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Andrea Paterniani814a8d52007-05-08 00:32:15 -07001SPI devices have a limited userspace API, supporting basic half-duplex
2read() and write() access to SPI slave devices. Using ioctl() requests,
3full duplex transfers and device I/O configuration are also available.
4
5 #include <fcntl.h>
6 #include <unistd.h>
7 #include <sys/ioctl.h>
8 #include <linux/types.h>
9 #include <linux/spi/spidev.h>
10
11Some reasons you might want to use this programming interface include:
12
13 * Prototyping in an environment that's not crash-prone; stray pointers
14 in userspace won't normally bring down any Linux system.
15
16 * Developing simple protocols used to talk to microcontrollers acting
17 as SPI slaves, which you may need to change quite often.
18
19Of course there are drivers that can never be written in userspace, because
20they need to access kernel interfaces (such as IRQ handlers or other layers
21of the driver stack) that are not accessible to userspace.
22
23
24DEVICE CREATION, DRIVER BINDING
25===============================
26The simplest way to arrange to use this driver is to just list it in the
27spi_board_info for a device as the driver it should use: the "modalias"
28entry is "spidev", matching the name of the driver exposing this API.
29Set up the other device characteristics (bits per word, SPI clocking,
30chipselect polarity, etc) as usual, so you won't always need to override
31them later.
32
33(Sysfs also supports userspace driven binding/unbinding of drivers to
34devices. That mechanism might be supported here in the future.)
35
36When you do that, the sysfs node for the SPI device will include a child
37device node with a "dev" attribute that will be understood by udev or mdev.
38(Larger systems will have "udev". Smaller ones may configure "mdev" into
39busybox; it's less featureful, but often enough.) For a SPI device with
40chipselect C on bus B, you should see:
41
42 /dev/spidevB.C ... character special device, major number 153 with
43 a dynamically chosen minor device number. This is the node
44 that userspace programs will open, created by "udev" or "mdev".
45
46 /sys/devices/.../spiB.C ... as usual, the SPI device node will
47 be a child of its SPI master controller.
48
49 /sys/class/spidev/spidevB.C ... created when the "spidev" driver
50 binds to that device. (Directory or symlink, based on whether
51 or not you enabled the "deprecated sysfs files" Kconfig option.)
52
53Do not try to manage the /dev character device special file nodes by hand.
54That's error prone, and you'd need to pay careful attention to system
55security issues; udev/mdev should already be configured securely.
56
57If you unbind the "spidev" driver from that device, those two "spidev" nodes
58(in sysfs and in /dev) should automatically be removed (respectively by the
59kernel and by udev/mdev). You can unbind by removing the "spidev" driver
60module, which will affect all devices using this driver. You can also unbind
61by having kernel code remove the SPI device, probably by removing the driver
62for its SPI controller (so its spi_master vanishes).
63
64Since this is a standard Linux device driver -- even though it just happens
65to expose a low level API to userspace -- it can be associated with any number
66of devices at a time. Just provide one spi_board_info record for each such
67SPI device, and you'll get a /dev device node for each device.
68
69
70BASIC CHARACTER DEVICE API
71==========================
72Normal open() and close() operations on /dev/spidevB.D files work as you
73would expect.
74
75Standard read() and write() operations are obviously only half-duplex, and
76the chipselect is deactivated between those operations. Full-duplex access,
77and composite operation without chipselect de-activation, is available using
78the SPI_IOC_MESSAGE(N) request.
79
80Several ioctl() requests let your driver read or override the device's current
81settings for data transfer parameters:
82
83 SPI_IOC_RD_MODE, SPI_IOC_WR_MODE ... pass a pointer to a byte which will
84 return (RD) or assign (WR) the SPI transfer mode. Use the constants
85 SPI_MODE_0..SPI_MODE_3; or if you prefer you can combine SPI_CPOL
86 (clock polarity, idle high iff this is set) or SPI_CPHA (clock phase,
87 sample on trailing edge iff this is set) flags.
Geert Uytterhoevendc64d392014-02-25 11:40:17 +010088 Note that this request is limited to SPI mode flags that fit in a
89 single byte.
90
91 SPI_IOC_RD_MODE32, SPI_IOC_WR_MODE32 ... pass a pointer to a uin32_t
92 which will return (RD) or assign (WR) the full SPI transfer mode,
93 not limited to the bits that fit in one byte.
Andrea Paterniani814a8d52007-05-08 00:32:15 -070094
95 SPI_IOC_RD_LSB_FIRST, SPI_IOC_WR_LSB_FIRST ... pass a pointer to a byte
96 which will return (RD) or assign (WR) the bit justification used to
97 transfer SPI words. Zero indicates MSB-first; other values indicate
98 the less common LSB-first encoding. In both cases the specified value
99 is right-justified in each word, so that unused (TX) or undefined (RX)
100 bits are in the MSBs.
101
102 SPI_IOC_RD_BITS_PER_WORD, SPI_IOC_WR_BITS_PER_WORD ... pass a pointer to
103 a byte which will return (RD) or assign (WR) the number of bits in
104 each SPI transfer word. The value zero signifies eight bits.
105
106 SPI_IOC_RD_MAX_SPEED_HZ, SPI_IOC_WR_MAX_SPEED_HZ ... pass a pointer to a
107 u32 which will return (RD) or assign (WR) the maximum SPI transfer
108 speed, in Hz. The controller can't necessarily assign that specific
109 clock speed.
110
111NOTES:
112
113 - At this time there is no async I/O support; everything is purely
114 synchronous.
115
116 - There's currently no way to report the actual bit rate used to
117 shift data to/from a given device.
118
119 - From userspace, you can't currently change the chip select polarity;
120 that could corrupt transfers to other devices sharing the SPI bus.
121 Each SPI device is deselected when it's not in active use, allowing
122 other drivers to talk to other devices.
123
124 - There's a limit on the number of bytes each I/O request can transfer
125 to the SPI device. It defaults to one page, but that can be changed
126 using a module parameter.
127
128 - Because SPI has no low-level transfer acknowledgement, you usually
129 won't see any I/O errors when talking to a non-existent device.
130
131
132FULL DUPLEX CHARACTER DEVICE API
133================================
134
Randy Dunlap31a16292008-04-28 02:14:18 -0700135See the spidev_fdx.c sample program for one example showing the use of the
136full duplex programming interface. (Although it doesn't perform a full duplex
Andrea Paterniani814a8d52007-05-08 00:32:15 -0700137transfer.) The model is the same as that used in the kernel spi_sync()
138request; the individual transfers offer the same capabilities as are
139available to kernel drivers (except that it's not asynchronous).
140
141The example shows one half-duplex RPC-style request and response message.
142These requests commonly require that the chip not be deselected between
143the request and response. Several such requests could be chained into
144a single kernel request, even allowing the chip to be deselected after
145each response. (Other protocol options include changing the word size
146and bitrate for each transfer segment.)
147
148To make a full duplex request, provide both rx_buf and tx_buf for the
149same transfer. It's even OK if those are the same buffer.