1=================
2SPI userspace API
3=================
4
5SPI devices have a limited userspace API, supporting basic half-duplex
6read() and write() access to SPI slave devices.  Using ioctl() requests,
7full duplex transfers and device I/O configuration are also available.
8
9::
10
11	#include <fcntl.h>
12	#include <unistd.h>
13	#include <sys/ioctl.h>
14	#include <linux/types.h>
15	#include <linux/spi/spidev.h>
16
17Some reasons you might want to use this programming interface include:
18
19 * Prototyping in an environment that's not crash-prone; stray pointers
20   in userspace won't normally bring down any Linux system.
21
22 * Developing simple protocols used to talk to microcontrollers acting
23   as SPI slaves, which you may need to change quite often.
24
25Of course there are drivers that can never be written in userspace, because
26they need to access kernel interfaces (such as IRQ handlers or other layers
27of the driver stack) that are not accessible to userspace.
28
29
30DEVICE CREATION, DRIVER BINDING
31===============================
32
33The spidev driver contains lists of SPI devices that are supported for
34the different hardware topology representations.
35
36The following are the SPI device tables supported by the spidev driver:
37
38    - struct spi_device_id spidev_spi_ids[]: list of devices that can be
39      bound when these are defined using a struct spi_board_info with a
40      .modalias field matching one of the entries in the table.
41
42    - struct of_device_id spidev_dt_ids[]: list of devices that can be
43      bound when these are defined using a Device Tree node that has a
44      compatible string matching one of the entries in the table.
45
46    - struct acpi_device_id spidev_acpi_ids[]: list of devices that can
47      be bound when these are defined using a ACPI device object with a
48      _HID matching one of the entries in the table.
49
50You are encouraged to add an entry for your SPI device name to relevant
51tables, if these don't already have an entry for the device. To do that,
52post a patch for spidev to the linux-spi@vger.kernel.org mailing list.
53
54It used to be supported to define an SPI device using the "spidev" name.
55For example, as .modalias = "spidev" or compatible = "spidev".  But this
56is no longer supported by the Linux kernel and instead a real SPI device
57name as listed in one of the tables must be used.
58
59Not having a real SPI device name will lead to an error being printed and
60the spidev driver failing to probe.
61
62Sysfs also supports userspace driven binding/unbinding of drivers to
63devices that do not bind automatically using one of the tables above.
64To make the spidev driver bind to such a device, use the following:
65
66    echo spidev > /sys/bus/spi/devices/spiB.C/driver_override
67    echo spiB.C > /sys/bus/spi/drivers/spidev/bind
68
69When the spidev driver is bound to a SPI device, the sysfs node for the
70device will include a child device node with a "dev" attribute that will
71be understood by udev or mdev (udev replacement from BusyBox; it's less
72featureful, but often enough).
73
74For a SPI device with chipselect C on bus B, you should see:
75
76    /dev/spidevB.C ...
77	character special device, major number 153 with
78	a dynamically chosen minor device number.  This is the node
79	that userspace programs will open, created by "udev" or "mdev".
80
81    /sys/devices/.../spiB.C ...
82	as usual, the SPI device node will
83	be a child of its SPI master controller.
84
85    /sys/class/spidev/spidevB.C ...
86	created when the "spidev" driver
87	binds to that device.  (Directory or symlink, based on whether
88	or not you enabled the "deprecated sysfs files" Kconfig option.)
89
90Do not try to manage the /dev character device special file nodes by hand.
91That's error prone, and you'd need to pay careful attention to system
92security issues; udev/mdev should already be configured securely.
93
94If you unbind the "spidev" driver from that device, those two "spidev" nodes
95(in sysfs and in /dev) should automatically be removed (respectively by the
96kernel and by udev/mdev).  You can unbind by removing the "spidev" driver
97module, which will affect all devices using this driver.  You can also unbind
98by having kernel code remove the SPI device, probably by removing the driver
99for its SPI controller (so its spi_master vanishes).
100
101Since this is a standard Linux device driver -- even though it just happens
102to expose a low level API to userspace -- it can be associated with any number
103of devices at a time.  Just provide one spi_board_info record for each such
104SPI device, and you'll get a /dev device node for each device.
105
106
107BASIC CHARACTER DEVICE API
108==========================
109Normal open() and close() operations on /dev/spidevB.D files work as you
110would expect.
111
112Standard read() and write() operations are obviously only half-duplex, and
113the chipselect is deactivated between those operations.  Full-duplex access,
114and composite operation without chipselect de-activation, is available using
115the SPI_IOC_MESSAGE(N) request.
116
117Several ioctl() requests let your driver read or override the device's current
118settings for data transfer parameters:
119
120    SPI_IOC_RD_MODE, SPI_IOC_WR_MODE ...
121	pass a pointer to a byte which will
122	return (RD) or assign (WR) the SPI transfer mode.  Use the constants
123	SPI_MODE_0..SPI_MODE_3; or if you prefer you can combine SPI_CPOL
124	(clock polarity, idle high iff this is set) or SPI_CPHA (clock phase,
125	sample on trailing edge iff this is set) flags.
126	Note that this request is limited to SPI mode flags that fit in a
127	single byte.
128
129    SPI_IOC_RD_MODE32, SPI_IOC_WR_MODE32 ...
130	pass a pointer to a uin32_t
131	which will return (RD) or assign (WR) the full SPI transfer mode,
132	not limited to the bits that fit in one byte.
133
134    SPI_IOC_RD_LSB_FIRST, SPI_IOC_WR_LSB_FIRST ...
135	pass a pointer to a byte
136	which will return (RD) or assign (WR) the bit justification used to
137	transfer SPI words.  Zero indicates MSB-first; other values indicate
138	the less common LSB-first encoding.  In both cases the specified value
139	is right-justified in each word, so that unused (TX) or undefined (RX)
140	bits are in the MSBs.
141
142    SPI_IOC_RD_BITS_PER_WORD, SPI_IOC_WR_BITS_PER_WORD ...
143	pass a pointer to
144	a byte which will return (RD) or assign (WR) the number of bits in
145	each SPI transfer word.  The value zero signifies eight bits.
146
147    SPI_IOC_RD_MAX_SPEED_HZ, SPI_IOC_WR_MAX_SPEED_HZ ...
148	pass a pointer to a
149	u32 which will return (RD) or assign (WR) the maximum SPI transfer
150	speed, in Hz.  The controller can't necessarily assign that specific
151	clock speed.
152
153NOTES:
154
155    - At this time there is no async I/O support; everything is purely
156      synchronous.
157
158    - There's currently no way to report the actual bit rate used to
159      shift data to/from a given device.
160
161    - From userspace, you can't currently change the chip select polarity;
162      that could corrupt transfers to other devices sharing the SPI bus.
163      Each SPI device is deselected when it's not in active use, allowing
164      other drivers to talk to other devices.
165
166    - There's a limit on the number of bytes each I/O request can transfer
167      to the SPI device.  It defaults to one page, but that can be changed
168      using a module parameter.
169
170    - Because SPI has no low-level transfer acknowledgement, you usually
171      won't see any I/O errors when talking to a non-existent device.
172
173
174FULL DUPLEX CHARACTER DEVICE API
175================================
176
177See the spidev_fdx.c sample program for one example showing the use of the
178full duplex programming interface.  (Although it doesn't perform a full duplex
179transfer.)  The model is the same as that used in the kernel spi_sync()
180request; the individual transfers offer the same capabilities as are
181available to kernel drivers (except that it's not asynchronous).
182
183The example shows one half-duplex RPC-style request and response message.
184These requests commonly require that the chip not be deselected between
185the request and response.  Several such requests could be chained into
186a single kernel request, even allowing the chip to be deselected after
187each response.  (Other protocol options include changing the word size
188and bitrate for each transfer segment.)
189
190To make a full duplex request, provide both rx_buf and tx_buf for the
191same transfer.  It's even OK if those are the same buffer.
192