4-bit SD Card mode

[Warner Losh]

From: Neil Bradley <nb at synthcom.com>
Subject: Re: 4-bit SD Card mode
Date: Thu, 7 Jun 2007 12:46:37 -0700 (PDT)

> > I'm not sure I like this at all.  The devices shouldn't be setting up
> > the pins for that device.  In some cases, they can't know how to do
> > that, and this is one of them.  The device has no way of knowing if
> > all 4 wires are connected, or just 1 and the 'spare' lines are used
> > for GPIO.
> 
> Not doing it will irritate more people (i.e. "I'm not getting full speed 
> out of the SD card). While it's a valid concern, I don't think it's 
> unrealistic to assume that all 4 DAT lines are hooked up. I've not yet 
> encountered a design that A) had SD capabilities, B) was only running 
> single bit because of a lack of GPIOs.

I'm not saying that we only hook up the one data.  I'm saying that we
force the boot loader to do it, or as a fallback a board specific
routine.  The driver shouldn't be in the business of doing it.

My hardware guys are somewhat agressive in their elimination of
"unused" lines.  We multiplex TX/RX from one of the serial ports to be
GPIO lines dynamically at run time depending on what add-in hardware
is connected to the box.  I want to keep things as flexible as
possible because I know that other designs are doing similar "icky"
things.

Warner