How to add customised ATAG variable in U-Boot and Linux kernel?

Question

I want to add customized atag variable in U-Boot and Linux kernel.
How can i achieve this?
Is there any procedure to add an ATAG variable i

Answer 1

1. U-Boot changes required :

   A.     Make sure the  CONFIG_CMDLINE_TAG/CONFIG_SETUP_MEMORY_TAGS/CONFIG_INITRD_TAG are defined in you project definition header file ( u-boot/include/configs/am335x_evm.h ), and we  can add our own tag here, eg. CONFIG_CUSTOM_TAG.
   
   B.     Add the structure definition you wanna append w/ ATAG in u-boot/include/asm-arm/setup.h, eg.
   
       #define ATAG_CUSTOM 0x5441000a
       struct tag_custom{
       unsigned char mac_addr[6];
       };
   
   C.    Add the struct at the tail of "u"...
   
       struct tag {
               struct tag_header hdr;
               union {
               struct tag_core         core;
               struct tag_mem32        mem;
               struct tag_videotext    videotext;
               struct tag_ramdisk      ramdisk;
               struct tag_initrd       initrd;
               struct tag_serialnr     serialnr;
               struct tag_revision     revision;
               struct tag_videolfb     videolfb;
               struct tag_cmdline      cmdline;
   
               /*
                * Acorn specific
                */
               struct tag_acorn        acorn;
   
               /*
                * DC21285 specific
                */
               struct tag_memclk       memclk;
   
               /****** INFOTECH Custom TAG ********/
   
              struct tag_custom custom;
               } u;
           };
   
   D.    Add implementation code in lib_arm/bootm.c:
   
       static void setup_custom_tag(bd_t *bd);
   
       static void setup_custom_tag(bd_t *bd) {
           params->hdr.tag = ATAG_CUSTOM;
              params->hdr.size = tag_size (tag_macaddr);
           params->u.custom.cmd =0;
           params = tag_next (params);
       }
   
   E.    Add "#ifdef CONFIG_CUSTOM_TAG / #endif" at every place you change the code.
   
   F.    Done of U-Boot modification.
   

   1. Linux Changes required:

      A. Add parse tag code in linux/arch/arm/kernel/setup.c:

      int cmd;
      static int __init parse_tag_custom(const struct tag *tag){
              printk("u.custom.cmd=%d\n",tag->u.custom.cmd);
          return 0;
      }
      
      __tagtable(ATAG_MACADDR, parse_tag_custom);
      

      B. Add the structure declaration as U-Boot did in linux/include/asm-arm/setup.h:

      #define ATAG_MACADDR 0x5441000a
      struct tag_custom {
          int cmd;
      };
      

      C. Add the struct at the tail of "u"...

      struct tag {
          struct tag_header hdr;
         union {
              struct tag_core         core;
           struct tag_mem32        mem;
          struct tag_videotext    videotext;
          struct tag_ramdisk      ramdisk;
          struct tag_initrd       initrd;
          struct tag_serialnr     serialnr;
          struct tag_revision     revision;
          struct tag_videolfb     videolfb;
          struct tag_cmdline      cmdline;
      
          /*
           * Acorn specific
           */
          struct tag_acorn        acorn;
      
          /*
           * DC21285 specific
           */
          struct tag_memclk       memclk;
      
          /* Add Infotech custom tag */
      
          struct tag_custom      custom;
          } u;
      };
      

      D. Done w/ Kernel parts.