[U-Boot] [RFC] mmc: Properly determine maximum supported bus width

[Stephen Warren <swarren@wwwdotorg.org>]

On 10/31/2012 11:02 PM, Andy Fleming wrote:
> At some point, a confusion arose about the use of the bit
> definitions in host_caps for bus widths, and the value
> in ext_csd. By coincidence, a simple shift could convert
> between one and the other:
> 
> MMC_MODE_1BIT = 0, EXT_CSD_BUS_WIDTH_1 = 0
> MMC_MODE_4BIT = 0x100, EXT_CSD_BUS_WIDTH_4 = 1
> MMC_MODE_8BIT = 0x200, EXT_CSD_BUS_WIDTH_8 = 2
> 
> However, as host_caps is a bitmask of supported things,
> there is not, in fact, a one-to-one correspondence. host_caps
> is capable of containing MODE_4BIT | MODE_8BIT, so nonsensical
> things were happening where we would try to set the bus width
> to 12.
> 
> The new code clarifies the very different namespaces:
> 
> host_caps/card_caps = bitmask (MMC_MODE_*)
> ext CSD fields are just an index (EXT_CSD_BUS_WIDTH_*)
> mmc->bus_width integer number of bits (1, 4, 8)
> 
> We create arrays to map between the namespaces, like in Linux.

(Coupled with a small change to the Tegra driver to correctly set
host_caps for 8-/4-bit, which I'll post shortly),

Tested-by: Stephen Warren <swarren@nvidia.com>

Interestingly, I just yesterday started working on a bug I'd filed
months ago to enable 8-bit eMMC support on Tegra, and found the 12-bit
issue, and came up with a very similar patch. Oh well!