XO-1 SD card access during boot-up

James Cameron quozl at laptop.org
Wed May 30 02:04:09 EDT 2012

On Wed, May 30, 2012 at 12:08:40AM +0200, Sascha Silbe wrote:
> James Cameron <quozl at laptop.org> writes:
> > The XO-1 SD card slot power supply circuit does not have a discharge
> > clamping function, and so when the firmware or software turns off the
> > power to the slot, the voltage falls slowly.
> >
> > The fall rate violates the specification for cards.
> [...]
> > When an SD card is used as the boot media, it remains powered, and so
> > one would typically not see this symptom.
> I understand this is only once Linux has taken over; OFW always powers
> down the card after each access?

Yes, after each access, where access is a directory search or a file
open and close.  That is, if a file is opened, the power is not
dropped until the file is closed.

> > Open Firmware briefly powers the SD card slot during boot, as part of
> > the search for an operating system to boot.
> [...]
> Would it be possible to avoid powering down the card after the first
> power-up in OFW?


> Or is a "software reset" (i.e. CMD0) insufficient for
> reinitialisation in Linux? (The standard suggests [2] it is, but actual
> cards may or may not be behave that way.)

I don't know.  Let me know if you test it.  We know that a power cycle
will reset a card.

> Would something like
> ' noop to card-power-off
> be sufficient or does OFW rely on the card getting powered down?

Open Firmware does not entirely rely on the card getting powered down,
in that on hardware that does not power the card down it works fine.

> I may have hit this problem again recently [1], if the SD card
> containing the image to be flashed indeed gets powered down
> intermittently during flashing.

It should not be powered down intermittently during flashing, but it
should be powered down at the end of flashing.

Regarding USB SD card readers in [1], these will not work with Open
Firmware if they present USB interfaces unlike a USB flash or hard
drive.  A card reader here has two interfaces, one of which is used by
the host to select which slot will be bound to the other interface.

James Cameron

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