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上一篇分析LVDS驱动程序移植过程的文章（文章链接为：移植Linux系统到iMX6开发板之LVDS显示屏驱动程序的框架分析与移植）中最后于有一点需要分析LVDS参数的匹配过程的，由于篇幅太长，所以另写一篇文章来记录。
核心函数fb_find_mode()，在分析之前先了解下几个参数。
重要参数说明：
一. ldb.c中的 ldb_modedb
在i.mx6中，关于lvds液晶屏的这个结构体参数（系统lvds接口支持的lcd时序参数都在此了）所属文件为：driver/video/mxc/ldb.c
static struct fb_videomode ldb_modedb[] = {
{
"LDB-WXGA", 60, 1280, 800, 14065,
40, 40,
10, 3,
80, 10,
0,
FB_VMODE_NONINTERLACED,
FB_MODE_IS_DETAILED,},
{
"LDB-XGA", 60, 1024, 768, 15385,
220, 40,
21, 7,
60, 10,
0,
FB_VMODE_NONINTERLACED,
FB_MODE_IS_DETAILED,},
{"LDB-WSVGA", 60, 1024, 600, 19528,
140, 160,
20, 12,
20, 3,
0,
FB_VMODE_NONINTERLACED,
FB_MODE_IS_DETAILED,},
{"LDB-WSVGA480", 60, 1024, 480, 23000,
140, 160,
20, 12,
20, 3,
0,
FB_VMODE_NONINTERLACED,
FB_MODE_IS_DETAILED,},
{
"LDB-1080P60", 60, 1920, 1080, 7692,
100, 40,
30, 3,
10, 2,
0,
FB_VMODE_NONINTERLACED,
FB_MODE_IS_DETAILED,},
{
"LDB-QXGA", 30, 2048, 1536, 9746,
5, 150,
9, 3,
5, 1,
0,
FB_VMODE_NONINTERLACED,
FB_MODE_IS_DETAILED,},
};
这些结构体参数的意义：
/* include/linux/fb.h */
struct fb_videomode {
2. const char *name; “LDB-WSVGA” /* 名字 */
3. u32 refresh; 60 /* 刷新频率 */
4. u32 xres; 1024 //行像素
5. u32 yres; 768 //列像素
6. u32 pixclock; 19528(14065) //时钟频率，单位ps，14430
7. u32 left_margin; 140 // HBPD(horizontal back porch)：80
8. u32 right_margin; 160 // HFPD(horizontal front porth)：48
9. u32 upper_margin; 20 // VBPD(vertical back porch),15
10. u32 lower_margin; 12 // VFBD(vertical front porch),2
11. u32 hsync_len; 20 // HSPW(horizontal sync pulse width)：32
12. u32 vsync_len; 3 // VSPW(vertical sync pulse width)：47
13. u32 sync; 0
14. u32 vmode;
15. u32 flag;
16.};
我们项目中用的屏幕参数如下：
整屏刷新频率 60M
屏幕分辨率 1024*768
时钟频率 14065
left_margin 40（单位像素）
right_margin 40（单位像素）
upper_margin 10（单位像素）
lower_margin 3（单位像素）
行扫描脉宽 hsync_len 80（单位像素时间）
场扫描脉宽vsync_len 10（单位像素时间）
二. arch\arm\mach-mx6\Board-mx6q_sabresd.c 中的 ipuv3_fb_platform_data结构。
我们的是：
static struct ipuv3_fb_platform_data sabresd_fb_data[] = {
{ /*fb0*/
.disp_dev = "ldb",
.interface_pix_fmt = IPU_PIX_FMT_RGB666,
.mode_str = "LDB-XGA",
.default_bpp = 16,
.int_clk = false,
.late_init = false,
}, {
.disp_dev = "ldb",
.interface_pix_fmt = IPU_PIX_FMT_RGB666,
.mode_str = "LDB-XGA",
.default_bpp = 16,
.int_clk = false,
}, {
.disp_dev = "lcd",
.interface_pix_fmt = IPU_PIX_FMT_RGB565,
.mode_str = "CLAA-WVGA",
.default_bpp = 16,
.int_clk = false,
.late_init = false,
}, {
.disp_dev = "ldb",
.interface_pix_fmt = IPU_PIX_FMT_RGB666,
.mode_str = "LDB-VGA",
.default_bpp = 16,
.int_clk = false,
.late_init = false,
},
};
fb_find_mode()函数就是匹配上面结构的参数mode_str 的值，然后再去ldb_modedb结构体看看有没有LVDS需要的时序参数。
此mode_str其实就是后面会提到的mode_options, 格式如下：
<xres>x<yres>[M][R][-<bpp>][@<refresh>][i][m] or
<name>[-<bpp>][@<refresh>]
所以有两种类型：
- 字符规则形， 如 “LDB-WXVGA”
- 数字规则形，如"1920*1080"
具体各个参数意义可参照fb_find_mode()函数注释。
三 .环境变量的设置（uboot环境变量cmdline的设置：）
我们拿到的源码，是由厂家直接提供的源码，可以直接通过uboot环境变量向内核代码覆盖一些参数：
如下：
单通道模式：
setenv bootargs_mmc 'setenv bootargs ${bootargs} ip=off root=/dev/mmcblk0p1 rootwait rw
video=mxcfb0:dev=ldb,LDB-WSVGA,if=RGB24,bpp=32
video=mxcfb1:off video=mxcfb2:off ldb=sin0 fbmem=28M fb0base=0x27b00000 ’
将上面参数通过uboot启动，输入进去，保存后重新启动就可以。
它会覆盖sabresd_fb_data[]的值，覆盖的规则根据mxcfb后面的值，比如 mxcfb0 覆盖sabresd_fb_data[0]
里的值，以此类推。了解了参数的意义后，下面就好理解了：
ldb.c中的ldb_disp_init函数有如下调用：
static int ldb_disp_init(struct mxc_dispdrv_handle *disp,
struct mxc_dispdrv_setting *setting) {
......
ret = fb_find_mode(&setting->fbi->var, setting->fbi, setting->dft_mode_str,
ldb_modedb, ldb_modedb_sz, NULL, setting->default_bpp);
......
}
我们fb_find_mode的参数为：
setting->dft_mode_str为： “LDB-WSVGA”
setting->default_bpp为： 32
fb_find_mode执行源代码为：
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)
{
int i;
/* Set up defaults */
/*如果db参数没有给，则使用modedb*/
if (!db) {
db = modedb;
dbsize = ARRAY_SIZE(modedb);
}
/*如果没有设置则使用db[0]的值，我们本身就是使用db[0]
(只不过我们是通过设置环境变量的值覆盖了它)的值*/
if (!default_mode)
default_mode = &db[0];
/*没有设置bpp则默认使用8bpp，本例是32*/
if (!default_bpp)
default_bpp = 8;
/* Did the user specify a video mode? */
if (!mode_option)
mode_option = fb_mode_option;
/*本例是“LDB-WSVGA”*/
if (mode_option) {
const char *name = mode_option;
unsigned int namelen = strlen(name);
int res_specified = 0, bpp_specified = 0, refresh_specified = 0;
unsigned int xres = 0, yres = 0, bpp = default_bpp, refresh = 0;
int yres_specified = 0, cvt = 0, rb = 0, interlace = 0, margins = 0;
u32 best, diff, tdiff;
/*数字格式规则形才会跑下面的循环*/
for (i = namelen-1; i >= 0; i--) {
switch (name[i]) {
/*@后面的是刷新频率*/
case '@':
namelen = i;
if (!refresh_specified && !bpp_specified &&
!yres_specified) {
refresh = simple_strtol(&name[i+1], NULL, 10);
refresh_specified = 1;
if (cvt || rb)
cvt = 0;
} else
goto done;
break;
/*后面是bpp*/
case '-':
namelen = i;
if (!bpp_specified && !yres_specified) {
bpp = simple_strtol(&name[i+1], NULL, 10);
bpp_specified = 1;
if (cvt || rb)
cvt = 0;
} else
goto done;
break;
/*获取yres*/
case 'x':
if (!yres_specified) {
yres = simple_strtol(&name[i+1], NULL, 10);
yres_specified = 1;
} else
goto done;
break;
case '0' ... '9':
break;
case 'M':
if (!yres_specified)
cvt = 1;
break;
case 'R':
if (!cvt)
rb = 1;
break;
case 'm':
if (!cvt)
margins = 1;
break;
case 'i':
if (!cvt)
interlace = 1;
break;
default:
goto done;
}
}
/*如果yres有值，那么也获取xres.*/
if (i < 0 && yres_specified) {
xres = simple_strtol(name, NULL, 10);
res_specified = 1;
}
done:
/*不会跑这里*/
if (cvt) {
struct fb_videomode cvt_mode;
int ret;
DPRINTK("CVT mode %dx%d@%dHz%s%s%s\n", xres, yres,
(refresh) ? refresh : 60, (rb) ? " reduced blanking" :
"", (margins) ? " with margins" : "", (interlace) ?
" interlaced" : "");
memset(&cvt_mode, 0, sizeof(cvt_mode));
cvt_mode.xres = xres;
cvt_mode.yres = yres;
cvt_mode.refresh = (refresh) ? refresh : 60;
if (interlace)
cvt_mode.vmode |= FB_VMODE_INTERLACED;
else
cvt_mode.vmode &= ~FB_VMODE_INTERLACED;
ret = fb_find_mode_cvt(&cvt_mode, margins, rb);
if (!ret && !fb_try_mode(var, info, &cvt_mode, bpp)) {
DPRINTK("modedb CVT: CVT mode ok\n");
return 1;
}
DPRINTK("CVT mode invalid, getting mode from database\n");
}
DPRINTK("Trying specified video mode%s %ix%i\n",
refresh_specified ? "" : " (ignoring refresh rate)", xres, yres);
/*如果刷新率没指定*/
if (!refresh_specified) {
/*
* If the caller has provided a custom mode database and a
* valid monspecs structure, we look for the mode with the
* highest refresh rate. Otherwise we play it safe it and
* try to find a mode with a refresh rate closest to the
* standard 60 Hz.
*/
if (db != modedb &&
info->monspecs.vfmin && info->monspecs.vfmax &&
info->monspecs.hfmin && info->monspecs.hfmax &&
info->monspecs.dclkmax) {
refresh = 1000;
} else {
/*默认使用60HZ*/
refresh = 60;
}
}
diff = -1;
best = -1;
/*根据名字或者分辨率来匹配。*/
for (i = 0; i < dbsize; i++) {
if ((name_matches(db[i], name, namelen) ||
(res_specified && res_matches(db[i], xres, yres))) &&
!fb_try_mode(var, info, &db[i], bpp)) {
/*刷新率也匹配的时候就认准你了！*/
if (refresh_specified && db[i].refresh == refresh) {
return 1;
} else {
/*刷新率不一样就找差得最少的*/
if (abs(db[i].refresh - refresh) < diff) {
diff = abs(db[i].refresh - refresh);
best = i;
}
}
}
}
/*得到刷新率差得最少的db，然后返回*/
if (best != -1) {
fb_try_mode(var, info, &db[best], bpp);
return (refresh_specified) ? 2 : 1;
}
/*跑到这里说明名字和分辨率都不匹配。*/
diff = 2 * (xres + yres);
best = -1;
DPRINTK("Trying best-fit modes\n");
/*找到分辨率最小的那组数据。*/
for (i = 0; i < dbsize; i++) {
DPRINTK("Trying %ix%i\n", db[i].xres, db[i].yres);
if (!fb_try_mode(var, info, &db[i], bpp)) {
tdiff = abs(db[i].xres - xres) +
abs(db[i].yres - yres);
/*
* Penalize modes with resolutions smaller
* than requested.
*/
if (xres > db[i].xres || yres > db[i].yres)
tdiff += xres + yres;
/*差值大的会被保留，说白了，最终就是找到分辨率最小的那组参数。*/
if (diff > tdiff) {
diff = tdiff;
best = i;
}
}
}
/*获取best对应的var参数。*/
if (best != -1) {
fb_try_mode(var, info, &db[best], bpp);
return 5;
}
}
/*运行到这里有两种情况，
1. 字母规则型(如LDB-WXVGA)，那就是名字不匹配，并且参数检查失败,。
2. 数字规则型(如1920x1080), 那就是名字不匹配 && 分辨率比ldb_modedb中的小上两倍以上(比如1920x1080 和 320x240)。 */
DPRINTK("Trying default video mode\n");
if (!fb_try_mode(var, info, default_mode, default_bpp))
return 3;
/*默认的还失败那只能随便找一个了。*/
DPRINTK("Trying all modes\n");
for (i = 0; i < dbsize; i++)
if (!fb_try_mode(var, info, &db[i], default_bpp))
return 4;
DPRINTK("No valid mode found\n");
return 0;
}
本例中fb_try_mode返回的都是0，看代码，这里的作用基本上看成是得到当前对应的db值然后放再var中供后面的framebuffer driver使用。
\drivers\video\modedb.c
static int fb_try_mode(struct fb_var_screeninfo *var, struct fb_info *info,
const struct fb_videomode *mode, unsigned int bpp)
{
int err = 0;
DPRINTK("Trying mode %s %dx%d-%d@%d\n", mode->name ? mode->name : "noname",
mode->xres, mode->yres, bpp, mode->refresh);
var->xres = mode->xres;
var->yres = mode->yres;
var->xres_virtual = mode->xres;
var->yres_virtual = mode->yres;
var->xoffset = 0;
var->yoffset = 0;
var->bits_per_pixel = bpp;
var->activate |= FB_ACTIVATE_TEST;
var->pixclock = mode->pixclock;
var->left_margin = mode->left_margin;
var->right_margin = mode->right_margin;
var->upper_margin = mode->upper_margin;
var->lower_margin = mode->lower_margin;
var->hsync_len = mode->hsync_len;
var->vsync_len = mode->vsync_len;
var->sync = mode->sync;
var->vmode = mode->vmode;
if (info->fbops->fb_check_var)
err = info->fbops->fb_check_var(var, info);
var->activate &= ~FB_ACTIVATE_TEST;
return err;
}
drivers\video\mxc\mxc_ipuv3_fb.c中的 mxcfb_check_var函数
static int mxcfb_check_var(struct fb_var_screeninfo *var, struct fb_info *info)
{
u32 vtotal;
u32 htotal;
struct mxcfb_info *mxc_fbi = (struct mxcfb_info *)info->par;
if (var->xres == 0 || var->yres == 0)
return 0;
/* fg should not bigger than bg */
if (mxc_fbi->ipu_ch == MEM_FG_SYNC) {
struct fb_info *fbi_tmp;
int bg_xres = 0, bg_yres = 0;
int16_t pos_x, pos_y;
bg_xres = var->xres;
bg_yres = var->yres;
fbi_tmp = found_registered_fb(MEM_BG_SYNC, mxc_fbi->ipu_id);
if (fbi_tmp) {
bg_xres = fbi_tmp->var.xres;
bg_yres = fbi_tmp->var.yres;
}
ipu_disp_get_window_pos(mxc_fbi->ipu, mxc_fbi->ipu_ch, &pos_x, &pos_y);
if ((var->xres + pos_x) > bg_xres)
var->xres = bg_xres - pos_x;
if ((var->yres + pos_y) > bg_yres)
var->yres = bg_yres - pos_y;
}
if (var->rotate > IPU_ROTATE_VERT_FLIP)
var->rotate = IPU_ROTATE_NONE;
if (var->xres_virtual < var->xres)
var->xres_virtual = var->xres;
if (var->yres_virtual < var->yres)
var->yres_virtual = var->yres * 3;
if ((var->bits_per_pixel != 32) && (var->bits_per_pixel != 24) &&
(var->bits_per_pixel != 16) && (var->bits_per_pixel != 12) &&
(var->bits_per_pixel != 8))
var->bits_per_pixel = 16;
if (check_var_pixfmt(var))
/* Fall back to default */
bpp_to_var(var->bits_per_pixel, var);
if (var->pixclock < 1000) {
htotal = var->xres + var->right_margin + var->hsync_len +
var->left_margin;
vtotal = var->yres + var->lower_margin + var->vsync_len +
var->upper_margin;
var->pixclock = (vtotal * htotal * 6UL) / 100UL;
var->pixclock = KHZ2PICOS(var->pixclock);
dev_dbg(info->device,
"pixclock set for 60Hz refresh = %u ps\n",
var->pixclock);
}
var->height = -1;
var->width = -1;
var->grayscale = 0;
return 0;
}
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