Linux kernel 在引导时无法从 SD 卡中检测到 rootfs

Question

I am trying to manually build a mainline kernel and boot in Pine64_Plus board. Linux kernel unable to mount the boot partition. And wierdly it doesn't display any partition under available partition in boot log. I did the below steps. Cloned the linux mainline git repo

git clone git://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git --depth=1

Installed the cross compiler tool chain.

and then..

ARCH=arm64 CROSS_COMPILE=aarch64-linux-gnu- make defconfig
ARCH=arm64 CROSS_COMPILE=aarch64-linux-gnu- make -j4 Image

Generated the dtb

ARCH=arm64 CROSS_COMPILE=aarch64-linux-gnu- make -j4 dtbs

Generated the kernel modules

ARCH=arm CROSS_COMPILE=arm-linux-gnueabihf- make -j4 modules
ARCH=arm CROSS_COMPILE=arm-linux-gnueabihf- INSTALL_MOD_PATH=ker_mod_temp_dir make modules modules_install

Partitioned the SD card like below

I kept the MLO, Uboot and firmware files in the boot partition.

I downloaded the ArchLinux rootfs from here

I copied the downloaded rootfs to the ROOTFS partition of my SD card.

sudo cp -rT <path-to-downloaded-rootfs> /media/user/ROOTFS

I copied the linux image and dts folder from /arch/arm64/boot to boot folder in the rootfs.

I copied the kernel modules to roots

sudo cp -rT ker_mod_temp_dir /media/user/ROOTFS/lib

I tried to boot the board with the SD card. Uboot successfully boots up. I gave the below commands in Uboot to load the kernel.

setenv kernel_addr_r 0x42000000
setenv dtb_addr_r 0x48000000
ext4load mmc 0:2 $kernel_addr_r /boot/Image
ext4load mmc 0:2 $dtb_addr_r /boot/dts/allwinner/sun50i-a64-pine64-plus.dtb
setenv bootargs console=ttyS0,115200 earlyprintk ignore_loglevel root=/dev/mmcblk0p2 rootfstype=ext4 rw
booti $kernel_addr_r - $dtb_addr_r

Linux kernel boots but it ends with error - [ end Kernel panic - not syncing: VFS: Unable to mount root fs on unknown-block(0,0) ]---

Boot log

sunxi#booti $kernel_addr_r - $dtb_addr_r
## Flattened Device Tree blob at 48000000
   Booting using the fdt blob at 0x48000000
   Loading Device Tree to 44ff7000, end 44fffba9 ... OK

Starting kernel ...

[mmc]: MMC Device 2 not found
[mmc]: mmc 2 not find, so not exit
INFO:    BL3-1: Next image address = 0x41080000
INFO:    BL3-1: Next image spsr = 0x3c9
[    0.000000] Booting Linux on physical CPU 0x0000000000 [0x410fd034]
[    0.000000] Linux version 5.4.0-rc3 (janani@janani-Vostro-2520) (gcc version 7.4.0 (Ubuntu/Linaro 7.4.0-1ubuntu1~18.04.1)) #2 SMP PREEMPT Fri Nov 1 13:01:35 IST 2019
[    0.000000] Machine model: Pine64+
[    0.000000] printk: debug: ignoring loglevel setting.
[    0.000000] efi: Getting EFI parameters from FDT:
[    0.000000] efi: UEFI not found.
[    0.000000] cma: Reserved 32 MiB at 0x000000007e000000
[    0.000000] NUMA: No NUMA configuration found
[    0.000000] NUMA: Faking a node at [mem 0x0000000041000000-0x000000007fffffff]
[    0.000000] NUMA: NODE_DATA [mem 0x7dde6800-0x7dde7fff]
[    0.000000] Zone ranges:
[    0.000000]   DMA32    [mem 0x0000000041000000-0x000000007fffffff]
[    0.000000]   Normal   empty
[    0.000000] Movable zone start for each node
[    0.000000] Early memory node ranges
[    0.000000]   node   0: [mem 0x0000000041000000-0x000000007fffffff]
[    0.000000] Initmem setup node 0 [mem 0x0000000041000000-0x000000007fffffff]
[    0.000000] On node 0 totalpages: 258048
[    0.000000]   DMA32 zone: 4032 pages used for memmap
[    0.000000]   DMA32 zone: 0 pages reserved
[    0.000000]   DMA32 zone: 258048 pages, LIFO batch:63
[    0.000000] psci: probing for conduit method from DT.
[    0.000000] psci: PSCIv0.2 detected in firmware.
[    0.000000] psci: Using standard PSCI v0.2 function IDs
[    0.000000] psci: Trusted OS migration not required
[    0.000000] percpu: Embedded 22 pages/cpu s52952 r8192 d28968 u90112
[    0.000000] pcpu-alloc: s52952 r8192 d28968 u90112 alloc=22*4096
[    0.000000] pcpu-alloc: [0] 0 [0] 1 [0] 2 [0] 3 
[    0.000000] Detected VIPT I-cache on CPU0
[    0.000000] CPU features: detected: ARM erratum 845719
[    0.000000] CPU features: detected: ARM erratum 843419
[    0.000000] Built 1 zonelists, mobility grouping on.  Total pages: 254016
[    0.000000] Policy zone: DMA32
[    0.000000] Kernel command line: console=ttyS0,115200 earlyprintk ignore_loglevel root=/dev/mmcblk0p2 rootfstype=ext4 rw
[    0.000000] Dentry cache hash table entries: 131072 (order: 8, 1048576 bytes, linear)
[    0.000000] Inode-cache hash table entries: 65536 (order: 7, 524288 bytes, linear)
[    0.000000] mem auto-init: stack:off, heap alloc:off, heap free:off
[    0.000000] Memory: 952956K/1032192K available (12156K kernel code, 1860K rwdata, 6420K rodata, 5056K init, 452K bss, 46468K reserved, 32768K cma-reserved)
[    0.000000] SLUB: HWalign=64, Order=0-3, MinObjects=0, CPUs=4, Nodes=1
[    0.000000] rcu: Preemptible hierarchical RCU implementation.
[    0.000000] rcu:     RCU restricting CPUs from NR_CPUS=256 to nr_cpu_ids=4.
[    0.000000]  Tasks RCU enabled.
[    0.000000] rcu: RCU calculated value of scheduler-enlistment delay is 25 jiffies.
[    0.000000] rcu: Adjusting geometry for rcu_fanout_leaf=16, nr_cpu_ids=4
[    0.000000] NR_IRQS: 64, nr_irqs: 64, preallocated irqs: 0
[    0.000000] GIC: Using split EOI/Deactivate mode
[    0.000000] random: get_random_bytes called from start_kernel+0x2b8/0x458 with crng_init=0
[    0.000000] arch_timer: Enabling global workaround for Allwinner erratum UNKNOWN1
[    0.000000] arch_timer: CPU0: Trapping CNTVCT access
[    0.000000] arch_timer: cp15 timer(s) running at 24.00MHz (phys).
[    0.000000] clocksource: arch_sys_counter: mask: 0xffffffffffffff max_cycles: 0x588fe9dc0, max_idle_ns: 440795202592 ns
[    0.000004] sched_clock: 56 bits at 24MHz, resolution 41ns, wraps every 4398046511097ns
[    0.000656] Console: colour dummy device 80x25
[    0.000738] Calibrating delay loop (skipped), value calculated using timer frequency.. 48.00 BogoMIPS (lpj=96000)
[    0.000748] pid_max: default: 32768 minimum: 301
[    0.000829] LSM: Security Framework initializing
[    0.000889] Mount-cache hash table entries: 2048 (order: 2, 16384 bytes, linear)
[    0.000903] Mountpoint-cache hash table entries: 2048 (order: 2, 16384 bytes, linear)
[    0.024021] ASID allocator initialised with 32768 entries
[    0.032014] rcu: Hierarchical SRCU implementation.
[    0.041639] EFI services will not be available.
[    0.048045] smp: Bringing up secondary CPUs ...
[    5.092138] CPU1: failed to come online
[    5.092145] CPU1: failed in unknown state : 0x0
[   10.216301] CPU2: failed to come online
[   10.216308] CPU2: failed in unknown state : 0x0
[   15.340451] CPU3: failed to come online
[   15.340457] CPU3: failed in unknown state : 0x0
[   15.340505] smp: Brought up 1 node, 1 CPU
[   15.340510] SMP: Total of 1 processors activated.
[   15.340521] CPU features: detected: 32-bit EL0 Support
[   15.340531] CPU features: detected: CRC32 instructions
[   15.346286] CPU: All CPU(s) started at EL2
[   15.346311] alternatives: patching kernel code
[   15.347526] devtmpfs: initialized
[   15.353004] clocksource: jiffies: mask: 0xffffffff max_cycles: 0xffffffff, max_idle_ns: 7645041785100000 ns
[   15.353025] futex hash table entries: 1024 (order: 4, 65536 bytes, linear)
[   15.354219] pinctrl core: initialized pinctrl subsystem
[   15.355443] DMI not present or invalid.
[   15.355817] NET: Registered protocol family 16
[   15.357162] DMA: preallocated 256 KiB pool for atomic allocations
[   15.357176] audit: initializing netlink subsys (disabled)
[   15.358361] cpuidle: using governor menu
[   15.358626] hw-breakpoint: found 6 breakpoint and 4 watchpoint registers.
[   15.359812] Serial: AMBA PL011 UART driver
[   15.364601] audit: type=2000 audit(15.344:1): state=initialized audit_enabled=0 res=1
[   15.380908] HugeTLB registered 1.00 GiB page size, pre-allocated 0 pages
[   15.380920] HugeTLB registered 32.0 MiB page size, pre-allocated 0 pages
[   15.380926] HugeTLB registered 2.00 MiB page size, pre-allocated 0 pages
[   15.380933] HugeTLB registered 64.0 KiB page size, pre-allocated 0 pages
[   15.388682] cryptd: max_cpu_qlen set to 1000
[   15.401330] ACPI: Interpreter disabled.
[   15.404770] iommu: Default domain type: Translated 
[   15.404955] vgaarb: loaded
[   15.405266] SCSI subsystem initialized
[   15.408762] libata version 3.00 loaded.
[   15.409025] usbcore: registered new interface driver usbfs
[   15.409065] usbcore: registered new interface driver hub
[   15.409108] usbcore: registered new device driver usb
[   15.409680] pps_core: LinuxPPS API ver. 1 registered
[   15.409686] pps_core: Software ver. 5.3.6 - Copyright 2005-2007 Rodolfo Giometti <giometti@linux.it>
[   15.409701] PTP clock support registered
[   15.409802] EDAC MC: Ver: 3.0.0
[   15.416828] FPGA manager framework
[   15.416910] Advanced Linux Sound Architecture Driver Initialized.
[   15.417650] clocksource: Switched to clocksource arch_sys_counter
[   15.417813] VFS: Disk quotas dquot_6.6.0
[   15.417869] VFS: Dquot-cache hash table entries: 512 (order 0, 4096 bytes)
[   15.418051] pnp: PnP ACPI: disabled
[   15.423643] thermal_sys: Registered thermal governor 'step_wise'
[   15.423647] thermal_sys: Registered thermal governor 'power_allocator'
[   15.423942] NET: Registered protocol family 2
[   15.424316] tcp_listen_portaddr_hash hash table entries: 512 (order: 1, 8192 bytes, linear)
[   15.424342] TCP established hash table entries: 8192 (order: 4, 65536 bytes, linear)
[   15.424417] TCP bind hash table entries: 8192 (order: 5, 131072 bytes, linear)
[   15.424565] TCP: Hash tables configured (established 8192 bind 8192)
[   15.424669] UDP hash table entries: 512 (order: 2, 16384 bytes, linear)
[   15.424699] UDP-Lite hash table entries: 512 (order: 2, 16384 bytes, linear)
[   15.424834] NET: Registered protocol family 1
[   15.437311] RPC: Registered named UNIX socket transport module.
[   15.437317] RPC: Registered udp transport module.
[   15.437322] RPC: Registered tcp transport module.
[   15.437326] RPC: Registered tcp NFSv4.1 backchannel transport module.
[   15.437338] PCI: CLS 0 bytes, default 64
[   15.438134] hw perfevents: enabled with armv8_cortex_a53 PMU driver, 7 counters available
[   15.438351] kvm [1]: IPA Size Limit: 40bits
[   15.439050] kvm [1]: vgic interrupt IRQ1
[   15.439134] kvm [1]: Hyp mode initialized successfully
[   15.445931] Initialise system trusted keyrings
[   15.446070] workingset: timestamp_bits=44 max_order=18 bucket_order=0
[   15.453479] squashfs: version 4.0 (2009/01/31) Phillip Lougher
[   15.458294] NFS: Registering the id_resolver key type
[   15.458319] Key type id_resolver registered
[   15.458323] Key type id_legacy registered
[   15.458335] nfs4filelayout_init: NFSv4 File Layout Driver Registering...
[   15.458518] 9p: Installing v9fs 9p2000 file system support
[   15.474148] Key type asymmetric registered
[   15.474156] Asymmetric key parser 'x509' registered
[   15.474191] Block layer SCSI generic (bsg) driver version 0.4 loaded (major 245)
[   15.474198] io scheduler mq-deadline registered
[   15.474204] io scheduler kyber registered
[   15.475689] sun50i-de2-bus 1000000.bus: Error couldn't map SRAM to device
[   15.476383] sun4i-usb-phy 1c19400.phy: failed to get clock usb0_phy
[   15.482035] sun50i-a64-r-pinctrl 1f02c00.pinctrl: initialized sunXi PIO driver
[   15.489405] EINJ: ACPI disabled.
[   15.502212] Serial: 8250/16550 driver, 4 ports, IRQ sharing enabled
[   15.504391] SuperH (H)SCI(F) driver initialized
[   15.504889] msm_serial: driver initialized
[   15.513007] loop: module loaded
[   15.517237] libphy: Fixed MDIO Bus: probed
[   15.517519] tun: Universal TUN/TAP device driver, 1.6
[   15.518323] thunder_xcv, ver 1.0
[   15.518361] thunder_bgx, ver 1.0
[   15.518396] nicpf, ver 1.0
[   15.518923] hclge is initializing
[   15.518930] hns3: Hisilicon Ethernet Network Driver for Hip08 Family - version
[   15.518935] hns3: Copyright (c) 2017 Huawei Corporation.
[   15.518990] e1000e: Intel(R) PRO/1000 Network Driver - 3.2.6-k
[   15.518994] e1000e: Copyright(c) 1999 - 2015 Intel Corporation.
[   15.519031] igb: Intel(R) Gigabit Ethernet Network Driver - version 5.6.0-k
[   15.519036] igb: Copyright (c) 2007-2014 Intel Corporation.
[   15.519073] igbvf: Intel(R) Gigabit Virtual Function Network Driver - version 2.4.0-k
[   15.519079] igbvf: Copyright (c) 2009 - 2012 Intel Corporation.
[   15.519367] sky2: driver version 1.30
[   15.519963] VFIO - User Level meta-driver version: 0.3
[   15.525385] ehci_hcd: USB 2.0 'Enhanced' Host Controller (EHCI) Driver
[   15.525393] ehci-pci: EHCI PCI platform driver
[   15.525431] ehci-platform: EHCI generic platform driver
[   15.525709] ehci-platform 1c1a000.usb: EHCI Host Controller
[   15.525733] ehci-platform 1c1a000.usb: new USB bus registered, assigned bus number 1
[   15.525833] ehci-platform 1c1a000.usb: irq 17, io mem 0x01c1a000
[   15.541660] ehci-platform 1c1a000.usb: USB 2.0 started, EHCI 1.00
[   15.542245] hub 1-0:1.0: USB hub found
[   15.542273] hub 1-0:1.0: 1 port detected
[   15.542788] ehci-orion: EHCI orion driver
[   15.542933] ehci-exynos: EHCI EXYNOS driver
[   15.543017] ohci_hcd: USB 1.1 'Open' Host Controller (OHCI) Driver
[   15.543039] ohci-pci: OHCI PCI platform driver
[   15.543101] ohci-platform: OHCI generic platform driver
[   15.543243] ohci-platform 1c1a400.usb: Generic Platform OHCI controller
[   15.543262] ohci-platform 1c1a400.usb: new USB bus registered, assigned bus number 2
[   15.543356] ohci-platform 1c1a400.usb: irq 18, io mem 0x01c1a400
[   15.606146] hub 2-0:1.0: USB hub found
[   15.606173] hub 2-0:1.0: 1 port detected
[   15.606637] ohci-exynos: OHCI EXYNOS driver
[   15.607181] usbcore: registered new interface driver usb-storage
[   15.610102] sun6i-rtc 1f00000.rtc: registered as rtc0
[   15.610111] sun6i-rtc 1f00000.rtc: RTC enabled
[   15.610335] i2c /dev entries driver
[   15.614705] sdhci: Secure Digital Host Controller Interface driver
[   15.614711] sdhci: Copyright(c) Pierre Ossman
[   15.615069] Synopsys Designware Multimedia Card Interface Driver
[   15.615953] sdhci-pltfm: SDHCI platform and OF driver helper
[   15.617242] ledtrig-cpu: registered to indicate activity on CPUs
[   15.618422] usbcore: registered new interface driver usbhid
[   15.618428] usbhid: USB HID core driver
[   15.624090] NET: Registered protocol family 17
[   15.624260] 9pnet: Installing 9P2000 support
[   15.624313] Key type dns_resolver registered
[   15.624694] registered taskstats version 1
[   15.624700] Loading compiled-in X.509 certificates
[   15.637906] sun50i-a64-r-pinctrl 1f02c00.pinctrl: 1f02c00.pinctrl supply vcc-pl not found, using dummy regulator
[   15.638123] sunxi-rsb 1f03400.rsb: RSB running at 3030303 Hz
[   15.638499] axp20x-rsb sunxi-rsb-3a3: AXP20x variant AXP803 found
[   15.650253] dcdc1: supplied by regulator-dummy
[   15.650589] dcdc2: supplied by regulator-dummy
[   15.650808] dcdc4: supplied by regulator-dummy
[   15.651048] dcdc5: supplied by regulator-dummy
[   15.651111] vcc-dram: Bringing 1500000uV into 1360000-1360000uV
[   15.651308] dcdc6: supplied by regulator-dummy
[   15.651509] dc1sw: supplied by regulator-dummy
[   15.651697] aldo1: supplied by regulator-dummy
[   15.651903] aldo2: supplied by regulator-dummy
[   15.652139] aldo3: supplied by regulator-dummy
[   15.652380] dldo1: supplied by regulator-dummy
[   15.652598] dldo2: supplied by regulator-dummy
[   15.652814] dldo3: supplied by regulator-dummy
[   15.653048] dldo4: supplied by regulator-dummy
[   15.653277] eldo1: supplied by regulator-dummy
[   15.653506] eldo2: supplied by regulator-dummy
[   15.653768] eldo3: supplied by regulator-dummy
[   15.653995] fldo1: supplied by regulator-dummy
[   15.654231] fldo2: supplied by regulator-dummy
[   15.654484] rtc-ldo: supplied by regulator-dummy
[   15.654666] ldo-io0: supplied by regulator-dummy
[   15.654876] ldo-io1: supplied by regulator-dummy
[   15.655073] axp20x-rsb sunxi-rsb-3a3: AXP20X driver loaded
[   15.659737] sun50i-a64-pinctrl 1c20800.pinctrl: initialized sunXi PIO driver
[   15.660680] sun50i-a64-pinctrl 1c20800.pinctrl: 1c20800.pinctrl supply vcc-pb not found, using dummy regulator
[   15.661132] printk: console [ttyS0] disabled
[   15.681709] 1c28000.serial: ttyS0 at MMIO 0x1c28000 (irq = 26, base_baud = 1500000) is a U6_16550A
[   16.928325] printk: console [ttyS0] enabled
[   16.934096] ehci-platform 1c1b000.usb: EHCI Host Controller
[   16.939689] ehci-platform 1c1b000.usb: new USB bus registered, assigned bus number 3
[   16.947533] ehci-platform 1c1b000.usb: irq 19, io mem 0x01c1b000
[   16.965665] ehci-platform 1c1b000.usb: USB 2.0 started, EHCI 1.00
[   16.972292] hub 3-0:1.0: USB hub found
[   16.976092] hub 3-0:1.0: 1 port detected
[   16.981558] ohci-platform 1c1b400.usb: Generic Platform OHCI controller
[   16.988223] ohci-platform 1c1b400.usb: new USB bus registered, assigned bus number 4
[   16.996091] ohci-platform 1c1b400.usb: irq 20, io mem 0x01c1b400
[   17.066206] hub 4-0:1.0: USB hub found
[   17.069997] hub 4-0:1.0: 1 port detected
[   17.075561] usb_phy_generic usb_phy_generic.0.auto: usb_phy_generic.0.auto supply vcc not found, using dummy regulator
[   17.086998] musb-hdrc musb-hdrc.1.auto: MUSB HDRC host driver
[   17.092774] musb-hdrc musb-hdrc.1.auto: new USB bus registered, assigned bus number 5
[   17.101105] hub 5-0:1.0: USB hub found
[   17.104901] hub 5-0:1.0: 1 port detected
[   17.111036] sun50i-a64-pinctrl 1c20800.pinctrl: 1c20800.pinctrl supply vcc-ph not found, using dummy regulator
[   17.123109] sun50i-a64-pinctrl 1c20800.pinctrl: 1c20800.pinctrl supply vcc-pf not found, using dummy regulator
[   17.133728] sunxi-mmc 1c0f000.mmc: Got CD GPIO
[   17.163428] sunxi-mmc 1c0f000.mmc: initialized, max. request size: 16384 KB, uses new timings mode
[   17.172748] sun6i-rtc 1f00000.rtc: setting system clock to 1970-01-01T00:01:49 UTC (109)
[   17.181341] ALSA device list:
[   17.184340]   No soundcards found.
[   17.188171] VFS: Cannot open root device "mmcblk0p2" or unknown-block(0,0): error -6
[   17.195947] Please append a correct "root=" boot option; here are the available partitions:
[   17.204326] Kernel panic - not syncing: VFS: Unable to mount root fs on unknown-block(0,0)
[   17.212579] CPU: 0 PID: 1 Comm: swapper/0 Not tainted 5.4.0-rc3 #2
[   17.218748] Hardware name: Pine64+ (DT)
[   17.222577] Call trace:
[   17.225026]  dump_backtrace+0x0/0x150
[   17.228685]  show_stack+0x14/0x20
[   17.231997]  dump_stack+0xb0/0xf8
[   17.235308]  panic+0x16c/0x37c
[   17.238360]  mount_block_root+0x1b0/0x2a8
[   17.242362]  mount_root+0x144/0x178
[   17.245845]  prepare_namespace+0x138/0x19c
[   17.249935]  kernel_init_freeable+0x22c/0x24c
[   17.254287]  kernel_init+0x10/0x108
[   17.257772]  ret_from_fork+0x10/0x18
[   17.261348] Kernel Offset: disabled
[   17.264831] CPU features: 0x0002,24002004
[   17.268832] Memory Limit: none
[   17.271888] ---[ end Kernel panic - not syncing: VFS: Unable to mount root fs on unknown-block(0,0) ]---

The below line

Please append a correct "root=" boot option; here are the available partitions:

doesn't display any partition. What could be the reason?

mmc drivers are selected in the config as below CONFIG_MMC_BLOCK=y and CONFIG_MMC_SUNXI=y

What am i doing wrong?

Answer 1

Try adding the kernel command line option rootwait to bootargs so that the kernel does not immediately give-up and instead waits forever hoping the root filesystem appears. It might be that kernel did not wait long enough during boot-up for the SD card to appear.

Answer 2

[mmc]: MMC Device 2 not found
[mmc]: mmc 2 not find, so not exit
...
Kernel command line: console=ttyS0,115200 earlyprintk ignore_loglevel root=/dev/mmcblk0p2 rootfstype=ext4 rw
...
VFS: Cannot open root device "mmcblk0p2" or unknown-block(0,0): error -6

It is possible that you need to delay and rootwait may solve it. However, this is the exact situation that initramfs is attempting to solve and is at least a 'debug' path for you. If you create an initramfs image and attach it to the Linux binary you can use a shell to probe Linux's partition layout and/or delay mounting the rootfs until it is ready. You can do other things in the initramfs like add a splash screen, parallel the USB infrastructure like module loading of detected devices with the rootfs mounting. This will make your boot faster as well as giving you a tool to diagnose booting.

It also look like u-boot can not see the /dev/mmcblk0p2 either. The fact that both u-boot and Linux seem to be complaining could indicate that something went wrong with the partition generation. It is definitely useful to compare boot logs versus a know working version as suggested by sawdust. For instance, timing/clocks, supplies or pinmux may not be set for the MMC and this can prevent the device from being recognized. It seems you have the driver, but not the device from the logs. Ie, no mmcblk0 anything. See: DTS mmc entries for some values that you may need to define for your board. The fact that Linux is found would seem to contradict this, but it would be useful to add the MMC device entries that the system is using. I would expect some output in the kernel logs. Another possibility is that something was built as a module and is expected to load to make the MMC accessible.

Getting a minimal initramfs will give you some shell tools to diagnose. You can use klibc or buildroot, etc to make a small initramfs that will let you look around the Linux device nodes, proc and sysfs value to diagnose things. Unfortunately, there are many things that may go wrong here. However, since you have Linux booting adding an initramfs is usually straight forward, even if you don't keep it for the final system.