SPI: Device API

Besides the transfer, we need an abstraction for the device
We may rely on the fact that read-only memory is cheaper
We must support various pin configurations

#define SPI_CS(x)               (-1 - (x)) /* arg is 0.., value is negative */
#define __SPI_IS_HW_CS(x)       ((x) < 0)
#define __SPI_GET_HW_CS(x)      (-(x) - 1)

/* This is an interface for SPI, master and slave */
struct spi_cfg {
        int gpio_cs;            /* Either GPIO number or SPI_CS(x) */
        int freq;               /* Suggested frequency */
        int timeout;            /* Jiffies */
        unsigned long flags;
        uint8_t pol, phase, bits, devn; /* devn selects spi0 or spi1 or more */
};

/* This would be opaque if we had malloc */
struct spi_dev {
        const   struct spi_cfg *cfg;
        unsigned long base;             int     current_freq;
};
extern struct spi_dev *spi_create(struct spi_dev *cfg);
extern void spi_destroy(struct spi_dev *dev);