spi: Add Analog Devices AXI SPI Engine controller support

This patch adds support for the AXI SPI Engine controller which is a FPGA
soft-peripheral which is used in some of Analog Devices' reference designs.

The AXI SPI Engine controller is part of the SPI Engine framework[1] and
allows memory mapped access to the SPI Engine control bus. This allows it
to be used as a general purpose software driven SPI controller. The SPI
Engine in addition offers some optional advanced acceleration and
offloading capabilities, which are not part of this patch though and will
be introduced separately.

At the core of the SPI Engine framework is a small sort of co-processor
that accepts a command stream and turns the commands into low-level SPI
transactions. Communication is done through three memory mapped FIFOs in
the register map of the AXI SPI Engine peripheral. One FIFO for the command
stream and one each for transmit and receive data.

The driver translates a spi_message in a command stream and writes it to
the peripheral which executes it asynchronously. This allows it to perform
very precise timings which are required for some SPI slave devices to
achieve maximum performance (e.g. analog-to-digital and digital-to-analog
converters). The execution flow is synchronized to the host system by a
special synchronize instruction which generates a interrupt.

[1] https://wiki.analog.com/resources/fpga/peripherals/spi_engine

Signed-off-by: Lars-Peter Clausen <lars@metafoo.de>
Signed-off-by: Mark Brown <broonie@kernel.org>

drivers/spi/Kconfig

@@ -83,6 +83,14 @@ config SPI_AU1550
 	  If you say yes to this option, support will be included for the
 	  PSC SPI controller found on Au1550, Au1200 and Au1300 series.
 
+config SPI_AXI_SPI_ENGINE
+	tristate "Analog Devices AXI SPI Engine controller"
+	depends on HAS_IOMEM
+	help
+	  This enables support for the Analog Devices AXI SPI Engine SPI controller.
+	  It is part of the SPI Engine framework that is used in some Analog Devices
+	  reference designs for FPGAs.
+
 config SPI_BCM2835
 	tristate "BCM2835 SPI controller"
 	depends on GPIOLIB

drivers/spi/Makefile

@@ -15,6 +15,7 @@ obj-$(CONFIG_SPI_ALTERA)		+= spi-altera.o
 obj-$(CONFIG_SPI_ATMEL)			+= spi-atmel.o
 obj-$(CONFIG_SPI_ATH79)			+= spi-ath79.o
 obj-$(CONFIG_SPI_AU1550)		+= spi-au1550.o
+obj-$(CONFIG_SPI_AXI_SPI_ENGINE)	+= spi-axi-spi-engine.o
 obj-$(CONFIG_SPI_BCM2835)		+= spi-bcm2835.o
 obj-$(CONFIG_SPI_BCM2835AUX)		+= spi-bcm2835aux.o
 obj-$(CONFIG_SPI_BCM53XX)		+= spi-bcm53xx.o