Linux kernel 无法保留512MB或更多的CMA

我正试图在内存为64GB的ARM64机器上保留512MB的CMA内存，在linux引导期间，我将收到“CMA:未能保留512MB”错误消息。预留384MB就可以了。如果您需要更多信息，请告诉我。我非常感谢你的帮助

[ESL_Start_OS]:[644L] Start to jump Linux kernel
    Booting Linux on physical CPU 0x10000
    Initializing cgroup subsys cpuset
    Initializing cgroup subsys cpu
    Initializing cgroup subsys cpuacct
    Linux version 4.1.0+ (s00327669@salem-linux) (gcc version 4.9.3 20141031 (prerelease) (Linaro GCC 2014.11) ) #4 SMP Mon Oct 5 12:00:57 EDT 2015
    CPU: AArch64 Processor [411fd071] revision 1
    Detected PIPT I-cache on CPU0
    alternatives: enabling workaround for ARM erratum 832075
    earlycon: Early serial console at MMIO32 0x60300000 (options '')
    bootconsole [uart0] enabled
    efi: Getting EFI parameters from FDT:
    efi: UEFI not found.
    cma: Failed to reserve 512 MiB
    PERCPU: Embedded 16 pages/cpu @ffffffd7bfa00000 s27648 r8192 d29696 u65536
    Built 1 zonelists in Zone order, mobility grouping on.  Total pages: 16514064
    Kernel command line: rdinit=/init console=ttyS0,115200 earlycon=uart8250,mmio32,0x60300000 cma=512M                                                                                                 
    log_buf_len individual max cpu contribution: 4096 bytes
    log_buf_len total cpu_extra contributions: 126976 bytes
    log_buf_len min size: 16384 bytes
    log_buf_len: 262144 bytes
    early log buf free: 14220(86%)
    PID hash table entries: 4096 (order: 3, 32768 bytes)
    Dentry cache hash table entries: 8388608 (order: 14, 67108864 bytes)
    Inode-cache hash table entries: 4194304 (order: 13, 33554432 bytes)
    software IO TLB [mem 0x3bbff000-0x3fbff000] (64MB) mapped at [ffffffc03bbff000-ffffffc03fbfefff]
    Memory: 65469716K/67104768K available (6109K kernel code, 537K rwdata, 2516K rodata, 436K init, 226K bss, 1635052K reserved, 0K cma-reserved)
    Virtual kernel memory layout:
        vmalloc : 0xffffff8000000000 - 0xffffffbdbfff0000   (   246 GB)
        vmemmap : 0xffffffbdc0000000 - 0xffffffbfc0000000   (     8 GB maximum)
                  0xffffffbdc0000000 - 0xffffffbe20000000   (  1536 MB actual)
        fixed   : 0xffffffbffabfd000 - 0xffffffbffac00000   (    12 KB)
        PCI I/O : 0xffffffbffae00000 - 0xffffffbffbe00000   (    16 MB)
        modules : 0xffffffbffc000000 - 0xffffffc000000000   (    64 MB)
        memory  : 0xffffffc000000000 - 0xffffffd800000000   ( 98304 MB)
          .init : 0xffffffc0008ef000 - 0xffffffc00095c000   (   436 KB)
          .text : 0xffffffc000080000 - 0xffffffc0008ee7c4   (  8634 KB)
          .data : 0xffffffc000965000 - 0xffffffc0009eb400   (   537 KB)
    SLUB: HWalign=64, Order=0-3, MinObjects=0, CPUs=32, Nodes=1
    Hierarchical RCU implementation.
            Additional per-CPU info printed with stalls.
    NR_IRQS:64 nr_irqs:64 0
    ITS: /interrupt-controller@6d000000/interrupt-controller@6c000000
    ITS: allocated 65536 Devices @17b9480000 (psz 4K, shr 2)
    ITS: allocated 512 Virtual CPUs @17b9418000 (psz 4K, shr 2)
    ITS: allocated 512 Interrupt Collections @17b9419000 (psz 4K, shr 2)
    ITS: /interrupt-controller@6d000000/interrupt-controller@4c000000
    ITS: allocated 65536 Devices @17b9500000 (psz 4K, shr 2)
    ITS: allocated 512 Virtual CPUs @17b9432000 (psz 4K, shr 2)
    ITS: allocated 512 Interrupt Collections @17b9433000 (psz 4K, shr 2)
    ITS: /interrupt-controller@6d000000/interrupt-controller@c6000000
    ITS: allocated 65536 Devices @17b9580000 (psz 4K, shr 2)
    ITS: allocated 512 Virtual CPUs @17b943d000 (psz 4K, shr 2)
    ITS: allocated 512 Interrupt Collections @17b943e000 (psz 4K, shr 2)
    ITS: /interrupt-controller@6d000000/interrupt-controller@a3000000
    ITS: allocated 65536 Devices @17b9600000 (psz 4K, shr 2)
    ITS: allocated 512 Virtual CPUs @17b9467000 (psz 4K, shr 2)
    ITS: allocated 512 Interrupt Collections @17b9468000 (psz 4K, shr 2)
    ITS: /interrupt-controller@6d000000/interrupt-controller@b7000000
    ITS: allocated 65536 Devices @17b9700000 (psz 4K, shr 2)
    ITS: allocated 512 Virtual CPUs @17b9681000 (psz 4K, shr 2)
    ITS: allocated 512 Interrupt Collections @17b9682000 (psz 4K, shr 2)
    GIC: using LPI property table @0x00000017b96a0000
    ITS: Allocated 1792 chunks for LPIs
    CPU0: found redistributor 10000 region 1:0x000000004d100000
    CPU0: using LPI pending table @0x00000017b96b0000
    MBIGEN: /peripherals/interrupt-controller@4c030000
    MBIGEN: /peripherals/interrupt-controller@6c030000
    MBIGEN: /peripherals/interrupt-controller@c6030000
    MBIGEN: /peripherals/interrupt-controller@a3030000
    MBIGEN: /peripherals/interrupt-controller@b7030000
    Architected cp15 timer(s) running at 50.00MHz (phys).
    clocksource arch_sys_counter: mask: 0xffffffffffffff max_cycles: 0xb8812736b, max_idle_ns: 440795202655 ns
    sched_clock: 56 bits at 50MHz, resolution 20ns, wraps every 4398046511100ns
    Console: colour dummy device 80x25
    Calibrating delay loop (skipped), value calculated using timer frequency.. 100.00 BogoMIPS (lpj=200000)
    pid_max: default: 32768 minimum: 301
    Security Framework initialized
    Mount-cache hash table entries: 131072 (order: 8, 1048576 bytes)
    Mountpoint-cache hash table entries: 131072 (order: 8, 1048576 bytes)
    Initializing cgroup subsys memory
    Initializing cgroup subsys devices
    Initializing cgroup subsys freezer
    Initializing cgroup subsys hugetlb
    hw perfevents: enabled with arm/armv8-pmuv3 PMU driver, 7 counters available
    EFI services will not be available.
    CPU1: Booted secondary processor
    Detected PIPT I-cache on CPU1
    CPU1: found redistributor 10001 region 1:0x000000004d130000
    CPU1: using LPI pending table @0x00000017b8ee0000
    CPU2: Booted secondary processor
    Detected PIPT I-cache on CPU2
    CPU2: found redistributor 10002 region 1:0x000000004d160000
    CPU2: using LPI pending table @0x00000017b8f10000
    CPU3: Booted secondary processor
    Detected PIPT I-cache on CPU3
    CPU3: found redistributor 10003 region 1:0x000000004d190000
    CPU3: using LPI pending table @0x00000017b8f40000
    CPU4: Booted secondary processor
    Detected PIPT I-cache on CPU4
    CPU4: found redistributor 10100 region 1:0x000000004d1c0000
    CPU4: using LPI pending table @0x00000017b8f80000
    CPU5: Booted secondary processor
    Detected PIPT I-cache on CPU5
    CPU5: found redistributor 10101 region 1:0x000000004d1f0000
    CPU5: using LPI pending table @0x00000017b8fa0000
    CPU6: Booted secondary processor
    Detected PIPT I-cache on CPU6
    CPU6: found redistributor 10102 region 1:0x000000004d220000
    CPU6: using LPI pending table @0x00000017b8fd0000
    CPU7: Booted secondary processor
    Detected PIPT I-cache on CPU7
    CPU7: found redistributor 10103 region 1:0x000000004d250000
    CPU7: using LPI pending table @0x00000017b8800000
    CPU8: Booted secondary processor
    Detected PIPT I-cache on CPU8
    CPU8: found redistributor 10200 region 1:0x000000004d280000
    CPU8: using LPI pending table @0x00000017b8840000
    CPU9: Booted secondary processor
    Detected PIPT I-cache on CPU9
    CPU9: found redistributor 10201 region 1:0x000000004d2b0000
    CPU9: using LPI pending table @0x00000017b8870000
    CPU10: Booted secondary processor
    Detected PIPT I-cache on CPU10
    CPU10: found redistributor 10202 region 1:0x000000004d2e0000
    CPU10: using LPI pending table @0x00000017b88b0000
    CPU11: Booted secondary processor
    Detected PIPT I-cache on CPU11
    CPU11: found redistributor 10203 region 1:0x000000004d310000
    CPU11: using LPI pending table @0x00000017b88e0000
    CPU12: Booted secondary processor
    Detected PIPT I-cache on CPU12
    CPU12: found redistributor 10300 region 1:0x000000004d340000
    CPU12: using LPI pending table @0x00000017b8910000
    CPU13: Booted secondary processor
    Detected PIPT I-cache on CPU13
    CPU13: found redistributor 10301 region 1:0x000000004d370000
    CPU13: using LPI pending table @0x00000017b8940000
    CPU14: Booted secondary processor
    Detected PIPT I-cache on CPU14
    CPU14: found redistributor 10302 region 1:0x000000004d3a0000
    CPU14: using LPI pending table @0x00000017b8970000
    CPU15: Booted secondary processor
    Detected PIPT I-cache on CPU15
    CPU15: found redistributor 10303 region 1:0x000000004d3d0000
    CPU15: using LPI pending table @0x00000017b89a0000
    CPU16: Booted secondary processor
    Detected PIPT I-cache on CPU16
    CPU16: found redistributor 20000 region 0:0x000000006d100000
    CPU16: using LPI pending table @0x00000017b89e0000
    CPU17: Booted secondary processor
    Detected PIPT I-cache on CPU17
    CPU17: found redistributor 20001 region 0:0x000000006d130000
    CPU17: using LPI pending table @0x00000017b8a10000
    CPU18: Booted secondary processor
    Detected PIPT I-cache on CPU18
    CPU18: found redistributor 20002 region 0:0x000000006d160000
    CPU18: using LPI pending table @0x00000017b8a40000
    CPU19: Booted secondary processor
    Detected PIPT I-cache on CPU19
    CPU19: found redistributor 20003 region 0:0x000000006d190000
    CPU19: using LPI pending table @0x00000017b8a80000
    CPU20: Booted secondary processor
    Detected PIPT I-cache on CPU20
    CPU20: found redistributor 20100 region 0:0x000000006d1c0000
    CPU20: using LPI pending table @0x00000017b8aa0000
    CPU21: Booted secondary processor
    Detected PIPT I-cache on CPU21
    CPU21: found redistributor 20101 region 0:0x000000006d1f0000
    CPU21: using LPI pending table @0x00000017b8ad0000
    CPU22: Booted secondary processor
    Detected PIPT I-cache on CPU22
    CPU22: found redistributor 20102 region 0:0x000000006d220000
    CPU22: using LPI pending table @0x00000017b8b10000
    CPU23: Booted secondary processor
    Detected PIPT I-cache on CPU23
    CPU23: found redistributor 20103 region 0:0x000000006d250000
    CPU23: using LPI pending table @0x00000017b8b40000
    CPU24: Booted secondary processor
    Detected PIPT I-cache on CPU24
    CPU24: found redistributor 20200 region 0:0x000000006d280000
    CPU24: using LPI pending table @0x00000017b8b70000
    CPU25: Booted secondary processor
    Detected PIPT I-cache on CPU25
    CPU25: found redistributor 20201 region 0:0x000000006d2b0000
    CPU25: using LPI pending table @0x00000017b8bb0000
    CPU26: Booted secondary processor
    Detected PIPT I-cache on CPU26
    CPU26: found redistributor 20202 region 0:0x000000006d2e0000
    CPU26: using LPI pending table @0x00000017b8be0000
    CPU27: Booted secondary processor
    Detected PIPT I-cache on CPU27
    CPU27: found redistributor 20203 region 0:0x000000006d310000
    CPU27: using LPI pending table @0x00000017b8400000
    CPU28: Booted secondary processor
    Detected PIPT I-cache on CPU28
    CPU28: found redistributor 20300 region 0:0x000000006d340000
    CPU28: using LPI pending table @0x00000017b8450000
    CPU29: Booted secondary processor
    Detected PIPT I-cache on CPU29
    CPU29: found redistributor 20301 region 0:0x000000006d370000
    CPU29: using LPI pending table @0x00000017b8470000
    CPU30: Booted secondary processor
    Detected PIPT I-cache on CPU30
    CPU30: found redistributor 20302 region 0:0x000000006d3a0000
    CPU30: using LPI pending table @0x00000017b84b0000
    CPU31: Booted secondary processor
    Detected PIPT I-cache on CPU31
    CPU31: found redistributor 20303 region 0:0x000000006d3d0000
    CPU31: using LPI pending table @0x00000017b84e0000
    Brought up 32 CPUs
    SMP: Total of 32 processors activated.
    CPU: All CPU(s) started at EL2
    alternatives: patching kernel code
    devtmpfs: initialized
    DMI not present or invalid.
    clocksource jiffies: mask: 0xffffffff max_cycles: 0xffffffff, max_idle_ns: 7645041785100000 ns
    atomic64_test: passed
    pinctrl core: initialized pinctrl subsystem
    NET: Registered protocol family 16
    cpuidle: using governor ladder
    cpuidle: using governor menu
    vdso: 2 pages (1 code @ ffffffc00096d000, 1 data @ ffffffc00096c000)
    hw-breakpoint: found 6 breakpoint and 4 watchpoint registers.
    DMA: preallocated 256 KiB pool for atomic allocations
    Serial: AMBA PL011 UART driver
    hisi-smmu 40040000.smmu_pa: registered 0 master devices
    hisi-smmu 40040000.smmu_pa: probing hardware configuration...
    hisi-smmu 40040000.smmu_pa:     nested translation
    hisi-smmu 40040000.smmu_pa:     coherent table walk
    hisi-smmu 40040000.smmu_pa:     39-bit VA, 39-bit IPA, 48-bit PA
    hisi-smmu 60040000.smmu_pc: registered 0 master devices
    hisi-smmu 60040000.smmu_pc: probing hardware configuration...
    hisi-smmu 60040000.smmu_pc:     nested translation
    hisi-smmu 60040000.smmu_pc:     coherent table walk
    hisi-smmu 60040000.smmu_pc:     39-bit VA, 39-bit IPA, 48-bit PA
    hisi-smmu c0040000.smmu_dsa: registered 0 master devices
    hisi-smmu c0040000.smmu_dsa: probing hardware configuration...
    hisi-smmu c0040000.smmu_dsa:    nested translation
    hisi-smmu c0040000.smmu_dsa:    coherent table walk
    hisi-smmu c0040000.smmu_dsa:    39-bit VA, 39-bit IPA, 48-bit PA
    Warning: Mbigen device id already exist.Old:0x2,new:0x20
    hisi-smmu a0040000.smmu_m3: registered 0 master devices
    hisi-smmu a0040000.smmu_m3: probing hardware configuration...
    hisi-smmu a0040000.smmu_m3:     nested translation
    hisi-smmu a0040000.smmu_m3:     coherent table walk
    hisi-smmu a0040000.smmu_m3:     39-bit VA, 39-bit IPA, 48-bit PA
    hisi-smmu b0040000.smmu_pcie: registered 0 master devices
    hisi-smmu b0040000.smmu_pcie: probing hardware configuration...
    hisi-smmu b0040000.smmu_pcie:   nested translation
    hisi-smmu b0040000.smmu_pcie:   coherent table walk
    hisi-smmu b0040000.smmu_pcie:   39-bit VA, 39-bit IPA, 48-bit PA
    vgaarb: loaded
    SCSI subsystem initialized
    usbcore: registered new interface driver usbfs
    usbcore: registered new interface driver hub
    usbcore: registered new device driver usb
    Warning: Mbigen device id already exist.Old:0x1,new:0x10
    Warning: Mbigen device id already exist.Old:0x3,new:0x30
    Warning: Mbigen device id already exist.Old:0x3,new:0x30
    Switched to clocksource arch_sys_counter
    NET: Registered protocol family 2
    TCP established hash table entries: 524288 (order: 10, 4194304 bytes)
    TCP bind hash table entries: 65536 (order: 8, 1048576 bytes)
    TCP: Hash tables configured (established 524288 bind 65536)
    UDP hash table entries: 32768 (order: 8, 1048576 bytes)
    UDP-Lite hash table entries: 32768 (order: 8, 1048576 bytes)
    NET: Registered protocol family 1
    RPC: Registered named UNIX socket transport module.
    RPC: Registered udp transport module.
    RPC: Registered tcp transport module.
    RPC: Registered tcp NFSv4.1 backchannel transport module.
    Unpacking initramfs...
    Freeing initrd memory: 409600K (ffffffc007000000 - ffffffc020000000)
    futex hash table entries: 8192 (order: 7, 524288 bytes)
    audit: initializing netlink subsys (disabled)
    audit: type=2000 audit(5.048:1): initialized
    HugeTLB registered 2 MB page size, pre-allocated 0 pages
    VFS: Disk quotas dquot_6.6.0
    VFS: Dquot-cache hash table entries: 512 (order 0, 4096 bytes)
    NFS: Registering the id_resolver key type
    Key type id_resolver registered
    Key type id_legacy registered
    fuse init (API version 7.23)
    9p: Installing v9fs 9p2000 file system support
    bounce: pool size: 64 pages
    Block layer SCSI generic (bsg) driver version 0.4 loaded (major 252)
    io scheduler noop registered
    io scheduler cfq registered (default)
    hisi-pcie b0080000.pcie: Link up
    PCI host bridge /pcie@0xb0080000 ranges:
      MEM 0xb4100000..0xb4ffffff -> 0xb4100000
    hisi-pcie b0080000.pcie: PCI host bridge to bus 0000:10
    pci_bus 0000:10: root bus resource [bus 10-1f]
    pci_bus 0000:10: root bus resource [mem 0xb4100000-0xb4ffffff]
    pci 0000:10:00.0: IOMMU is currently not supported for PCI
    pci 0000:10:00.0: of_irq_parse_pci() failed with rc=-22
    pci 0000:10:00.0: Primary bus is hard wired to 0
    pci 0000:10:00.0: bridge configuration invalid ([bus 01-01]), reconfiguring
    pci 0000:11:00.0: IOMMU is currently not supported for PCI
    pci 0000:11:00.0: of_irq_parse_pci() failed with rc=-22
    pci 0000:10:00.0: BAR 0: no space for [mem size 0x10000000]
    pci 0000:10:00.0: BAR 0: failed to assign [mem size 0x10000000]
    pci 0000:10:00.0: BAR 8: assigned [mem 0xb4200000-0xb43fffff]
    pci 0000:11:00.0: BAR 0: assigned [mem 0xb4200000-0xb43fffff]
    pci 0000:10:00.0: PCI bridge to [bus 11]
    pci 0000:10:00.0:   bridge window [mem 0xb4200000-0xb43fffff]
    Serial: 8250/16550 driver, 4 ports, IRQ sharing disabled
    console [ttyS0] disabled
    60300000.uart: ttyS0 at MMIO 0x60300000 (irq = 12, base_baud = 12500000) is a 16550A
    console [ttyS0] enabled
    console [ttyS0] enabled
    bootconsole [uart0] disabled
    bootconsole [uart0] disabled
    msm_serial: driver initialized
    Unable to detect cache hierarcy from DT for CPU 0
    loop: module loaded
    at24 1-0052: 16384 byte 24c128 EEPROM, writable, 64 bytes/write
    Loading iSCSI transport class v2.0-870.
    rdac: device handler registered
    hp_sw: device handler registered
    emc: device handler registered
    alua: device handler registered

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在启用CONFIG_ZONE_DMA的情况下，与arm64上的默认情况一样，CMA仅限于具有32位寻址功能的设备可以访问的物理内存。因此，如果4GB边界下没有足够的可用RAM来查找连续的512MB块（），那么无论在更高的地址上存在多少RAM，它都将失败。这将取决于机器的精确物理内存映射、内核加载的位置、引导加载程序所做的保留，等等

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作为比较，我可以很高兴地用

cma=1024M

启动我的一个开发板——那一个，内核在底部加载，在顶部只保留了一个小的固件，显然在两者之间找到了足够大的空间。同一个禁用ZONE_DMA的内核很高兴地分配了4GB的CMA（只能从上6GB块中分配），但我没有仔细观察所有外围设备的损坏情况，因为它们可能会被分配到无法访问的DMA地址。

谢谢。但很明显，在ARM64上，CONFIG_ZONE_DMA被固定为y，并且它不是用户可设置的。我错过了什么吗？不是真的；这就是为什么你会看到你所看到的东西的一个解释——你对它所做的一切都被“视情况而定”所包裹。如果你的所有外设都能处理4GB以上的地址（或者你不在乎破坏不能处理的地址），你可以关闭ZONE_DMA（手动编辑Kconfig）；您可以查看引导加载程序，看看是否有任何内存布局调整可以在那里进行；或者，正如你在问题中提到的那样，你可以一开始就不尝试分配这么大的CMA区域（特别是考虑到该区域充满了IOMMUs…）