Mount root from SD card?

[M. Warner Losh]

In message: <49787314.3070004 at FreeBSD.org>
            Alexander Motin <mav at freebsd.org> writes:
: Bernd Walter wrote:
: > On Wed, Jan 21, 2009 at 12:28:42PM -0700, M. Warner Losh wrote:
: >> In message: <49776734.8030805 at FreeBSD.org>
: >>             Alexander Motin <mav at FreeBSD.org> writes:
: >> : 
: >> : This part looks quire strange for plain FIFO explanation. Several 
: >> : consequential commands give different results:
: >> : 
: >> : CMD: 37 ARG 10000 len 0
: >> : RES: 0
: >> : CMD: d ARG 0 len 64
: >> : 
: >> : ff ff ff ff ff ff ff ff 00 00 00 00 00 00 00 28
: >> : 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
: >> : 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
: >> : 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 ...
: >> : RES: 2
: >> : CMD: 37 ARG 10000 len 0
: >> : RES: 0
: >> : CMD: d ARG 0 len 64
: >> : 
: >> : ff ff ff ff ff ff ff ff ff ff ff ff 00 00 00 00
: >> : 00 00 00 28 00 00 00 00 00 00 00 00 00 00 00 00
: >> : 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
: >> : 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 ...
: >> : RES: 2
: >> : CMD: 37 ARG 10000 len 0
: >> : RES: 0
: >> : CMD: d ARG 0 len 64
: >> : 
: >> : ff ff ff ff 00 00 00 00 00 00 00 28 00 00 00 00
: >> : 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
: >> : 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
: >> : 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 ...
: >> : RES: 2
: >> : CMD: 37 ARG 10000 len 0
: >> : RES: 0
: >> : CMD: d ARG 0 len 64
: >> : 
: >> : ff ff ff ff ff ff ff ff 00 00 00 00 00 00 00 28
: >> : 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
: >> : 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
: >> : 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 ...
: >> : RES: 2
: >> : 
: >> : While working on sdhci driver for on ENE chip I have found that for 
: >> : short transfers (less then 1K) DMA engine returns set of zeroes instead 
: >> : of data. I haven't found better solution and just handling short 
: >> : transfers by PIO. Same problem exists for PIO also, but there it was 
: >> : masked by adding short delay before reading from port.
: >>
: >> I'll have to take a look at the code in more detail to make sure that
: >> we're doing the right thing.  I noticed all the ff's, but didn't
: >> notice until now what they were shifted the same way that the data
: >> blocks were later.  In this case, you'll see there's three of them.
: >>
: >> I believe that this is the first use of a CMD that generates data that
: >> isn't a full block of data...
: > 
: > Havn't read everything, so maybe I writing nonsense in respect to
: > this problem.
: > The multiblock read trouble is a problem in the MMC design.
: > The first read always works fine, but you need to stop the transfer
: > and an exact time, otherwise it starts reading the next block.
: > The first bytes of the next block then stucks in the DMA fifo and
: > the next read then starts with the stuck uint32.
: > If you see broken reads like this then I would assume the previous
: > command was problematic.
: > The official workaround to get multiblock reading is to reset the MMC
: > and clean the fifo after each multiblock command, but I would assume
: > it is enough to just read the data register a few time before setting
: > up the DMA for the new request.
: 
: Haven't look deep at that arm controller operation, but standard SD host 
: has special counter register which terminates DMA transfer after 
: specified amount of blocks. sdhci driver even uses it's Auth-CMD12 
: feature, where controller even sends STOP command to the card by itself.

The standard host adapter spec has this.  The Atmel part doesn't.

Warner