BeagleBone Black MMC ordering clashes

[Dr. Rolf Jansen]

The BeagleBone Black comes with 2 MMC facilities, one built-in (internal 4 GB) and one provided by a removable SD card. For unknown reasons the BBB defined on the removable SD card to be MMC0 and the internal flash memory is MMC0.

  1) In the first go, I dd'd the FreeBSD-12 BBB snapshot (20161221) onto a SD card and I was able to start the device from that card. gpart showed me that the device identifier of the SD card is mmcsd0 and that of the internal flash memory is mmcsd1. OK, that matches the already mentioned definitions.

  2) Now, I destroyed the partition of the internal mmcsd1 and created a new one:

=>     63  7552961  mmcsd1  MBR  (3.6G)
       63     8129          - free -  (4.0M)
     8192     8192       1  fat32  [active]  (4.0M)
    16384  7536640       2  freebsd  (3.6G)

=>      0  7536640  mmcsd1s2  BSD  (3.6G)
        0  7536640         1  freebsd-ufs  (3.6G)

  3) I copied over the contents of /boot/msdos from the external SD card to the msdosfs on mmcsd1s1 and I installed the FreeBSD 12 snapshot on mmcsd1s1a. Restarting the device from the internal flash and no external SD inserted with FreeBSD 12-CURRENT works well at the first glance.

  4) However, the internal flash got now the device identifier mmcsd0:

   ...
   mmc0: No compatible cards found on bus
   ...
   mmcsd0: 4GB <MMCHC MMC04G 5.8 SN 82390DEC MFG 11/1998 by 112 0x0000> at mmc1 48.0MHz/8bit/65535-block
   ...

I know, this is how FreeBSD deals with the device numbering, i.e. serial numbers starting at zero without particular meaning, and I know that we should not rely on the number ordering.

But it seems that the MMC device driver does cont on the external SD card is at MMC0 when I insert it into the BBB once it has been started from the internal flash. It seems to insist to assign the device ID mmcsd0, which results in the device ordering clash because mmcsd0 has been assigned to the internal flash at MMC1 (s. above).

In the moment, I can have both flash device active at the same time only when I start the BBB from the external SD.

I would be glad to hear suggestions on how to deal with the issue. At the end of the day, I want to start the device from the mostly static OS file system on the internal flash and keep the volatile data on the external SD.

Best regards

Rolf