Mount root from SD card?

Bernd Walter ticso at
Thu Jan 22 06:20:24 PST 2009

On Thu, Jan 22, 2009 at 03:22:28PM +0200, Alexander Motin wrote:
> Bernd Walter wrote:
> >On Wed, Jan 21, 2009 at 12:28:42PM -0700, M. Warner Losh wrote:
> >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.

Yes - and this is broken in the Atmel design.
This is not the only nasty bug that Atmel has left int he chip.
You need to issue the STOP command at the absolut right timing, which
is more or less impossible.
The design puts every 32bit word in a receive register, which has a small
You can setup a DMA enginge to pull the words from the register into RAM.
Now what happens is that if you issue the STOP too late it starts reading
the next block and then stops - the card can stop at every location, so
there is no problem about this, but now you have the first words in the
receive fifo.
The DMA doesn't work anymore, because it was setup with a limited number
of words to pull, so the additional words are kept in the register.
Once you start reading the next block and setup the DMA it pulls the
stuck words first.
It that case it looks as if the timing is one word too late therefor
you get 4 additonal bytes after each transaction.
It shouldn't be a problem to read the receive register without data in
it, so 4 or 5 dummy reads befor DMA setup should be enough.
IIRC the fifo can only hold up to 3 words.

B.Walter <bernd at>
Modbus/TCP Ethernet I/O Baugruppen, ARM basierte FreeBSD Rechner uvm.

More information about the freebsd-arm mailing list