external SD card failure to boot

James Cameron quozl at laptop.org
Tue Dec 7 18:26:38 EST 2010

New SD cards are appearing that are showing errors in OpenFirmware.
Using them for booting sometimes or always doesn't work.

These have a lower current draw than previous cards, or a response to
partial power down that is different to what we have previously

XO-1, XO-1.5 and XO-1.75 A1 hardware does not properly quench the power
pin for external SD cards.  This is fixed for XO-1.75 A2, where we
should need to wait only 1ms.

Therefore OpenFirmware must wait longer after powering down the card,
otherwise the card may not properly reset, and may not respond
afterwards until it is properly reset.  This is what causes the errors
or boot failure.

I'd like to know if anyone has any SD cards they've avoided using with
an XO-1.5, because of errors displayed by OpenFirmware, or because they
would not boot.  If so, I've a version of OpenFirmware that can be used
to test whether the errors were due to insufficient power off time.  The
previous power off time was 20ms.


With this firmware installed, the amount of time spent with the card
power off can be changed at the ok prompt:

	ok dev /sd d# 60 to power-off-time

Causing an error with one of these cards takes multiple accesses, which
is what boot does.  Reproducing this manually sometimes works with the
following command:

	ok dir ext:\security

... because one power up is needed for the root directory, and another
power up for the security directory.

Reminder: this only affects the external SD slot on XO-1, XO-1.5 and
XO-1.75 A1, and only where the card is present on laptop power up.

(I've no idea if there is any impact while Linux is running.  I can't
see any sign of a minimum power off time in sdhci_set_ios.)


James Cameron

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