看一下dmar_table_init这个函数:

int __init dmar_table_init(void)
{
    static int dmar_table_initialized;
    int ret;

    if (dmar_table_initialized == 0) {
        ret = parse_dmar_table();
        if (ret < 0) {
            if (ret != -ENODEV)
                pr_info("parse DMAR table failure.\n");
        } else  if (list_empty(&dmar_drhd_units)) {
            pr_info("No DMAR devices found\n");
            ret = -ENODEV;
        }

        if (ret < 0)
            dmar_table_initialized = ret;
        else
            dmar_table_initialized = 1;
    }

    return dmar_table_initialized < 0 ? dmar_table_initialized : 0;
}

dmar_table_init其实就是调用了parse_dmar_table这个函数来解析 DMAR ACPI tableparse_dmar_table代码如下:

static int __init
parse_dmar_table(void)
{
    struct acpi_table_dmar *dmar;
    struct acpi_dmar_header *entry_header;
    int ret = 0;
    int drhd_count = 0;

    /*
     * Do it again, earlier dmar_tbl mapping could be mapped with
     * fixed map.
     */
    dmar_table_detect();

    /*
     * ACPI tables may not be DMA protected by tboot, so use DMAR copy
     * SINIT saved in SinitMleData in TXT heap (which is DMA protected)
     */
    dmar_tbl = tboot_get_dmar_table(dmar_tbl);

    dmar = (struct acpi_table_dmar *)dmar_tbl;
    if (!dmar)
        return -ENODEV;

    if (dmar->width < PAGE_SHIFT - 1) {
        pr_warn("Invalid DMAR haw\n");
        return -EINVAL;
    }

    pr_info("Host address width %d\n", dmar->width + 1);

    entry_header = (struct acpi_dmar_header *)(dmar + 1);
    while (((unsigned long)entry_header) <
            (((unsigned long)dmar) + dmar_tbl->length)) {
        /* Avoid looping forever on bad ACPI tables */
        if (entry_header->length == 0) {
            pr_warn("Invalid 0-length structure\n");
            ret = -EINVAL;
            break;
        }

        dmar_table_print_dmar_entry(entry_header);

        switch (entry_header->type) {
        case ACPI_DMAR_TYPE_HARDWARE_UNIT:
            drhd_count++;
            ret = dmar_parse_one_drhd(entry_header);
            break;
        case ACPI_DMAR_TYPE_RESERVED_MEMORY:
            ret = dmar_parse_one_rmrr(entry_header);
            break;
        case ACPI_DMAR_TYPE_ATSR:
            ret = dmar_parse_one_atsr(entry_header);
            break;
        case ACPI_DMAR_HARDWARE_AFFINITY:
#ifdef CONFIG_ACPI_NUMA
            ret = dmar_parse_one_rhsa(entry_header);
#endif
            break;
        case ACPI_DMAR_TYPE_ANDD:
            ret = dmar_parse_one_andd(entry_header);
            break;
        default:
            pr_warn("Unknown DMAR structure type %d\n",
                entry_header->type);
            ret = 0; /* for forward compatibility */
            break;
        }
        if (ret)
            break;

        entry_header = ((void *)entry_header + entry_header->length);
    }
    if (drhd_count == 0)
        pr_warn(FW_BUG "No DRHD structure found in DMAR table\n");
    return ret;
}

parse_dmar_table函数中重要的工作有以下两点:
(1)检查host机器地址总线宽度:

if (dmar->width < PAGE_SHIFT - 1) {
        pr_warn("Invalid DMAR haw\n");
        return -EINVAL;
}

(2)遍历并解析DMAR table中的每一项,并加到相应的列表中:

while (((unsigned long)entry_header) <
            (((unsigned long)dmar) + dmar_tbl->length)) {
            ......
}

DMA Remapping Hardware单元为例:

static int __init
dmar_parse_one_drhd(struct acpi_dmar_header *header)
{
    struct acpi_dmar_hardware_unit *drhd;
    struct dmar_drhd_unit *dmaru;
    int ret = 0;

    drhd = (struct acpi_dmar_hardware_unit *)header;
    dmaru = kzalloc(sizeof(*dmaru), GFP_KERNEL);
    if (!dmaru)
        return -ENOMEM;

    dmaru->hdr = header;
    dmaru->reg_base_addr = drhd->address;
    dmaru->segment = drhd->segment;
    dmaru->include_all = drhd->flags & 0x1; /* BIT0: INCLUDE_ALL */
    dmaru->devices = dmar_alloc_dev_scope((void *)(drhd + 1),
                          ((void *)drhd) + drhd->header.length,
                          &dmaru->devices_cnt);
    if (dmaru->devices_cnt && dmaru->devices == NULL) {
        kfree(dmaru);
        return -ENOMEM;
    }

    ret = alloc_iommu(dmaru);
    if (ret) {
        dmar_free_dev_scope(&dmaru->devices,
                    &dmaru->devices_cnt);
        kfree(dmaru);
        return ret;
    }
    dmar_register_drhd_unit(dmaru);
    return 0;
}

最后dmar_register_drhd_unit这个函数会把解析好的DMA Remapping Hardware单元加到dmar_drhd_units这个列表中。