fb_fix_screeninfo and fb_var_screeninfo structure respectively. | |
150 | ||
151 | struct fb_fix_screeninfo stores device independent unchangeable information | |
152 | about the frame buffer device and the current format. Those information can't | |
153 | be directly modified by applications, but can be changed by the driver when an | |
154 | application modifies the format. | |
155 | ||
156 | struct fb_fix_screeninfo { | |
157 | char id[16]; /* identification string eg "TT Builtin" */ | |
158 | unsigned long smem_start; /* Start of frame buffer mem */ | |
159 | /* (physical address) */ | |
160 | __u32 smem_len; /* Length of frame buffer mem */ | |
161 | __u32 type; /* see FB_TYPE_* */ | |
162 | __u32 type_aux; /* Interleave for interleaved Planes */ | |
163 | __u32 visual; /* see FB_VISUAL_* */ | |
164 | __u16 xpanstep; /* zero if no hardware panning */ | |
165 | __u16 ypanstep; /* zero if no hardware panning */ | |
166 | __u16 ywrapstep; /* zero if no hardware ywrap */ | |
167 | __u32 line_length; /* length of a line in bytes */ | |
168 | unsigned long mmio_start; /* Start of Memory Mapped I/O */ | |
169 | /* (physical address) */ | |
170 | __u32 mmio_len; /* Length of Memory Mapped I/O */ | |
171 | __u32 accel; /* Indicate to driver which */ | |
172 | /* specific chip/card we have */ | |
173 | __u16 capabilities; /* see FB_CAP_* */ | |
174 | __u16 reserved[2]; /* Reserved for future compatibility */ | |
175 | }; | |
176 | ||
177 | struct fb_var_screeninfo stores device independent changeable information | |
178 | about a frame buffer device, its current format and video mode, as well as | |
179 | other miscellaneous parameters. | |
180 | ||
181 | struct fb_var_screeninfo { | |
182 | __u32 xres; /* visible resolution */ | |
183 | __u32 yres; | |
184 | __u32 xres_virtual; /* virtual resolution */ | |
185 | __u32 yres_virtual; | |
186 | __u32 xoffset; /* offset from virtual to visible */ | |
187 | __u32 yoffset; /* resolution */ | |
188 | ||
189 | __u32 bits_per_pixel; /* guess what */ | |
190 | __u32 grayscale; /* 0 = color, 1 = grayscale, */ | |
191 | /* >1 = FOURCC */ | |
192 | struct fb_bitfield red; /* bitfield in fb mem if true color, */ | |
193 | struct fb_bitfield green; /* else only length is significant */ | |
194 | struct fb_bitfield blue; | |
195 | struct fb_bitfield transp; /* transparency */ | |
196 | ||
197 | __u32 nonstd; /* != 0 Non standard pixel format */ | |
198 | ||
199 | __u32 activate; /* see FB_ACTIVATE_* */ | |
200 | ||
201 | __u32 height; /* height of picture in mm */ | |
202 | __u32 width; /* width of picture in mm */ | |
203 | ||
204 | __u32 accel_flags; /* (OBSOLETE) see fb_info.flags */ | |
205 | ||
206 | /* Timing: All values in pixclocks, except pixclock (of course) */ | |
207 | __u32 pixclock; /* pixel clock in ps (pico seconds) */ | |
208 | __u32 left_margin; /* time from sync to picture */ | |
209 | __u32 right_margin; /* time from picture to sync */ | |
210 | __u32 upper_margin; /* time from sync to picture */ | |
211 | __u32 lower_margin; | |
212 | __u32 hsync_len; /* length of horizontal sync */ | |
213 | __u32 vsync_len; /* length of vertical sync */ | |
214 | __u32 sync; /* see FB_SYNC_* */ | |
215 | __u32 vmode; /* see FB_VMODE_* */ | |
216 | __u32 rotate; /* angle we rotate counter clockwise */ | |
217 | __u32 colorspace; /* colorspace for FOURCC-based modes */ | |
218 | __u32 reserved[4]; /* Reserved for future compatibility */ | |
219 | }; | |
220 | ||
221 | To modify variable information, applications call the FBIOPUT_VSCREENINFO | |
222 | ioctl with a pointer to a fb_var_screeninfo structure. If the call is | |
223 | successful, the driver will update the fixed screen information accordingly. | |
224 | ||
225 | Instead of filling the complete fb_var_screeninfo structure manually, | |
226 | applications should call the FBIOGET_VSCREENINFO ioctl and modify only the | |
227 | fields they care about. | |
228 | ||
229 | ||
230 | 4. Format configuration | |
231 | ----------------------- | |
232 | ||
233 | Frame buffer devices offer two ways to configure the frame buffer format: the | |
234 | legacy API and the FOURCC-based API. | |
235 | ||
236 | ||
237 | The legacy API has been the only frame buffer format configuration API for a | |
238 | long time and is thus widely used by application. It is the recommended API | |
239 | for applications when using RGB and grayscale formats, as well as legacy | |
240 | non-standard formats. | |
241 | ||
242 | To select a format, applications set the fb_var_screeninfo bits_per_pixel field | |
243 | to the desired frame buffer depth. Values up to 8 will usually map to | |
244 | monochrome, grayscale or pseudocolor visuals, although this is not required. | |
245 | ||
246 | - For grayscale formats, applications set the grayscale field to one. The red, | |
247 | blue, green and transp fields must be set to 0 by applications and ignored by | |
248 | drivers. Drivers must fill the red, blue and green offsets to 0 and lengths | |
249 | to the bits_per_pixel value. | |
250 | ||
251 | - For pseudocolor formats, applications set the grayscale field to zero. The | |
252 | red, blue, green and transp fields must be set to 0 by applications and | |
253 | ignored by drivers. Drivers must fill the red, blue and green offsets to 0 | |
254 | and lengths to the bits_per_pixel value. | |
255 | ||
256 | - For truecolor and directcolor formats, applications set the grayscale field | |
257 | to zero, and the red, blue, green and transp fields to describe the layout of | |
258 | color components in memory. | |
259 | ||
260 | struct fb_bitfield { | |
261 | __u32 offset; /* beginning of bitfield */ | |
262 | __u32 length; /* length of bitfield */ | |
263 | __u32 msb_right; /* != 0 : Most significant bit is */ | |
264 | /* right */ | |
265 | }; | |
266 | ||
267 | Pixel values are bits_per_pixel wide and are split in non-overlapping red, | |
268 | green, blue and alpha (transparency) components. Location and size of each | |
269 | component in the pixel value are described by the fb_bitfield offset and | |
270 | length fields. Offset are computed from the right. | |
271 | ||
272 | Pixels are always stored in an integer number of bytes. If the number of | |
273 | bits per pixel is not a multiple of 8, pixel values are padded to the next | |
274 | multiple of 8 bits. | |
275 | ||
276 | Upon successful format configuration, drivers update the fb_fix_screeninfo | |
277 | type, visual and line_length fields depending on the selected format. | |
278 | ||
279 | ||
280 | The FOURCC-based API replaces format descriptions by four character codes | |
281 | (FOURCC). FOURCCs are abstract identifiers that uniquely define a format | |
282 | without explicitly describing it. This is the only API that supports YUV | |
283 | formats. Drivers are also encouraged to implement the FOURCC-based API for RGB | |
284 | and grayscale formats. | |
285 | ||
286 | Drivers that support the FOURCC-based API report this capability by setting | |
287 | the FB_CAP_FOURCC bit in the fb_fix_screeninfo capabilities field. | |
288 | ||
289 | FOURCC definitions are located in the linux/videodev2.h header. However, and | |
290 | despite starting with the V4L2_PIX_FMT_prefix, they are not restricted to V4L2 | |
291 | and don't require usage of the V4L2 subsystem. FOURCC documentation is | |
292 | available in Documentation/DocBook/v4l/pixfmt.xml. | |
293 | ||
294 | To select a format, applications set the grayscale field to the desired FOURCC. | |
295 | For YUV formats, they should also select the appropriate colorspace by setting | |
296 | the colorspace field to one of the colorspaces listed in linux/videodev2.h and | |
297 | documented in Documentation/DocBook/v4l/colorspaces.xml. | |
298 | ||
299 | The red, green, blue and transp fields are not used with the FOURCC-based API. | |
300 | For forward compatibility reasons applications must zero those fields, and | |
301 | drivers must ignore them. Values other than 0 may get a meaning in future | |
302 | extensions. | |
303 | ||
304 | Upon successful format configuration, drivers update the fb_fix_screeninfo | |
305 | type, visual and line_length fields depending on the selected format. The type | |
306 | and visual fields are set to FB_TYPE_FOURCC and FB_VISUAL_FOURCC respectively. |