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, 84 insertions, 0 deletions

diff --git a/Documentation/spi/spi-lm70llp.rst b/Documentation/spi/spi-lm70llp.rst
new file mode 100644
index 000000000000..07631aef4343
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+++ b/Documentation/spi/spi-lm70llp.rst
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+==============================================
+spi_lm70llp :  LM70-LLP parport-to-SPI adapter
+==============================================
+
+Supported board/chip:
+
+  * National Semiconductor LM70 LLP evaluation board
+
+    Datasheet: http://www.national.com/pf/LM/LM70.html
+
+Author:
+        Kaiwan N Billimoria <kaiwan@designergraphix.com>
+
+Description
+-----------
+This driver provides glue code connecting a National Semiconductor LM70 LLP
+temperature sensor evaluation board to the kernel's SPI core subsystem.
+
+This is a SPI master controller driver. It can be used in conjunction with
+(layered under) the LM70 logical driver (a "SPI protocol driver").
+In effect, this driver turns the parallel port interface on the eval board
+into a SPI bus with a single device, which will be driven by the generic
+LM70 driver (drivers/hwmon/lm70.c).
+
+
+Hardware Interfacing
+--------------------
+The schematic for this particular board (the LM70EVAL-LLP) is
+available (on page 4) here:
+
+  http://www.national.com/appinfo/tempsensors/files/LM70LLPEVALmanual.pdf
+
+The hardware interfacing on the LM70 LLP eval board is as follows:
+
+   ======== == =========   ==========
+   Parallel                 LM70 LLP
+     Port   .  Direction   JP2 Header
+   ======== == =========   ==========
+      D0     2      -         -
+      D1     3     -->      V+   5
+      D2     4     -->      V+   5
+      D3     5     -->      V+   5
+      D4     6     -->      V+   5
+      D5     7     -->      nCS  8
+      D6     8     -->      SCLK 3
+      D7     9     -->      SI/O 5
+     GND    25      -       GND  7
+    Select  13     <--      SI/O 1
+   ======== == =========   ==========
+
+Note that since the LM70 uses a "3-wire" variant of SPI, the SI/SO pin
+is connected to both pin D7 (as Master Out) and Select (as Master In)
+using an arrangement that lets either the parport or the LM70 pull the
+pin low.  This can't be shared with true SPI devices, but other 3-wire
+devices might share the same SI/SO pin.
+
+The bitbanger routine in this driver (lm70_txrx) is called back from
+the bound "hwmon/lm70" protocol driver through its sysfs hook, using a
+spi_write_then_read() call.  It performs Mode 0 (SPI/Microwire) bitbanging.
+The lm70 driver then inteprets the resulting digital temperature value
+and exports it through sysfs.
+
+A "gotcha": National Semiconductor's LM70 LLP eval board circuit schematic
+shows that the SI/O line from the LM70 chip is connected to the base of a
+transistor Q1 (and also a pullup, and a zener diode to D7); while the
+collector is tied to VCC.
+
+Interpreting this circuit, when the LM70 SI/O line is High (or tristate
+and not grounded by the host via D7), the transistor conducts and switches
+the collector to zero, which is reflected on pin 13 of the DB25 parport
+connector.  When SI/O is Low (driven by the LM70 or the host) on the other
+hand, the transistor is cut off and the voltage tied to it's collector is
+reflected on pin 13 as a High level.
+
+So: the getmiso inline routine in this driver takes this fact into account,
+inverting the value read at pin 13.
+
+
+Thanks to
+---------
+
+- David Brownell for mentoring the SPI-side driver development.
+- Dr.Craig Hollabaugh for the (early) "manual" bitbanging driver version.
+- Nadir Billimoria for help interpreting the circuit schematic.