I'm having the following error message with my Dell E6440 laptop:
[    2.468473] mmc0: Unknown controller version (3). You may experience
problems.

Seems to be related to the SD card reader, however my SD card reader works
fine.

Some useful outputs:

$ lsmod | grep mmc
mmc_core              139264  3 sdhci,sdhci_acpi,sdhci_pc

$ dmesg | grep controller
[    2.468091] sdhci-pci 0000:0e:00.0: SDHCI controller found [1217:8520] (rev
1)
[    2.468473] mmc0: Unknown controller version (3). You may experience
problems.
[    2.472614] mmc0: SDHCI controller on PCI [0000:0e:00.0] using ADMA

- After inserting card:
$ dmesg | grep mmc
[    2.539618] mmc0: Unknown controller version (3). You may experience
problems.
[    2.539630] sdhci-pci 0000:0e:00.0: No vmmc regulator found
[    2.539632] sdhci-pci 0000:0e:00.0: No vqmmc regulator found
[    2.542498] mmc0: SDHCI controller on PCI [0000:0e:00.0] using ADMA
[    2.553335] sr 1:0:0:0: [sr0] scsi3-mmc drive: 24x/24x writer dvd-ram cd/rw
xa/form2 cdda tray
[    2.846323] mmc0: new SD card at address aaaa
[    6.379072] mmcblk0: mmc0:aaaa SU01G 942 MiB 
[    6.380567]  mmcblk0: p1

- After removing it
$ dmesg | grep mmc
[    2.539618] mmc0: Unknown controller version (3). You may experience
problems.
[    2.539630] sdhci-pci 0000:0e:00.0: No vmmc regulator found
[    2.539632] sdhci-pci 0000:0e:00.0: No vqmmc regulator found
[    2.542498] mmc0: SDHCI controller on PCI [0000:0e:00.0] using ADMA
[    2.553335] sr 1:0:0:0: [sr0] scsi3-mmc drive: 24x/24x writer dvd-ram cd/rw
xa/form2 cdda tray
[    2.846323] mmc0: new SD card at address aaaa
[    6.379072] mmcblk0: mmc0:aaaa SU01G 942 MiB 
[    6.380567]  mmcblk0: p1
[  260.519930] mmc0: card aaaa removed