Frame Buffer Library¶

The frame buffer drivers depend heavily on four data structures. These structures are declared in include/linux/fb.h. They are fb_info, fb_var_screeninfo, fb_fix_screeninfo and fb_monospecs. The last three can be made available to and from userland.

fb_info defines the current state of a particular video card. Inside fb_info, there exists a fb_ops structure which is a collection of needed functions to make fbdev and fbcon work. fb_info is only visible to the kernel.

fb_var_screeninfo is used to describe the features of a video card that are user defined. With fb_var_screeninfo, things such as depth and the resolution may be defined.

The next structure is fb_fix_screeninfo. This defines the properties of a card that are created when a mode is set and can’t be changed otherwise. A good example of this is the start of the frame buffer memory. This “locks” the address of the frame buffer memory, so that it cannot be changed or moved.

The last structure is fb_monospecs. In the old API, there was little importance for fb_monospecs. This allowed for forbidden things such as setting a mode of 800x600 on a fix frequency monitor. With the new API, fb_monospecs prevents such things, and if used correctly, can prevent a monitor from being cooked. fb_monospecs will not be useful until kernels 2.5.x.

Frame Buffer Memory¶

int register_framebuffer(struct fb_info *fb_info)¶: registers a frame buffer device

Parameters

struct fb_info *fb_info

frame buffer info structure

Registers a frame buffer device fb_info.

Returns negative errno on error, or zero for success.

void unregister_framebuffer(struct fb_info *fb_info)¶: releases a frame buffer device

Parameters

struct fb_info *fb_info

frame buffer info structure

Unregisters a frame buffer device fb_info.

Returns negative errno on error, or zero for success.

This function will also notify the framebuffer console to release the driver.

This is meant to be called within a driver’s module_exit() function. If this is called outside module_exit(), ensure that the driver implements fb_open() and fb_release() to check that no processes are using the device.

int devm_register_framebuffer(struct device *dev, struct fb_info *fb_info)¶: resource-managed frame buffer device registration

Parameters

struct device *dev

device the framebuffer belongs to

struct fb_info *fb_info

frame buffer info structure

Registers a frame buffer device fb_info to device dev.

Returns negative errno on error, or zero for success.

void fb_set_suspend(struct fb_info *info, int state)¶: low level driver signals suspend

Parameters

struct fb_info *info

framebuffer affected

int state

0 = resuming, !=0 = suspending

This is meant to be used by low level drivers to signal suspend/resume to the core & clients. It must be called with the console semaphore held

Frame Buffer Colormap¶

void fb_dealloc_cmap(struct fb_cmap *cmap)¶: deallocate a colormap

Parameters

struct fb_cmap *cmap

frame buffer colormap structure

Deallocates a colormap that was previously allocated with fb_alloc_cmap().

int fb_copy_cmap(const struct fb_cmap *from, struct fb_cmap *to)¶: copy a colormap

Parameters

const struct fb_cmap *from

frame buffer colormap structure

struct fb_cmap *to

frame buffer colormap structure

Copy contents of colormap from from to to.

int fb_set_cmap(struct fb_cmap *cmap, struct fb_info *info)¶: set the colormap

Parameters

struct fb_cmap *cmap

frame buffer colormap structure

struct fb_info *info

frame buffer info structure

Sets the colormap cmap for a screen of device info.

Returns negative errno on error, or zero on success.

const struct fb_cmap *fb_default_cmap(int len)¶: get default colormap

Parameters

int len

size of palette for a depth

Gets the default colormap for a specific screen depth. len is the size of the palette for a particular screen depth.

Returns pointer to a frame buffer colormap structure.

void fb_invert_cmaps(void)¶: invert all defaults colormaps

Parameters

void: no arguments

Description

Invert all default colormaps.

Frame Buffer Video Mode Database¶

int fb_try_mode(struct fb_var_screeninfo *var, struct fb_info *info, const struct fb_videomode *mode, unsigned int bpp)¶: test a video mode

Parameters

struct fb_var_screeninfo *var

frame buffer user defined part of display

struct fb_info *info

frame buffer info structure

const struct fb_videomode *mode

frame buffer video mode structure

unsigned int bpp

color depth in bits per pixel

Tries a video mode to test it’s validity for device info.

Returns 1 on success.

void fb_delete_videomode(const struct fb_videomode *mode, struct list_head *head)¶: removed videomode entry from modelist

Parameters

const struct fb_videomode *mode: videomode to remove
struct list_head *head: struct list_head of modelist

NOTES

Will remove all matching mode entries

int fb_find_mode(struct fb_var_screeninfo *var, struct fb_info *info, const char *mode_option, const struct fb_videomode *db, unsigned int dbsize, const struct fb_videomode *default_mode, unsigned int default_bpp)¶: finds a valid video mode

Parameters

struct fb_var_screeninfo *var: frame buffer user defined part of display
struct fb_info *info: frame buffer info structure
const char *mode_option: string video mode to find
const struct fb_videomode *db: video mode database
unsigned int dbsize: size of db
const struct fb_videomode *default_mode: default video mode to fall back to
unsigned int default_bpp: default color depth in bits per pixel

Description

Finds a suitable video mode, starting with the specified mode in mode_option with fallback to default_mode. If default_mode fails, all modes in the video mode database will be tried.

Valid mode specifiers for mode_option:

<xres>x<yres>[M][R][-<bpp>][@<refresh>][i][p][m]

or

<name>[-<bpp>][@<refresh>]

with <xres>, <yres>, <bpp> and <refresh> decimal numbers and <name> a string.

If ‘M’ is present after yres (and before refresh/bpp if present), the function will compute the timings using VESA(tm) Coordinated Video Timings (CVT). If ‘R’ is present after ‘M’, will compute with reduced blanking (for flatpanels). If ‘i’ or ‘p’ are present, compute interlaced or progressive mode. If ‘m’ is present, add margins equal to 1.8% of xres rounded down to 8 pixels, and 1.8% of yres. The char ‘i’, ‘p’ and ‘m’ must be after ‘M’ and ‘R’. Example:

1024x768MR-8@60m - Reduced blank with margins at 60Hz.

Returns zero for failure, 1 if using specified mode_option, 2 if using specified mode_option with an ignored refresh rate, 3 if default mode is used, 4 if fall back to any valid mode.

NOTE

The passed struct var is _not_ cleared! This allows you to supply values for e.g. the grayscale and accel_flags fields.

void fb_var_to_videomode(struct fb_videomode *mode, const struct fb_var_screeninfo *var)¶: convert fb_var_screeninfo to fb_videomode

Parameters

struct fb_videomode *mode: pointer to struct fb_videomode
const struct fb_var_screeninfo *var: pointer to struct fb_var_screeninfo

void fb_videomode_to_var(struct fb_var_screeninfo *var, const struct fb_videomode *mode)¶: convert fb_videomode to fb_var_screeninfo

Parameters

struct fb_var_screeninfo *var: pointer to struct fb_var_screeninfo
const struct fb_videomode *mode: pointer to struct fb_videomode

int fb_mode_is_equal(const struct fb_videomode *mode1, const struct fb_videomode *mode2)¶: compare 2 videomodes

Parameters

const struct fb_videomode *mode1: first videomode
const struct fb_videomode *mode2: second videomode

Return

1 if equal, 0 if not

const struct fb_videomode *fb_find_best_mode(const struct fb_var_screeninfo *var, struct list_head *head)¶: find best matching videomode

Parameters

const struct fb_var_screeninfo *var: pointer to struct fb_var_screeninfo
struct list_head *head: pointer to struct list_head of modelist

Return

struct fb_videomode, NULL if none found

Description

IMPORTANT: This function assumes that all modelist entries in info->modelist are valid.

NOTES

Finds best matching videomode which has an equal or greater dimension than var->xres and var->yres. If more than 1 videomode is found, will return the videomode with the highest refresh rate

const struct fb_videomode *fb_find_nearest_mode(const struct fb_videomode *mode, struct list_head *head)¶: find closest videomode

Parameters

const struct fb_videomode *mode: pointer to struct fb_videomode
struct list_head *head: pointer to modelist

Description

Finds best matching videomode, smaller or greater in dimension. If more than 1 videomode is found, will return the videomode with the closest refresh rate.

const struct fb_videomode *fb_match_mode(const struct fb_var_screeninfo *var, struct list_head *head)¶: find a videomode which exactly matches the timings in var

Parameters

const struct fb_var_screeninfo *var: pointer to struct fb_var_screeninfo
struct list_head *head: pointer to struct list_head of modelist

Return

struct fb_videomode, NULL if none found

int fb_add_videomode(const struct fb_videomode *mode, struct list_head *head)¶: adds videomode entry to modelist

Parameters

const struct fb_videomode *mode: videomode to add
struct list_head *head: struct list_head of modelist

NOTES

Will only add unmatched mode entries

void fb_destroy_modelist(struct list_head *head)¶: destroy modelist

Parameters

struct list_head *head: struct list_head of modelist

void fb_videomode_to_modelist(const struct fb_videomode *modedb, int num, struct list_head *head)¶: convert mode array to mode list

Parameters

const struct fb_videomode *modedb: array of struct fb_videomode
int num: number of entries in array
struct list_head *head: struct list_head of modelist

Frame Buffer Macintosh Video Mode Database¶

int mac_vmode_to_var(int vmode, int cmode, struct fb_var_screeninfo *var)¶: converts vmode/cmode pair to var structure

Parameters

int vmode

MacOS video mode

int cmode

MacOS color mode

struct fb_var_screeninfo *var

frame buffer video mode structure

Converts a MacOS vmode/cmode pair to a frame buffer video mode structure.

Returns negative errno on error, or zero for success.

int mac_map_monitor_sense(int sense)¶: Convert monitor sense to vmode

Parameters

int sense

Macintosh monitor sense number

Converts a Macintosh monitor sense number to a MacOS vmode number.

Returns MacOS vmode video mode number.

int mac_find_mode(struct fb_var_screeninfo *var, struct fb_info *info, const char *mode_option, unsigned int default_bpp)¶: find a video mode

Parameters

struct fb_var_screeninfo *var

frame buffer user defined part of display

struct fb_info *info

frame buffer info structure

const char *mode_option

video mode name (see mac_modedb[])

unsigned int default_bpp

default color depth in bits per pixel

Finds a suitable video mode. Tries to set mode specified by mode_option. If the name of the wanted mode begins with ‘mac’, the Mac video mode database will be used, otherwise it will fall back to the standard video mode database.

Note

Function marked as __init and can only be used during

system boot.

Returns error code from fb_find_mode (see fb_find_mode function).

Frame Buffer Fonts¶

Refer to the file lib/fonts/fonts.c for more information.