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

Follow this procedure ,

To achieve this goal, there're 2 parts need to be modified. One is the U-Boot, and the other one is the Linux kernel.

    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.

2. 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.