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1. Linux Device Drivers Ioctl
2. Linux Drivers Ioctl
3. Drivers/tty/tty Ioctl.c

SPI devices have a limited userspace API, supporting basic half-duplexread() and write() access to SPI slave devices. Using ioctl() requests,full duplex transfers and device I/O configuration are also available.

Some reasons you might want to use this programming interface include:

• Prototyping in an environment that’s not crash-prone; stray pointersin userspace won’t normally bring down any Linux system.
• Developing simple protocols used to talk to microcontrollers actingas SPI slaves, which you may need to change quite often.

Of course there are drivers that can never be written in userspace, becausethey need to access kernel interfaces (such as IRQ handlers or other layersof 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 thespi_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 overridethem later.

(Sysfs also supports userspace driven binding/unbinding of drivers todevices. That mechanism might be supported here in the future.)

When you do that, the sysfs node for the SPI device will include a childdevice node with a “dev” attribute that will be understood by udev or mdev.(Larger systems will have “udev”. Smaller ones may configure “mdev” intobusybox; it’s less featureful, but often enough.) For a SPI device withchipselect C on bus B, you should see:

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 systemsecurity 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 thekernel and by udev/mdev). You can unbind by removing the “spidev” drivermodule, which will affect all devices using this driver. You can also unbindby having kernel code remove the SPI device, probably by removing the driverfor its SPI controller (so its spi_master vanishes).

Since this is a standard Linux device driver – even though it just happensto expose a low level API to userspace – it can be associated with any numberof devices at a time. Just provide one spi_board_info record for each suchSPI device, and you’ll get a /dev device node for each device.

Linux Device Drivers Ioctl

BASIC CHARACTER DEVICE API¶

Normal open() and close() operations on /dev/spidevB.D files work as youwould expect.

Standard read() and write() operations are obviously only half-duplex, andthe chipselect is deactivated between those operations. Full-duplex access,and composite operation without chipselect de-activation, is available usingthe SPI_IOC_MESSAGE(N) request.

Several ioctl() requests let your driver read or override the device’s currentsettings for data transfer parameters:

NOTES:

• At this time there is no async I/O support; everything is purelysynchronous.
• There’s currently no way to report the actual bit rate used toshift 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, allowingother drivers to talk to other devices.
• There’s a limit on the number of bytes each I/O request can transferto the SPI device. It defaults to one page, but that can be changedusing a module parameter.
• Because SPI has no low-level transfer acknowledgement, you usuallywon’t see any I/O errors when talking to a non-existent device.

FULL DUPLEX CHARACTER DEVICE API¶

Linux Drivers Ioctl

See the spidev_fdx.c sample program for one example showing the use of thefull duplex programming interface. (Although it doesn’t perform a full duplextransfer.) The model is the same as that used in the kernel spi_sync()request; the individual transfers offer the same capabilities as areavailable 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 betweenthe request and response. Several such requests could be chained intoa single kernel request, even allowing the chip to be deselected aftereach response. (Other protocol options include changing the word sizeand bitrate for each transfer segment.)

Drivers/tty/tty Ioctl.c

To make a full duplex request, provide both rx_buf and tx_buf for thesame transfer. It’s even OK if those are the same buffer.