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+SPI devices have a limited userspace API, supporting basic half-duplex
+read() and write() access to SPI slave devices. Using ioctl() requests,
+full duplex transfers and device I/O configuration are also available.
+
+ #include <fcntl.h>
+ #include <unistd.h>
+ #include <sys/ioctl.h>
+ #include <linux/types.h>
+ #include <linux/spi/spidev.h>
+
+Some reasons you might want to use this programming interface include:
+
+ * Prototyping in an environment that's not crash-prone; stray pointers
+ in userspace won't normally bring down any Linux system.
+
+ * Developing simple protocols used to talk to microcontrollers acting
+ as SPI slaves, which you may need to change quite often.
+
+Of course there are drivers that can never be written in userspace, because
+they need to access kernel interfaces (such as IRQ handlers or other layers
+of the driver stack) that are not accessible to userspace.
+
+
+DEVICE CREATION, DRIVER BINDING
+===============================
+The simplest way to arrange to use this driver is to just list it in the
+spi_board_info for a device as the driver it should use: the "modalias"
+entry is "spidev", matching the name of the driver exposing this API.
+Set up the other device characteristics (bits per word, SPI clocking,
+chipselect polarity, etc) as usual, so you won't always need to override
+them later.
+
+(Sysfs also supports userspace driven binding/unbinding of drivers to
+devices. That mechanism might be supported here in the future.)
+
+When you do that, the sysfs node for the SPI device will include a child
+device node with a "dev" attribute that will be understood by udev or mdev.
+(Larger systems will have "udev". Smaller ones may configure "mdev" into
+busybox; it's less featureful, but often enough.) For a SPI device with
+chipselect C on bus B, you should see:
+
+ /dev/spidevB.C ... character special device, major number 153 with
+ a dynamically chosen minor device number. This is the node
+ that userspace programs will open, created by "udev" or "mdev".
+
+ /sys/devices/.../spiB.C ... as usual, the SPI device node will
+ be a child of its SPI master controller.
+
+ /sys/class/spidev/spidevB.C ... created when the "spidev" driver
+ binds to that device. (Directory or symlink, based on whether
+ or not you enabled the "deprecated sysfs files" Kconfig option.)
+
+Do not try to manage the /dev character device special file nodes by hand.
+That's error prone, and you'd need to pay careful attention to system
+security issues; udev/mdev should already be configured securely.
+
+If you unbind the "spidev" driver from that device, those two "spidev" nodes
+(in sysfs and in /dev) should automatically be removed (respectively by the
+kernel and by udev/mdev). You can unbind by removing the "spidev" driver
+module, which will affect all devices using this driver. You can also unbind
+by having kernel code remove the SPI device, probably by removing the driver
+for its SPI controller (so its spi_master vanishes).
+
+Since this is a standard Linux device driver -- even though it just happens
+to expose a low level API to userspace -- it can be associated with any number
+of devices at a time. Just provide one spi_board_info record for each such
+SPI device, and you'll get a /dev device node for each device.
+
+
+BASIC CHARACTER DEVICE API
+==========================
+Normal open() and close() operations on /dev/spidevB.D files work as you
+would expect.
+
+Standard read() and write() operations are obviously only half-duplex, and
+the chipselect is deactivated between those operations. Full-duplex access,
+and composite operation without chipselect de-activation, is available using
+the SPI_IOC_MESSAGE(N) request.
+
+Several ioctl() requests let your driver read or override the device's current
+settings for data transfer parameters:
+
+ SPI_IOC_RD_MODE, SPI_IOC_WR_MODE ... pass a pointer to a byte which will
+ return (RD) or assign (WR) the SPI transfer mode. Use the constants
+ SPI_MODE_0..SPI_MODE_3; or if you prefer you can combine SPI_CPOL
+ (clock polarity, idle high iff this is set) or SPI_CPHA (clock phase,
+ sample on trailing edge iff this is set) flags.
+
+ SPI_IOC_RD_LSB_FIRST, SPI_IOC_WR_LSB_FIRST ... pass a pointer to a byte
+ which will return (RD) or assign (WR) the bit justification used to
+ transfer SPI words. Zero indicates MSB-first; other values indicate
+ the less common LSB-first encoding. In both cases the specified value
+ is right-justified in each word, so that unused (TX) or undefined (RX)
+ bits are in the MSBs.
+
+ SPI_IOC_RD_BITS_PER_WORD, SPI_IOC_WR_BITS_PER_WORD ... pass a pointer to
+ a byte which will return (RD) or assign (WR) the number of bits in
+ each SPI transfer word. The value zero signifies eight bits.
+
+ SPI_IOC_RD_MAX_SPEED_HZ, SPI_IOC_WR_MAX_SPEED_HZ ... pass a pointer to a
+ u32 which will return (RD) or assign (WR) the maximum SPI transfer
+ speed, in Hz. The controller can't necessarily assign that specific
+ clock speed.
+
+NOTES:
+
+ - At this time there is no async I/O support; everything is purely
+ synchronous.
+
+ - There's currently no way to report the actual bit rate used to
+ shift data to/from a given device.
+
+ - From userspace, you can't currently change the chip select polarity;
+ that could corrupt transfers to other devices sharing the SPI bus.
+ Each SPI device is deselected when it's not in active use, allowing
+ other drivers to talk to other devices.
+
+ - There's a limit on the number of bytes each I/O request can transfer
+ to the SPI device. It defaults to one page, but that can be changed
+ using a module parameter.
+
+ - Because SPI has no low-level transfer acknowledgement, you usually
+ won't see any I/O errors when talking to a non-existent device.
+
+
+FULL DUPLEX CHARACTER DEVICE API
+================================
+
+See the spidev_fdx.c sample program for one example showing the use of the
+full duplex programming interface. (Although it doesn't perform a full duplex
+transfer.) The model is the same as that used in the kernel spi_sync()
+request; the individual transfers offer the same capabilities as are
+available to kernel drivers (except that it's not asynchronous).
+
+The example shows one half-duplex RPC-style request and response message.
+These requests commonly require that the chip not be deselected between
+the request and response. Several such requests could be chained into
+a single kernel request, even allowing the chip to be deselected after
+each response. (Other protocol options include changing the word size
+and bitrate for each transfer segment.)
+
+To make a full duplex request, provide both rx_buf and tx_buf for the
+same transfer. It's even OK if those are the same buffer.