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1=================2SPI userspace API3=================4 5SPI devices have a limited userspace API, supporting basic half-duplex6read() 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 pointers20 in userspace won't normally bring down any Linux system.21 22 * Developing simple protocols used to talk to microcontrollers acting23 as SPI slaves, which you may need to change quite often.24 25Of course there are drivers that can never be written in userspace, because26they need to access kernel interfaces (such as IRQ handlers or other layers27of the driver stack) that are not accessible to userspace.28 29 30DEVICE CREATION, DRIVER BINDING31===============================32 33The spidev driver contains lists of SPI devices that are supported for34the 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 be39 bound when these are defined using a struct spi_board_info with a40 .modalias field matching one of the entries in the table.41 42 - struct of_device_id spidev_dt_ids[]: list of devices that can be43 bound when these are defined using a Device Tree node that has a44 compatible string matching one of the entries in the table.45 46 - struct acpi_device_id spidev_acpi_ids[]: list of devices that can47 be bound when these are defined using a ACPI device object with a48 _HID matching one of the entries in the table.49 50You are encouraged to add an entry for your SPI device name to relevant51tables, 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 this56is no longer supported by the Linux kernel and instead a real SPI device57name 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 and60the spidev driver failing to probe.61 62Sysfs also supports userspace driven binding/unbinding of drivers to63devices 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_override67 echo spiB.C > /sys/bus/spi/drivers/spidev/bind68 69When the spidev driver is bound to a SPI device, the sysfs node for the70device will include a child device node with a "dev" attribute that will71be understood by udev or mdev (udev replacement from BusyBox; it's less72featureful, 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 with78 a dynamically chosen minor device number. This is the node79 that userspace programs will open, created by "udev" or "mdev".80 81 /sys/devices/.../spiB.C ...82 as usual, the SPI device node will83 be a child of its SPI master controller.84 85 /sys/class/spidev/spidevB.C ...86 created when the "spidev" driver87 binds to that device. (Directory or symlink, based on whether88 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 system92security issues; udev/mdev should already be configured securely.93 94If you unbind the "spidev" driver from that device, those two "spidev" nodes95(in sysfs and in /dev) should automatically be removed (respectively by the96kernel and by udev/mdev). You can unbind by removing the "spidev" driver97module, which will affect all devices using this driver. You can also unbind98by having kernel code remove the SPI device, probably by removing the driver99for its SPI controller (so its spi_master vanishes).100 101Since this is a standard Linux device driver -- even though it just happens102to expose a low level API to userspace -- it can be associated with any number103of devices at a time. Just provide one spi_board_info record for each such104SPI device, and you'll get a /dev device node for each device.105 106 107BASIC CHARACTER DEVICE API108==========================109Normal open() and close() operations on /dev/spidevB.D files work as you110would expect.111 112Standard read() and write() operations are obviously only half-duplex, and113the chipselect is deactivated between those operations. Full-duplex access,114and composite operation without chipselect de-activation, is available using115the SPI_IOC_MESSAGE(N) request.116 117Several ioctl() requests let your driver read or override the device's current118settings for data transfer parameters:119 120 SPI_IOC_RD_MODE, SPI_IOC_WR_MODE ...121 pass a pointer to a byte which will122 return (RD) or assign (WR) the SPI transfer mode. Use the constants123 SPI_MODE_0..SPI_MODE_3; or if you prefer you can combine SPI_CPOL124 (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 a127 single byte.128 129 SPI_IOC_RD_MODE32, SPI_IOC_WR_MODE32 ...130 pass a pointer to a uin32_t131 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 byte136 which will return (RD) or assign (WR) the bit justification used to137 transfer SPI words. Zero indicates MSB-first; other values indicate138 the less common LSB-first encoding. In both cases the specified value139 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 to144 a byte which will return (RD) or assign (WR) the number of bits in145 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 a149 u32 which will return (RD) or assign (WR) the maximum SPI transfer150 speed, in Hz. The controller can't necessarily assign that specific151 clock speed.152 153NOTES:154 155 - At this time there is no async I/O support; everything is purely156 synchronous.157 158 - There's currently no way to report the actual bit rate used to159 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, allowing164 other drivers to talk to other devices.165 166 - There's a limit on the number of bytes each I/O request can transfer167 to the SPI device. It defaults to one page, but that can be changed168 using a module parameter.169 170 - Because SPI has no low-level transfer acknowledgement, you usually171 won't see any I/O errors when talking to a non-existent device.172 173 174FULL DUPLEX CHARACTER DEVICE API175================================176 177See the spidev_fdx.c sample program for one example showing the use of the178full duplex programming interface. (Although it doesn't perform a full duplex179transfer.) The model is the same as that used in the kernel spi_sync()180request; the individual transfers offer the same capabilities as are181available 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 between185the request and response. Several such requests could be chained into186a single kernel request, even allowing the chip to be deselected after187each response. (Other protocol options include changing the word size188and bitrate for each transfer segment.)189 190To make a full duplex request, provide both rx_buf and tx_buf for the191same transfer. It's even OK if those are the same buffer.192