BeagleBone Black MMC ordering clashes

Fri Jan 6 17:09:27 UTC 2017

On Fri, 2017-01-06 at 14:33 -0200, Dr. Rolf Jansen wrote:
> > 
> > Am 06.01.2017 um 13:54 schrieb Ian Lepore <ian at freebsd.org>:
> > 
> > On Fri, 2017-01-06 at 12:37 -0200, Dr. Rolf Jansen wrote:
> > > 
> > > 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
> > The mmcsd0 identifier will be assigned to the first mmc device
> > found
> > that has a valid mmc or sd card.  If  the external slot has a card
> > in
> > it, it will become mmcsd0 because it gets probed first.  If it has
> > no
> > card in it, the onboard eMMC becomes mmcsd0 because the sd slot
> > didn't
> > claim that device id.  There is no way to change the order of
> > probing;
> > probing happens in order of the devices in the chip.  The BBB
> > makers
> > decided to connect the external sd to the first device and the
> > onboard
> > eMMC to the second one.
> Thank you for rephrasing of what I wanted to say in my initial post:
> 
> > 
> > > 
> > > 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.
> So, yes, this part is absolutely clear and understood.
> 
> > 
> > If you want the eMMC to be the root filesystem whether there is an
> > external sd card installed or not, the only solution is to use UFS
> > or
> > GPT labels instead of device names to refer to partitions in fstab.
> I do not rely on device identifiers in fstab on non of my numerous
> FreeBSD boxes, instead, I use the various sorts of labels ever since,
> and I continue to do it also with the BBB installation(s).
> 
> # df -h
> Filesystem         Size    Used   Avail Capacity  Mounted on
> /dev/ufs/SYSTEM     13G    875M     11G     7%    /
> devfs              1.0K    1.0K      0B   100%    /dev
> /dev/label/BOOT    4.0M    924K    3.1M    23%    /boot/msdos
> tmpfs               50M    4.0K     50M     0%    /tmp
> 
> However, this does not help in the given case, because the MMC device
> driver refuses to enumerate the SD card when inserted into a BBB that
> has been already started-up from the internal flash. So the question
> is, how can I get the SD card activated once inserted into a running
> system, and I would happily live with any device identifier for it,
> even let it mmcsd77 if only the device would be accessible by this.
> 
> Best regards
> 
> Rolf

Oh, you mean you boot from eMMC without an sd card, then later when you
insert an sd card it's not detected?  I've never tried that (I've never
used the onboard eMMC at all), but it wouldn't surprise me that it's
broken.  It would imply we're not handling the card-detect properly,
probably because it's a gpio pin, and we didn't have good gpio support
back when I was working on the TI sdcard driver.

Yep, I just looked at the source, and there's no handling for gpio-
based sd card detect in the TI driver.  Maybe I can find some time this
weekend to add it, now that we have a better gpio infrastructure for
drivers to use.

-- Ian