spi: docs: convert to ReST and add it to the kABI bookset

While there's one file there with briefily describes the uAPI,
the documentation was written just like most subsystems: focused
on kernel developers. So, add it together with driver-api books.

Signed-off-by: Mauro Carvalho Chehab <mchehab+samsung@kernel.org>
Acked-by: Jonathan Cameron <Jonathan.Cameron@huawei.com> # for iio
Signed-off-by: Jonathan Corbet <corbet@lwn.net>

1 files changed, 74 insertions, 0 deletions

diff --git a/Documentation/spi/butterfly.rst b/Documentation/spi/butterfly.rst
new file mode 100644
index 000000000000..e614a589547c
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+++ b/Documentation/spi/butterfly.rst
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+===================================================
+spi_butterfly - parport-to-butterfly adapter driver
+===================================================
+
+This is a hardware and software project that includes building and using
+a parallel port adapter cable, together with an "AVR Butterfly" to run
+firmware for user interfacing and/or sensors.  A Butterfly is a $US20
+battery powered card with an AVR microcontroller and lots of goodies:
+sensors, LCD, flash, toggle stick, and more.  You can use AVR-GCC to
+develop firmware for this, and flash it using this adapter cable.
+
+You can make this adapter from an old printer cable and solder things
+directly to the Butterfly.  Or (if you have the parts and skills) you
+can come up with something fancier, providing ciruit protection to the
+Butterfly and the printer port, or with a better power supply than two
+signal pins from the printer port.  Or for that matter, you can use
+similar cables to talk to many AVR boards, even a breadboard.
+
+This is more powerful than "ISP programming" cables since it lets kernel
+SPI protocol drivers interact with the AVR, and could even let the AVR
+issue interrupts to them.  Later, your protocol driver should work
+easily with a "real SPI controller", instead of this bitbanger.
+
+
+The first cable connections will hook Linux up to one SPI bus, with the
+AVR and a DataFlash chip; and to the AVR reset line.  This is all you
+need to reflash the firmware, and the pins are the standard Atmel "ISP"
+connector pins (used also on non-Butterfly AVR boards).  On the parport
+side this is like "sp12" programming cables.
+
+	======	  =============	  ===================
+	Signal	  Butterfly	  Parport (DB-25)
+	======	  =============	  ===================
+	SCK	  J403.PB1/SCK	  pin 2/D0
+	RESET	  J403.nRST	  pin 3/D1
+	VCC	  J403.VCC_EXT	  pin 8/D6
+	MOSI	  J403.PB2/MOSI	  pin 9/D7
+	MISO	  J403.PB3/MISO	  pin 11/S7,nBUSY
+	GND	  J403.GND	  pin 23/GND
+	======	  =============	  ===================
+
+Then to let Linux master that bus to talk to the DataFlash chip, you must
+(a) flash new firmware that disables SPI (set PRR.2, and disable pullups
+by clearing PORTB.[0-3]); (b) configure the mtd_dataflash driver; and
+(c) cable in the chipselect.
+
+	======	  ============	  ===================
+	Signal	  Butterfly	  Parport (DB-25)
+	======	  ============	  ===================
+	VCC	  J400.VCC_EXT	  pin 7/D5
+	SELECT	  J400.PB0/nSS	  pin 17/C3,nSELECT
+	GND	  J400.GND	  pin 24/GND
+	======	  ============	  ===================
+
+Or you could flash firmware making the AVR into an SPI slave (keeping the
+DataFlash in reset) and tweak the spi_butterfly driver to make it bind to
+the driver for your custom SPI-based protocol.
+
+The "USI" controller, using J405, can also be used for a second SPI bus.
+That would let you talk to the AVR using custom SPI-with-USI firmware,
+while letting either Linux or the AVR use the DataFlash.  There are plenty
+of spare parport pins to wire this one up, such as:
+
+	======	  =============	  ===================
+	Signal	  Butterfly	  Parport (DB-25)
+	======	  =============	  ===================
+	SCK	  J403.PE4/USCK	  pin 5/D3
+	MOSI	  J403.PE5/DI	  pin 6/D4
+	MISO	  J403.PE6/DO	  pin 12/S5,nPAPEROUT
+	GND	  J403.GND	  pin 22/GND
+
+	IRQ	  J402.PF4	  pin 10/S6,ACK
+	GND	  J402.GND(P2)	  pin 25/GND
+	======	  =============	  ===================