[Thorough understanding linux kernel] -Linux frame buffer device driver

series	content
Depth understanding of linux kernel	Linux frame buffer device driver

surroundings:

platform	Kernel version	Android version
RK3399	Linux4.4	Android7.1

Article Directory

1, Linux framebuffer introduction
2, Framebuffer critical data structures

2.1, fb_info
2.2、fb_ops

3, Framebuffer internal function analysis
4, vfb analysis

1, Linux framebuffer introduction

framebuffer The user interface is a bridge support system and display system hardware part
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2, Framebuffer critical data structures

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2.1, fb_info

fb_infoIt is framebufferthe base structure.
File Location:include\linx\fb.h

Important member	content
`.node`	In `framebuffer`architecture, there is one `struct fb_info *registered_fb[32]`; it represents the most support
`32`A `framebuffer`device, `node`represents an array of several members of the first array `ID`from the `0`start.
`.screen_bas`	Screen starting virtual address, by pre-registering `DMA`been assigned way
`.screen_size`	Buf screen size, in the size, and all the display modes supported by the system,`xres``yres``bpp`
`.var:`	Screen variable parameters
`.fix`	Screen fixed parameters
`.monspecs`	Monitor describes the parameters
`.pixmap`	Hardware mapping image
`.sprite`	Mapping the hardware cursor
`.cmap`	Color table
`.modelist`	Display mode list, a `fb`drive that supports multiple devices, each device displays the relevant parameters are different, so it is necessary to maintain a list of
`.mode`	The current display mode

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struct fb_var_screeninfo

Important member	content
`.xres`	Line the number of points, as `xres`expressed
`.yres`	There are a number of points, as `yres`expressed
`.bits_per_pixel`	`bpp`Each pixel represents how many bits
`.red`	Offset and length occupied by red
`.green`	Offset and length occupied by green
`.blue`	Offset and length occupied by blue
`.nonstd`	Whether the pixel is a standard format, `0`standard formats for representing the non- `0`representation is not a standard format
`.pixclock`	Pixel clock, reading (the unit? Bits / byte / word) the time spent
`.lef_margin`	Line switching, the delay between synchronizing to the drawing from
`.right_margin`	Line switching, the delay between drawing from the synchronization
`.upper_margin`	Frame switching, from the synchronization delay between drawing
`.lower_margin`	Frame switching, the delay between the sync from the drawing to
`.hsync_len`	水平同步长度
`.vsync_len`	垂直同步长度

struct fb_fix_screeninfo

重要成员	内容
`id[16]`	字符串形式的标识符
`.smem_start`	`fb` 缓存开始位置，物理地址
`.smem_len`	`fb` 缓存的长度
`.type`	type 可取如下值: `FB_TYPE_PACKED_PIXELS`、`FB_TYPE_PACKED_PIXELS`、`FB_TYPE_INTERLEAVED_PLANES`、`FB_TYPE_TEXT`、`FB_TYPE_VGA_PLANES`
`.type_aux`	硬件显示点的方式
`.visual`	色彩模式，很多种
`.xpanstep`	硬件平移，无硬件平移值为 `0`
`.ypanstep`	硬件平移，无硬件平移值为 `0`
`.ywrapstep`
`.line_length`	一行多少个字节
`.mmio_start`	内存映射`I/O`开始位置，物理地址
`.mmio_len`	内存映射`I/O` 长度
`.accel:`	特定的芯片

2.2、fb_ops

功能：实现 Framebuffer 操作。
定义：include\linux\fb.h

3、Framebuffer内部函数分析

framebuffer_alloc()函数分析：

调用者：platform_driver->probe，例如 vfb_probe()。
定义：drivers\video\fbsysfb.c
原型：include\linux\fb.h：
extern struct fb_info *framebuffer_alloc(size_t size, struct device *dev);
参数：

参数名	内容
`size`	驱动私有数据的大小，可以是 0；
`dev`	`设备结构指针，可以是 NULL`

该函数分配 fb_info 结构体和设备私有数据。如果存在设备私有数据，分配空间时，会保证 fb_info 和用户私有数据的起点差距是 4 字节的整数倍——Fb_info+PADDING+设备私有数据。

设备私有数据的指针是 fb_info->par
info->device = dev;
device 结构体定义于include\linux\device.h
kzalloc 是分配空间并把分配的空间都设为零。
GFP_KERNEL 的作用：分配空间的方式是一个在内核空间运行的进程的方式。
当内核可分配的 memory 不足时，内核会使 GFP_KERNEL 的使用者睡眠