CMD19 -- The offical way to validate bus widths from the JEDEC spec
does not work on all platforms. Some platforms that use PCI/PCIe
to connect their SD controllers are known to fail.
If the quirk MMC_BUS_WIDTH_TEST is not defined we try to figure out
the bus width by reading the ext_csd at different bus widths and
compare this against the ext_csd read in 1 bit mode. If no ext_csd
is available we default to 1 bit operations.
Code has been tested on mmp2 against 8 bit eMMC and Transcend 2GB
card that is known to not work in 4 bit mode. The physical pins
on the card are not present to support 4 bit operation.
Signed-off-by: Philip Rakity <prakity@marvell.com>
Signed-off-by: Chris Ball <cjb@laptop.org>
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