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 in U-Boot and Linux ?

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.

Answer 2

The latest Linux kernel is attempting to obsolete ATAGS with device trees . However, the setup.h file defines the different ATAG values and structures. To parse these, you need to add them with something like,

static int __init parse_tag_custom(const struct tag *tag)
{
    if (tag->hdr.size > CUSTOM_SIZE) {
         /* Use, storing or acting on passed values */ 
         tag->u.custom;
    }
    return 0;
}

__tagtable(ATAG_CUSTOM, parse_tag_custom);

as found in atags_parse.c . Of course, you need to add these to the values in setup.h .

u-boot is probably less defined as for the most part, it passes arguments via the kernel command line as this is not ARM specific. A command argument or device trees is probably the preferred method. If you gave an example of what type of configuration you need, someone could probably give better guidance.

Answer 3

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.