Mount root from SD card?

[M. Warner Losh]

In message: <49760E8E.1000609 at FreeBSD.org>
            Alexander Motin <mav at FreeBSD.org> writes:
: Krassimir Slavchev wrote:
: > -----BEGIN PGP SIGNED MESSAGE-----
: > Hash: SHA1
: > 
: > M. Warner Losh wrote:
: > ...
: >> mmcsd0: 1983MB <SD Memory Card> at mmc0 30MHz/1bit
: >> Trying to mount root from ufs:/dev/mmcsd0s1
: >>
: >> Manual root filesystem specification:
: >>   <fstype>:<device>  Mount <device> using filesystem <fstype>
: >>                        eg. ufs:/dev/da0a
: >>   ?                  List valid disk boot devices
: >>   <empty line>       Abort manual input
: >>
: >> mountroot> ?
: >>
: >> List of GEOM managed disk devices:
: >>   mmcsd0
: >>
: >>> Looks like that should be working.
: >>> mav@ has done a lot of hacking on the mmc code...
: >>> Do you have 1 wire or 4 wires for your mmc bus on your board?
: > 
: > On the board all 4 bus wires are connected (MCD A0-A3) but I've never
: > seen working 4-bit mode on AT91RM9200 (See PR128987 too).
: 
: I have just committed MMCBR_IVAR_CAPS implementation into CURRENT. 
: Without having it implemented, results can be unpredictable. For 
: example, mmc layer could enable high-speed timings to reach 30MHz, but 
: this mode is not implemented for this controller. Booting with verbose 
: messages enabled could give a bit more information.

This controller's driver should do the right thing when given a too
high bus speed: clamp it to the maximum...

However, maybe the clamps are right.  The above symptom was what I'd
see when the data read in was corrupted.  It is the whole reason I
never enabled the multi-block read for this controller.  I could never
make it work well enough to even make mountroot happy.

: What's about 4-bit mode, I see some sc->wire4 variable checked by the 
: driver, which is never initialized. I don't very understand how this 
: thing expected to work.

It is initialized to zero.  It is expected that there will be a
different mechanism in the future to set it generically on a per-board
basis.

: PS: For MMC cards bus width testing routine implemented. May be we could 
: do something alike for SD cards. It is not part of SD specification, but 
: may be we could just issue some other command, transferring data, to 
: check effective bus width.

That would be cool.

Warner