bootm命令執行過程中調用了bootm_start函數，這個函數比較重要，所以先分析它。

1.common/cmd_bootm.c

? static?int?bootm_start(cmd_tbl_t?*cmdtp,?int?flag,?int?argc,?char?*argv[])?? {?? ????void????????*os_hdr;?? ????int?????ret;?? ????memset?((void?*)&images,?0,?sizeof?(images));//images是一個bootm_headers_t類型的全局變量。見下面的分析。??? ????images.verify?=?getenv_yesno?("verify");//從環境變量中檢查是否要對鏡像的數據（不是鏡像頭）進行校驗。??? ????bootm_start_lmb();//不做任何有意義的工作，除了定義#?define?lmb_reserve(lmb,?base,?size)??? ????/*?get?kernel?image?header,?start?address?and?length?*/尋找可用的內核鏡像，見下面的分析。主要根據傳入的參數檢查鏡像的合法性并獲取信息。?? ????os_hdr?=?boot_get_kernel?(cmdtp,?flag,?argc,?argv,?? ????????????&images,?&images.os.image_start,?&images.os.image_len);//返回指向內存中鏡像頭的指針??? ????if?(images.os.image_len?==?0)?{?? ????????puts?("ERROR:?can't?get?kernel?image!/n");?? ????????return?1;?? ????}?? ????/*?get?image?parameters?*/?? ????switch?(genimg_get_format?(os_hdr))?{//根據鏡像魔數獲取鏡像類型??? ????case?IMAGE_FORMAT_LEGACY:?? ????????images.os.type?=?image_get_type?(os_hdr);//鏡像類型??? ????????images.os.comp?=?image_get_comp?(os_hdr);//壓縮類型??? ????????images.os.os?=?image_get_os?(os_hdr);//操作系統類型??? ????????images.os.end?=?image_get_image_end?(os_hdr);//當前鏡像的尾地址??? ????????images.os.load?=?image_get_load?(os_hdr);//鏡像數據的載入地址??? ????????break;?? ????default:?? ????????puts?("ERROR:?unknown?image?format?type!/n");?? ????????return?1;?? ????}?? ????/*?find?kernel?entry?point?*/?? ????if?(images.legacy_hdr_valid)?{//如果鏡像已經通過驗證??? ????????images.ep?=?image_get_ep?(&images.legacy_hdr_os_copy);//獲取入口地址，填充images.ep?。??? ????}?else?{?? ????????puts?("Could?not?find?kernel?entry?point!/n");?? ????????return?1;?? ????}?? ????if?(((images.os.type?==?IH_TYPE_KERNEL)?||?? ?????????(images.os.type?==?IH_TYPE_MULTI))?&&?? ????????(images.os.os?==?IH_OS_LINUX))?{?? ????????/*?find?ramdisk?*/3250的配置中這個函數不做任何工作?? ????????ret?=?boot_get_ramdisk?(argc,?argv,?&images,?IH_INITRD_ARCH,?? ????????????????&images.rd_start,?&images.rd_end);?? ????????if?(ret)?{?? ????????????puts?("Ramdisk?image?is?corrupt?or?invalid/n");?? ????????????return?1;?? ????????}?? ????}?? ????images.os.start?=?(ulong)os_hdr;//指向內存中鏡像的頭地址??? ????images.state?=?BOOTM_STATE_START;//標記引導狀態??? ????return?0;?? }??

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總結一下這個函數的主要工作：第一步校驗鏡像的正確性，獲取鏡像的信息（根據鏡像頭），第二部將第一步獲取的信息存入images（主要是填充image_info_t類型的os成員）

2. bootm_headers_t

? typedef?struct?bootm_headers?{?? ????/*? ?????*?Legacy?os?image?header,?if?it?is?a?multi?component?image? ?????*?then?boot_get_ramdisk()?and?get_fdt()?will?attempt?to?get? ?????*?data?from?second?and?third?component?accordingly.? ?????*/?? ????image_header_t????*legacy_hdr_os;????????/*?image?header?pointer?*/?? ????image_header_t????legacy_hdr_os_copy;????/*?header?copy?*/?? ????ulong????????legacy_hdr_valid;?? #ifndef?USE_HOSTCC??? ????image_info_t????os;????????/*?os?image?info?*/?? ????ulong????????ep;????????/*?entry?point?of?OS?*/?? ????ulong????????rd_start,?rd_end;/*?ramdisk?start/end?*/?? ????ulong????????ft_len;????????/*?length?of?flat?device?tree?*/?? ????ulong????????initrd_start;?? ????ulong????????initrd_end;?? ????ulong????????cmdline_start;?? ????ulong????????cmdline_end;?? ????bd_t????????*kbd;?? #endif??? ????int????????verify;????????/*?getenv("verify")[0]?!=?'n'?*/?? #define????BOOTM_STATE_START????????(0x00000001)??? #define????BOOTM_STATE_LOADOS????????(0x00000002)??? #define????BOOTM_STATE_RAMDISK????(0x00000004)??? #define????BOOTM_STATE_FDT????????(0x00000008)??? #define????BOOTM_STATE_OS_CMDLINE????(0x00000010)??? #define????BOOTM_STATE_OS_BD_T????(0x00000020)??? #define????BOOTM_STATE_OS_PREP????(0x00000040)??? #define????BOOTM_STATE_OS_GO????????(0x00000080)??? ????int????????state;?? }?bootm_headers_t;?? /*? ?*?Legacy?format?image?header,? ?*?all?data?in?network?byte?order?(aka?natural?aka?bigendian).? ?*/內核鏡像頭??include/image.h?? typedef?struct?image_header?{?? ????uint32_t????ih_magic;????????/*?Image?Header?Magic?Number????*/鏡像頭部幻數，為#define?IH_MAGIC????0x27051956?? ????uint32_t????ih_hcrc;????????????/*?Image?Header?CRC?Checksum????*/鏡像頭部crc校驗碼?? ????uint32_t????ih_time;????????????/*?Image?Creation?Timestamp????*/鏡像創建時間戳?? ????uint32_t????ih_size;????????????/*?Image?Data?Size????????*/鏡像數據大小（不算頭部）1828536?? ????uint32_t????ih_load;????????????/*?Data?????Load??Address????????*/數據將要載入的內存地址???80008000?? ????uint32_t????ih_ep;????????????/*?Entry?Point?Address????????*/鏡像入口地址????????80008000????????????? ????uint32_t????ih_dcrc;????????????/*?Image?Data?CRC?Checksum????*/鏡像數據校驗碼?? ????uint8_t????????ih_os;????????/*?Operating?System????????*/操作系統類型????#define?IH_OS_LINUX????????5?? ????uint8_t????????ih_arch;????????/*?CPU?architecture????????*/CPU架構類型????#define?IH_ARCH_ARM????????2?? ????uint8_t????????ih_type;????????/*?Image?Type????????????*/鏡像類型????????IH_TYPE_KERNEL?? ????uint8_t????????ih_comp;????/*?Compression?Type????????*/壓縮類型????????IH_COMP_NONE?? ????uint8_t????????ih_name[IH_NMLEN];????/*?Image?Name????????*/鏡像名字Linux-2.6.27.8，#define?IH_NMLEN????????32?? }?image_header_t;?? 鏡像信息??????include/image.h?? typedef?struct?image_info?{?? ????ulong????????start,?end;????????????/*?start/end?of?blob?*/鏡像的起始地址和尾地址?? ????ulong????????image_start,?image_len;?????????/*?start?of?image?within?blob,?len?of?image?*/鏡像數據的開始地址和長度?? ????ulong????????load;????????????????/*?load?addr?for?the?image?*/鏡像數據的裝載地址?? ????uint8_t????????comp,?type,?os;????????????/*?compression,?type?of?image,?os?type?*/壓縮類型，鏡像類型和操作系統類型?? }?image_info_t;??

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3。boot_get_kernel

? /**? ?*?boot_get_kernel?-?find?kernel?image? ?*?@os_data:?pointer?to?a?ulong?variable,?will?hold?os?data?start?address? ?*?@os_len:?pointer?to?a?ulong?variable,?will?hold?os?data?length? ?*? ?*?boot_get_kernel()?tries?to?find?a?kernel?image,?verifies?its?integrity? ?*?and?locates?kernel?data.? ?*? ?*?returns:? ?*?????pointer?to?image?header?if?valid?image?was?found,?plus?kernel?start? ?*?????address?and?length,?otherwise?NULL? ?*/?? 尋找可用的內核鏡像?? static?void?*boot_get_kernel?(cmd_tbl_t?*cmdtp,?int?flag,?int?argc,?char?*argv[],?? ????????bootm_headers_t?*images,?ulong?*os_data,?ulong?*os_len)?? {?? ????image_header_t????*hdr;?? ????ulong????????img_addr;?? ????/*?find?out?kernel?image?address?*/?? ????if?(argc?<?2)?{//如果參數太少??? ????????img_addr?=?load_addr;????//使用默認的鏡像載入地址，這個地址是在配置頭文件中定義的?ulong?load_addr?=?CONFIG_SYS_LOAD_ADDR;??? ????????debug?("*??kernel:?default?image?load?address?=?0x%08lx/n",?? ????????????????load_addr);?? ????}?else?{?? ????????img_addr?=?simple_strtoul(argv[1],?NULL,?16);//參數足夠的話，把第二個參數轉化為16進制作為地址??? ????????debug?("*??kernel:?cmdline?image?address?=?0x%08lx/n",?img_addr);?? ????}?? ????show_boot_progress?(1);?? ????/*?copy?from?dataflash?if?needed?*/?? ????img_addr?=?genimg_get_image?(img_addr);//對于3250，這個函數什么都沒做。因為沒有dataflash.??? ????/*?check?image?type,?for?FIT?images?get?FIT?kernel?node?*/?? ????*os_data?=?*os_len?=?0;?? ????switch?(genimg_get_format?((void?*)img_addr))?{//根據上面得到的鏡像的地址，獲取鏡像的類型。這個函數根據鏡像頭部的魔數返回類型。??? ????case?IMAGE_FORMAT_LEGACY:?? ????????printf?("##?Booting?kernel?from?Legacy?Image?at?%08lx?.../n",???//打印引導消息??? ????????????????img_addr);?? ????????hdr?=?image_get_kernel?(img_addr,?images->verify);//檢驗鏡像的合法性（校驗碼、魔數、架構類型等），并打印信息。見下面的分析。??? ????????if?(!hdr)?? ????????????return?NULL;?? ????????show_boot_progress?(5);?? ????????/*?get?os_data?and?os_len?*/?? ????????switch?(image_get_type?(hdr))?{?? ????????case?IH_TYPE_KERNEL:?? ????????????*os_data?=?image_get_data?(hdr);//鏡像數據的地址，也就是緊挨著鏡像頭的地址。??? ????????????*os_len?=?image_get_data_size?(hdr);//鏡像數據部分大小??? ????????????break;?? ????????case?IH_TYPE_MULTI:?? ????????????image_multi_getimg?(hdr,?0,?os_data,?os_len);?? ????????????break;?? ????????case?IH_TYPE_STANDALONE:?? ????????????*os_data?=?image_get_data?(hdr);?? ????????????*os_len?=?image_get_data_size?(hdr);?? ????????????break;?? ????????default:?? ????????????printf?("Wrong?Image?Type?for?%s?command/n",?cmdtp->name);?? ????????????show_boot_progress?(-5);?? ????????????return?NULL;?? ????????}?? ????????/*? ?????????*?copy?image?header?to?allow?for?image?overwrites?during?kernel? ?????????*?decompression.? ?????????*/拷貝一份鏡像的頭部到images中。?? ????????memmove?(&images->legacy_hdr_os_copy,?hdr,?sizeof(image_header_t));?? ????????/*?save?pointer?to?image?header?*/?? ????????images->legacy_hdr_os?=?hdr;//images中指針指向鏡像的頭部??? ????????images->legacy_hdr_valid?=?1;//鏡像已經檢驗合格，置標志量??? ????????show_boot_progress?(6);?? ????????break;?? ????default:?? ????????printf?("Wrong?Image?Format?for?%s?command/n",?cmdtp->name);?? ????????show_boot_progress?(-108);?? ????????return?NULL;?? ????}?? ????debug?("???kernel?data?at?0x%08lx,?len?=?0x%08lx?(%ld)/n",?? ????????????*os_data,?*os_len,?*os_len);?? ????return?(void?*)img_addr;?? }??

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4。image_get_kernel

? common/cmd_bootm.c?? static?image_header_t?*image_get_kernel?(ulong?img_addr,?int?verify)?? {?? ????image_header_t?*hdr?=?(image_header_t?*)img_addr;?? ????if?(!image_check_magic(hdr))?{????//檢查鏡像頭部的魔數是否等于?IH_MAGIC??? ????????puts?("Bad?Magic?Number/n");?? ????????show_boot_progress?(-1);?? ????????return?NULL;?? ????}?? ????show_boot_progress?(2);?? ????if?(!image_check_hcrc?(hdr))?{//檢查鏡像頭部的校驗碼（hcrc為0時鏡像頭的校驗碼）??? ????????puts?("Bad?Header?Checksum/n");?? ????????show_boot_progress?(-2);?? ????????return?NULL;?? ????}?? ????show_boot_progress?(3);?? ????image_print_contents?(hdr);//根據傳入的鏡像頭地址，打印鏡像的信息，見下面的分析。??? /*? ???Image?Name:???Linux-2.6.27.8? ???Image?Type:???ARM?Linux?Kernel?Image?(uncompressed)? ???Data?Size:????1828536?Bytes?=?1.7?MiB? ???Load?Address:?80008000? ???Entry?Point:??80008000? */?? ????if?(verify)?{//是否要對鏡像的數據部分進行校驗??? ????????puts?("???Verifying?Checksum?...?");?? ????????if?(!image_check_dcrc?(hdr))?{?? ????????????printf?("Bad?Data?CRC/n");?? ????????????show_boot_progress?(-3);?? ????????????return?NULL;?? ????????}?? ????????puts?("OK/n");?? ????}?? ????show_boot_progress?(4);?? ????if?(!image_check_target_arch?(hdr))?{//檢查是否是IH_ARCH_ARM架構??? ????????printf?("Unsupported?Architecture?0x%x/n",?image_get_arch?(hdr));?? ????????show_boot_progress?(-4);?? ????????return?NULL;?? ????}?? ????return?hdr;?? } ?

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