Based on kernel version 3.9. Page generated on 2013-05-02 23:15 EST.
1 SPI devices have a limited userspace API, supporting basic half-duplex 2 read() and write() access to SPI slave devices. Using ioctl() requests, 3 full 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 11 Some 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 19 Of course there are drivers that can never be written in userspace, because 20 they need to access kernel interfaces (such as IRQ handlers or other layers 21 of the driver stack) that are not accessible to userspace. 22 23 24 DEVICE CREATION, DRIVER BINDING 25 =============================== 26 The simplest way to arrange to use this driver is to just list it in the 27 spi_board_info for a device as the driver it should use: the "modalias" 28 entry is "spidev", matching the name of the driver exposing this API. 29 Set up the other device characteristics (bits per word, SPI clocking, 30 chipselect polarity, etc) as usual, so you won't always need to override 31 them later. 32 33 (Sysfs also supports userspace driven binding/unbinding of drivers to 34 devices. That mechanism might be supported here in the future.) 35 36 When you do that, the sysfs node for the SPI device will include a child 37 device 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 39 busybox; it's less featureful, but often enough.) For a SPI device with 40 chipselect 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 53 Do not try to manage the /dev character device special file nodes by hand. 54 That's error prone, and you'd need to pay careful attention to system 55 security issues; udev/mdev should already be configured securely. 56 57 If 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 59 kernel and by udev/mdev). You can unbind by removing the "spidev" driver 60 module, which will affect all devices using this driver. You can also unbind 61 by having kernel code remove the SPI device, probably by removing the driver 62 for its SPI controller (so its spi_master vanishes). 63 64 Since this is a standard Linux device driver -- even though it just happens 65 to expose a low level API to userspace -- it can be associated with any number 66 of devices at a time. Just provide one spi_board_info record for each such 67 SPI device, and you'll get a /dev device node for each device. 68 69 70 BASIC CHARACTER DEVICE API 71 ========================== 72 Normal open() and close() operations on /dev/spidevB.D files work as you 73 would expect. 74 75 Standard read() and write() operations are obviously only half-duplex, and 76 the chipselect is deactivated between those operations. Full-duplex access, 77 and composite operation without chipselect de-activation, is available using 78 the SPI_IOC_MESSAGE(N) request. 79 80 Several ioctl() requests let your driver read or override the device's current 81 settings 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. 88 89 SPI_IOC_RD_LSB_FIRST, SPI_IOC_WR_LSB_FIRST ... pass a pointer to a byte 90 which will return (RD) or assign (WR) the bit justification used to 91 transfer SPI words. Zero indicates MSB-first; other values indicate 92 the less common LSB-first encoding. In both cases the specified value 93 is right-justified in each word, so that unused (TX) or undefined (RX) 94 bits are in the MSBs. 95 96 SPI_IOC_RD_BITS_PER_WORD, SPI_IOC_WR_BITS_PER_WORD ... pass a pointer to 97 a byte which will return (RD) or assign (WR) the number of bits in 98 each SPI transfer word. The value zero signifies eight bits. 99 100 SPI_IOC_RD_MAX_SPEED_HZ, SPI_IOC_WR_MAX_SPEED_HZ ... pass a pointer to a 101 u32 which will return (RD) or assign (WR) the maximum SPI transfer 102 speed, in Hz. The controller can't necessarily assign that specific 103 clock speed. 104 105 NOTES: 106 107 - At this time there is no async I/O support; everything is purely 108 synchronous. 109 110 - There's currently no way to report the actual bit rate used to 111 shift data to/from a given device. 112 113 - From userspace, you can't currently change the chip select polarity; 114 that could corrupt transfers to other devices sharing the SPI bus. 115 Each SPI device is deselected when it's not in active use, allowing 116 other drivers to talk to other devices. 117 118 - There's a limit on the number of bytes each I/O request can transfer 119 to the SPI device. It defaults to one page, but that can be changed 120 using a module parameter. 121 122 - Because SPI has no low-level transfer acknowledgement, you usually 123 won't see any I/O errors when talking to a non-existent device. 124 125 126 FULL DUPLEX CHARACTER DEVICE API 127 ================================ 128 129 See the spidev_fdx.c sample program for one example showing the use of the 130 full duplex programming interface. (Although it doesn't perform a full duplex 131 transfer.) The model is the same as that used in the kernel spi_sync() 132 request; the individual transfers offer the same capabilities as are 133 available to kernel drivers (except that it's not asynchronous). 134 135 The example shows one half-duplex RPC-style request and response message. 136 These requests commonly require that the chip not be deselected between 137 the request and response. Several such requests could be chained into 138 a single kernel request, even allowing the chip to be deselected after 139 each response. (Other protocol options include changing the word size 140 and bitrate for each transfer segment.) 141 142 To make a full duplex request, provide both rx_buf and tx_buf for the 143 same transfer. It's even OK if those are the same buffer.