[RFC] New MMC driver model

[Pierre Ossman]

I've been looking at how we can support SDIO cards and how we need to
adapt our driver model for it.

Functions
=========

First of all, we need to have multiple drivers for one physical card.
This is needed to handle both "combo cards" (mem and I/O) and because
SDIO cards can have seven distinct functions in one card.

For this I propose we add the concept of "function" and that each "card"
has 1 to 8 of these. The drivers then bind to these functions, not to
the card.

Identification
==============

SDIO uses the PCMCIA CIS structure for its generic fields. This includes
the CISTPL_MANFID tuple, which has one 16-bit value for manufacturer and
one 16-bit value for card id. The standard also has a special field for
"standard" interfaces, which are similar to PCI classes.

This scheme would allow us to handle storage cards quite nicely:

#define SDIO_ID_ANY                  0xFFFFFFFF

#define SDIO_VENDOR_STORAGE          0xFFFFFFFE
#define SDIO_DEVICE_ID_STORAGE_MMC   0x00000000
#define SDIO_DEVICE_ID_STORAGE_SD    0x00000001

(If the prefix makes the MMC layer a bit SDIO centric, feel free to come
with other suggestions)

Interrupts
==========

SDIO has generic interrupts that cards can use how they damn well
please. The interrupts are also level triggered and have the nice
"feature" of being active when there is no card in the slot.

So I propose the following:

 * We add a "interrupt enable" field to the ios structure so that hosts
know when a SDIO card has been inserted and card interrupts should be
caught.

 * When a interrupt is caught, the host driver masks it and tells the
MMC layer that a interrupt is pending. The MMC layer then calls a card
interrupt handler in some deferred manner (suggestions welcome).

 * When the card driver feels that it has handled the interrupt, it
calls a special acknowledge command that removes the mask the host has set.

Since SDIO cards can have seven distinct functions, there is a generic
register that tells which of the seven that currently has a pending
interrupt. This allows us to call only the relevant handlers.

The "interrupt pending" register also allows us to do a polled solution
for non-SDIO capable hosts. I'm unsure how to get a good balance between
latency and resource usage though.

Register functions
==================

I also intend to write a couple of register functions (sdio_read[bwl])
so that card drivers doesn't have to deal with MMC requests more than
necessary. Endianness can also be handled there (SDIO are always LE).


Comment away! :)

Rgds
Pierre
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