ANDROID Porting系列七、Display Drivers

•功能

•实现您自己的驱动程序（驱动程序模板）

•故障排除

本节介绍了如何显示驱动功能，并提供一个功能模板，旨在帮助您建立自己的特定于设备的驱动程序。

Android依靠在Linux / fb.h内核头文件的标准帧缓冲设备（/ dev/fb0或/ dev/graphics/fb0）和驱动程序。欲了解更多有关标准的Linux帧缓冲区信息，请参阅http://kernel.org的帧缓冲设备。

功能

在Android，每个窗口得到Surface对象实施，即SurfaceFlinger 将对象放置于framebuffer上(这就是system-wide screen composer)。每个Surface是双缓冲。后台缓冲区--drawing takes, 前台缓冲区--composition。

当unlockCanvas（）被调用时，后台缓冲区是已经画完，这意味着它将被呈现，且可以再次使用。 Android翻转前后缓冲区，缓冲区确保最少量的复制，而且总是有一个缓冲被SurfaceFlinger使用, （即确保屏幕没有闪烁或显示痕迹）。

Android提出两项要求的驱动程序：一个可映射内存的线性地址空间，它可以直接写入和支持rgb_565像素格式。一个典型的帧显示包括：

•访问通过调用/ dev/fb0打开驱动

•使用FBIOGET_FSCREENINFO和FBIOGET_VSCREENINFO输入/输出控制（IOCTL）调用以检索有关屏幕信息

•使用FBIOPUT_VSCREENINFO的ioctl，试图建立一个虚拟显示两倍的物理屏幕大小和设置像素格式rgb_565。如果成功，实现双缓冲的视频内存。

当一个页面需要翻盖，Android就会再做FBIOPUT_VSCREENINFO的ioctl调用一个新的y偏移指向其他视频内存缓冲区。这个ioctl反过来调用驱动程序的fb_pan_display()，以进行实际的翻转。如果没有足够的视频内存，内存使用和定期只是到视频内存复制时，到时候做翻转。分配显存和设置像素格式后，Android使用的mmap（）映射到进程的地址空间的内存。所有写入到帧缓冲通过此mmaped内存中完成。

为了保持足够的性能，帧缓冲内存应该缓存。如果您使用write-back，刷新前帧从DMA缓冲区写入到液晶缓存。如果这是不可能的，你可以使用write-through。作为最后的手段，你也可以使用未缓存的write-bugger内存，但性能会受到影响。

实现自己的驱动程序（驱动程序模板）

下面的示例驱动程序提供了一个功能的例子，以帮助您建立自己的显示驱动程序。修改PGUIDE_FB ...宏根据需要以匹配您自己的设备硬件的要求。

/*

 * pguidefb.c

 * 

* Copyright 2007, Google Inc.

 *

 * This program is free software; you can redistribute it and/or modify

 * it under the terms of the GNU General Public License version 2 as

 * published by the Free Software Foundation.

 */

/*

 * ANDROID PORTING GUIDE: FRAME BUFFER DRIVER TEMPLATE

 *

 * This template is designed to provide the minimum frame buffer

 * functionality necessary for Android to display properly on a new

 * device. The PGUIDE_FB macros are meant as pointers indicating

 * where to implement the hardware specific code necessary for the new

 * device. The existence of the macros is not meant to trivialize the

 * work required, just as an indication of where the work needs to be

 * done.

 */

#include <linux/module.h>

#include <linux/kernel.h>

#include <linux/errno.h>

#include <linux/string.h>

#include <linux/slab.h>

#include <linux/delay.h>

#include <linux/mm.h>

#include <linux/fb.h>

#include <linux/init.h>

#include <linux/platform_device.h>

/* Android currently only uses rgb565 in the hardware framebuffer */

#define ANDROID_BYTES_PER_PIXEL 2

/* Android will use double buffer in video if there is enough */

#define ANDROID_NUMBER_OF_BUFFERS 2

/* Modify these macros to suit the hardware */

#define PGUIDE_FB_ROTATE 

 /* Do what is necessary to cause the rotation */

#define PGUIDE_FB_PAN 

 /* Do what is necessary to cause the panning */

#define PGUIDE_FB_PROBE_FIRST 

 /* Do any early hardware initialization */

#define PGUIDE_FB_PROBE_SECOND

 /* Do any later hardware initialization */

#define PGUIDE_FB_WIDTH 320

 /* Return the width of the screen */

#define PGUIDE_FB_HEIGHT 240

 /* Return the heighth of the screen */

#define PGUIDE_FB_SCREEN_BASE 0

 /* Return the virtual address of the start of fb memory */

#define PGUIDE_FB_SMEM_START PGUIDE_FB_SCREEN_BASE

 /* Return the physical address of the start of fb memory */

#define PGUIDE_FB_REMOVE 

 /* Do any hardware shutdown */

struct pguide_fb {

 int rotation;

 struct fb_info fb;

 u32 cmap[16];

};

static inline u32 convert_bitfield(int val, struct fb_bitfield *bf)

{

 unsigned int mask = (1 << bf->length) - 1;

 return (val >> (16 - bf->length) & mask) << bf->offset;

}

/* set the software color map. Probably doesn't need modifying. */

static int

pguide_fb_setcolreg(unsigned int regno, unsigned int red, unsigned int green,

  unsigned int blue, unsigned int transp, struct fb_info *info)

{

 struct pguide_fb *fb = container_of(info, struct pguide_fb, fb);

 if (regno < 16) {

 fb->cmap[regno] = convert_bitfield(transp, &fb->fb.var.transp) |

  convert_bitfield(blue, &fb->fb.var.blue) |

  convert_bitfield(green, &fb->fb.var.green) |

  convert_bitfield(red, &fb->fb.var.red);

 return 0;

 }

 else {

 return 1;

 }

}

/* check var to see if supported by this device. Probably doesn't

 * need modifying.

 */

static int pguide_fb_check_var(struct fb_var_screeninfo *var, struct fb_info *info)

{

 if((var->rotate & 1) != (info->var.rotate & 1)) {

 if((var->xres != info->var.yres) ||

  (var->yres != info->var.xres) ||

  (var->xres_virtual != info->var.yres) ||

  (var->yres_virtual > 

 info->var.xres * ANDROID_NUMBER_OF_BUFFERS) ||

  (var->yres_virtual < info->var.xres )) {

 return -EINVAL;

 }

 }

 else {

 if((var->xres != info->var.xres) ||

  (var->yres != info->var.yres) ||

  (var->xres_virtual != info->var.xres) ||

  (var->yres_virtual > 

 info->var.yres * ANDROID_NUMBER_OF_BUFFERS) ||

  (var->yres_virtual < info->var.yres )) {

 return -EINVAL;

 }

 }

 if((var->xoffset != info->var.xoffset) ||

  (var->bits_per_pixel != info->var.bits_per_pixel) ||

  (var->grayscale != info->var.grayscale)) {

 return -EINVAL;

 }

 return 0;

}

/* Handles screen rotation if device supports it. */

static int pguide_fb_set_par(struct fb_info *info)

{

 struct pguide_fb *fb = container_of(info, struct pguide_fb, fb);

 if(fb->rotation != fb->fb.var.rotate) {

 info->fix.line_length = 

 info->var.xres * ANDROID_BYTES_PER_PIXEL;

 fb->rotation = fb->fb.var.rotate;

 PGUIDE_FB_ROTATE;

 }

 return 0;

}

/* Pan the display if device supports it. */

static int pguide_fb_pan_display(struct fb_var_screeninfo *var, struct fb_info *info)

{

 struct pguide_fb *fb __attribute__ ((unused)) 

 = container_of(info, struct pguide_fb, fb);

 /* Set the frame buffer base to something like:

  fb->fb.fix.smem_start + fb->fb.var.xres * 

 ANDROID_BYTES_PER_PIXEL * var->yoffset

 */

 PGUIDE_FB_PAN;

 return 0;

}

static struct fb_ops pguide_fb_ops = {

 .owner = THIS_MODULE,

 .fb_check_var = pguide_fb_check_var,

 .fb_set_par = pguide_fb_set_par,

 .fb_setcolreg = pguide_fb_setcolreg,

 .fb_pan_display = pguide_fb_pan_display,

 /* These are generic software based fb functions */

 .fb_fillrect = cfb_fillrect,

 .fb_copyarea = cfb_copyarea,

 .fb_imageblit = cfb_imageblit,

};

static int pguide_fb_probe(struct platform_device *pdev)

{

 int ret;

 struct pguide_fb *fb;

 size_t framesize;

 uint32_t width, height;

 fb = kzalloc(sizeof(*fb), GFP_KERNEL);

 if(fb == NULL) {

 ret = -ENOMEM;

 goto err_fb_alloc_failed;

 }

 platform_set_drvdata(pdev, fb);

 PGUIDE_FB_PROBE_FIRST;

 width = PGUIDE_FB_WIDTH;

 height = PGUIDE_FB_HEIGHT;

 fb->fb.fbops = &pguide_fb_ops;

 /* These modes are the ones currently required by Android */

 fb->fb.flags = FBINFO_FLAG_DEFAULT;

 fb->fb.pseudo_palette = fb->cmap;

 fb->fb.fix.type = FB_TYPE_PACKED_PIXELS;

 fb->fb.fix.visual = FB_VISUAL_TRUECOLOR;

 fb->fb.fix.line_length = width * ANDROID_BYTES_PER_PIXEL;

 fb->fb.fix.accel = FB_ACCEL_NONE;

 fb->fb.fix.ypanstep = 1;

 fb->fb.var.xres = width;

 fb->fb.var.yres = height;

 fb->fb.var.xres_virtual = width;

 fb->fb.var.yres_virtual = height * ANDROID_NUMBER_OF_BUFFERS;

 fb->fb.var.bits_per_pixel = 16;

 fb->fb.var.activate = FB_ACTIVATE_NOW;

 fb->fb.var.height = height;

 fb->fb.var.width = width;

 fb->fb.var.red.offset = 11;

 fb->fb.var.red.length = 5;

 fb->fb.var.green.offset = 5;

 fb->fb.var.green.length = 6;

 fb->fb.var.blue.offset = 0;

 fb->fb.var.blue.length = 5;

 framesize = width * height * 

 ANDROID_BYTES_PER_PIXEL * ANDROID_NUMBER_OF_BUFFERS;

 fb->fb.screen_base = PGUIDE_FB_SCREEN_BASE;

 fb->fb.fix.smem_start = PGUIDE_FB_SMEM_START;

 fb->fb.fix.smem_len = framesize;

 ret = fb_set_var(&fb->fb, &fb->fb.var);

 if(ret)

 goto err_fb_set_var_failed;

 PGUIDE_FB_PROBE_SECOND;

 ret = register_framebuffer(&fb->fb);

 if(ret)

 goto err_register_framebuffer_failed;

 return 0;

err_register_framebuffer_failed:

err_fb_set_var_failed:

 kfree(fb);

err_fb_alloc_failed:

 return ret;

}

static int pguide_fb_remove(struct platform_device *pdev)

{

 struct pguide_fb *fb = platform_get_drvdata(pdev);

 PGUIDE_FB_REMOVE;

 kfree(fb);

 return 0;

}

static struct platform_driver pguide_fb_driver = {

 .probe = pguide_fb_probe,

 .remove = pguide_fb_remove,

 .driver = {

 .name = "pguide_fb"

 }

};

static int __init pguide_fb_init(void)

{

 return platform_driver_register(&pguide_fb_driver);

}

static void __exit pguide_fb_exit(void)

{

 platform_driver_unregister(&pguide_fb_driver);

}

module_init(pguide_fb_init);

module_exit(pguide_fb_exit);

MODULE_LICENSE("GPL");

疑难解答

下面的问题都具有类似的原因：

•数字键：在拨号的应用程序，当按下数字键拨打一个电话号码，该号码不显示在屏幕上，直到下一个数字后，已被按下。

•箭头键：当按下箭头键，所需的图标没有得到强调。例如，如果您通过在应用程序菜单图标浏览，您可能会注意到，当您使用箭头键来浏览选项之间时，图标预期不按您预期突出显示。

这两个问题是由于不正确的执行情况的帧缓冲区的页面翻动。关键事件被捕获，但图形界面呈现每一帧滞后。

Android上的双缓冲依靠顺利呈现页面翻转（详情请参阅功能）。