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