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SubjectRe: SDIO drivers?
Maximus wrote:
> Hi,
> Just going through the linux kernel sources which has support for
> SD and MMC.
> Wondering whats the difference between SD and SDIO.
> What is meant by SDIO ?. (SD + IO) ?.
> what is meant by IO part?.
> Are any SDIO drivers open source?.

I found the PDF at of great use
for understanding the differences.

Basically - SD and MMC are not really 'normal' memory - due to their
connection, they act more like USB, or even network attached stuff.

At the top level that you'd want to present to the user, you've got the
logical SD and SDIO device.
This can be one of 8 functions in a SD/SDIO card.
A SDIO card is a SD-I/O card - it does input or output. An SD card is
just memory, with some area that may be encrypted - see the S of SD.

Now, there are many ways to connect to an SD(I'll omit the I/O from here
in). First, you've got the 'low speed' 400kbps serial mode - that is
common to MMC and SD, now you've got 20Mhz 1 bit serial that is SDs base
mode, and 4 bit 25Mhz that's the faster mode - there may have been
faster clocks released since the above doc - I haven't checkes.

Each card can now either be connected directly to the controller, or it
can be connected to a bus, sharing access between cards. This mode is of
course slower, and can slow all devices on the bus to the speed of the
slowest, and is hard to do well.

This is not a 'traditional' memory/IO device, it's more like USB, or IO
across the network.
For example, a single 8 byte write to a configuration register takes
about 64 clocks, which can be many tens of microseconds. In the slowest
mode, you can't even reliably run a 16550 (which is what one of the
standard class drivers - GPS - is defined as, a 16550 in register space)
at the highest baud rates, without losing chars.

So - briefly, an SD card can have up to 8 functions, with varying
bandwidth back to the host, depending on card and connection from a
shared 32KiB/s to unshared 25MiB/s. The controller can vary from
_really_ dumb - practically bit-banging, to one that does scatter-gather
DMA, and much of the protocol work for you.
AIUI, all of the controllers available do not expose the device as a
'simple' register/memory interface, but as a message passing interface -
with the exception of DMA.
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