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AN2641

Linux frame buffer

Doc ID 14027 Rev 1

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6.4

More details on the frame buffer in Linux

6.4.1

Data structures

The 'fb_info' structure defines the current state of the video card. 'fb_info' is only visible

from the kernel. Inside fb_info, there is a structure called 'fb_ops' which is a collection

of functions needed to make the driver work.

'struct fb_info' is the central data structure used by frame buffer drivers. This structure

is defined in include/linux/fb.h as follows:

Figure 16.

Structure fb_info

The memory for fb_info is allocated by framebuffer_alloc(), a library routine provided by

the frame buffer core. This function also takes the size of a private area as an argument

and appends it to the end of the allocated fb_info. This private area can be referenced

using the par pointer in the fb_info structure.

The user application program can use the ioctl() system call to operate low level LCD

hardware. Methods defined in structure 'fb_ops' are used to support these operations.

The 'fb_ops' structure contains the addresses of all entry points provided by the low-

level frame buffer driver. The first few methods in fb_ops are necessary for the

functioning of the driver, while those remaining are optional ones that provide for

graphics acceleration. The responsibility of each function is briefly explained in the

comments:

struct fb_info {

/* ... */

struct fb_var_screeninfo var; /* Variable screen information*/

struct fb_fix_screeninfo fix; /* Fixed screen information*/

/* ... */

struct fb_cmap cmap; /* Color map*/

/* ... */

struct fb_ops *fbops; /* Driver operations*/

/* ... */

char __iomem *screen_base; /* Frame buffer's virtual address */

unsigned long screen_size; /* Frame buffer's size */

/* ... */

/* From here on everything is device dependent */

void *par; /* Private area */

};

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