support DUAL and QUAD[patch v1]

[thomas.langer at lantiq.com]

Hello Pekon,

Gupta, Pekon wrote on 2013-07-16:
> >
> > Hi, Gupta
> >
> >
> > > [Pekon]: Instead of adding new fields you can use existing 'mode' field to
> > > pass on the platform specific configurations. And if 'u8 mode' does not
> > > suffice you can increase it to 'u32'.
> > > #define QSPI_MODE       1 << 5; // just check which bit-fields are un-used
> > > spi_board_info->mode |= QSPI_MODE;
> > >
> >
> > well, can dual and quad be regarded as a spi mode? if so, your comment
> > seems
> > to be right.
> 
> Yes, Quad and Dual modes should be regarded as extension of SPI protocol.
> - They follow the same basic principle of synchronous data transfer. Right ?
> - These modes are not adding any extra side-band | In-band signaling or
> controls to modify the protocol. They are just increasing the width of
>  data-channel for better throughput.
> 
> with regards, pekon
> 

In general, yes. But I think, for the interface we have to take care of more details.

For example, what happens in the following situation:
We have a spi-controller, which supports the QSPI mode, and a spi-flash, which fulfils 
the requirements, but the board has not connected all signals?

And the interface for the slave-driver (like m25p80) should allow to specify the transfer mode
for each spi_message.
This will be necessary, because it depends on the flash and mode, how each phase of "cmd",
"address", and "data" will be transferred.

I don't know, if it is only me, but I would like to see some more abstract description for all 
these scenarios, before the details of an interface really could be discussed and decided.

Best Regards,
Thomas