Difference between revisions of "Category:SPI"

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== Overview ==
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== Overo SPI ==
  
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Kernel documentation for the SPI framework is excellent and should be your starting point. You can find it under the Documentation directory of your linux source tree or here [http://www.kernel.org/doc/Documentation/spi/spi-summary Documentation/spi/spi-summary].
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Kernel documentation for the SPI framework can be found in the Documentation directory of your linux source tree or here [http://www.kernel.org/doc/Documentation/spi/spi-summary Documentation/spi/spi-summary].
  
 
Linux SPI drivers are broken into two parts. A controller driver that handles direct communication with the hardware and a protocol driver to handle the details of the data for a particular device. The interface defined in spi.c/spi.h is the bridge between the two.
 
Linux SPI drivers are broken into two parts. A controller driver that handles direct communication with the hardware and a protocol driver to handle the details of the data for a particular device. The interface defined in spi.c/spi.h is the bridge between the two.
  
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For the OMAP3's, the controller driver is omap2_mcspi.c. The OMAP3's have 4 SPI busses which the omap2_mcspi driver handles. The kernel config option to include the omap2_mcspi driver is CONFIG_SPI_OMAP24XX.
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For the OMAP3's, the controller driver is omap2_mcspi.c. The OMAP3's have 4 SPI busses which the omap2_mcspi driver handles. The kernel config option to include the omap2_mcspi driver is CONFIG_SPI_OMAP24XX and is already built into standard Gumstix Overo kernels.
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The OMAP3 SPI controller has the ability to act as either a SPI master or SPI slave, but the current kernel code only supports the SPI master configuration.
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Gumstix brings out SPI bus 1 chip select pins 0 and 1 on most of the Overo expansion boards.
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SPI pins for the OMAP3 are multi-functional and need to be multiplexed for SPI use. The SPI bus 1 MOSI, SIMO, CLK, CS0 and CS1 brought out on the Overo expansion boards are correctly setup for SPI use. This configuration is done in the bootloader U-Boot.
  
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The OMAP3 SPI controller has the ability to act as either a SPI master or SPI slave, but the current kernel code only supports the SPI master configuration. There are some new checkins to the kernel code indicating that SPI slave functionality might be coming.
 
  
 
=== Protocol Drivers ===
 
=== Protocol Drivers ===
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A protocol driver is probably what you want to write.
 
  
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There are a couple of steps you need to follow in writing a SPI protocol driver.
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A protocol driver is what you need to use the SPI system on Linux.
  
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# You need to tell the SPI subsystem about the slave device(s) on each SPI bus that you want to use. This gets passed along to the controller driver when it's needed. You can do this during kernel init in a board file like board-overo.c or it can be done at run time in module code.  A device name (modalias) is specified as a key part of this process.
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There are a couple of steps that need to happen.
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# You need to register a SPI protocol driver. This driver should have the same name (modalias) as was specified above. You register a spi_device using the spi_register_driver() call. When this internal mapping connection is made by the SPI subsystem, the result is a probe() call to your protocol driver.
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# Slave devices must be registered for each SPI bus and chip select position used.
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# A protocol driver needs to be registered with the system using the same name as the devices it will be handling so the kernel can link the two.
  
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It doesn't matter which order you perform these steps. But after both are completed and your probe() function returns success, your driver will be ready for SPI communication. You are given a spi_device in the probe() function that you need to hang onto.
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It doesn't matter which order these steps are performed. Most existing SPI drivers do the device registration completely outside the driver code in a board file. If you are writing your own driver, it is probably more convenient to do it in the driver code itself. When both device and driver registration are completed, the probe() function for the protocol driver will be called and SPI communications can start.
  
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SPI data communication happens using I/O queues. One or more SPI transfers are bundled up into SPI messages in the protocol driver. The protocol driver then submits the SPI messages to the controller driver's I/O queue with calls to spi_async(). These messages are processed in FIFO order asynchronously by the controller driver. As each message finishes, the controller notifies the protocol driver via 'completion' callbacks. The protocol driver can then process the raw data as necessary.
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SPI data communication happens using I/O queues. One or more SPI transfers are bundled up into SPI messages in the protocol driver. The protocol driver submits the SPI messages to the controller driver's I/O queue. These messages are then processed asynchronously by the controller driver in FIFO order. As each message finishes, the controller notifies the protocol driver via a callback function.  
  
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There is also a simple synchronous interface exposed by spi.h/spi.c. Here the SPI framework puts the caller, a protocol driver, to sleep while the same asynchronous interface to the controller driver is used behind the scenes. When the controller is done, the protocol driver is woken up and can then proceed to process the data.  
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There are some synchronous helper functions exposed by the interface to simplify the asynchronous behaviour of the Linux SPI system for protocol driver writers.
  
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=== Example Driver ===
 
  
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A sample OMAP3 Linux SPI driver can be found [http://www.github.com/scottellis/overo-adc-mcp3002 here].
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=== Spidev ===
  
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The adc.c driver demonstrates how to add SPI devices at runtime instead of at boot time in order to simplify development. It also demonstrates how to handle asynchronous I/O with the SPI controller.  
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Spidev is a standard Linux module that implements a generic SPI protocol driver while exposing a char device interface to userland.
  
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There is no attempt to do anything useful with the data, a couple of ADC slaves, it's just for testing out the SPI framework.  
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The documentation for spidev can be found here - [http://www.mjmwired.net/kernel/Documentation/spi/spidev Documentation/spi/spidev].  
  
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The driver exposes a simple character device interface to interact with userland.
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The spidev driver does not register devices so a board file patch like this one [https://github.com/scottellis/spike/blob/0d633a22da47022073abd9224ec8709c5b9db932/patches/spidev-enable.patch spidev-enable.patch] is required to use spidev with the Overos. You should customize that patch for the particular device(s) you are using. Things you might want to change are the bus speed and the mode.
  
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=== Spidev ===
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The spidev driver is already built into the standard Overo kernels.
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Only one protocol driver at a time can manage a particular CS line on a given SPI bus.
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As you can see from the above patch, the standard Overo kernels already have devices registering for SPI bus 1 CS0 (ADS7846 touchscreen controller) and SPI bus 1 CS1 (LGPHILIPS LB035Q02 display driver). You will have to disable one or both of these drivers in your kernel config if you want to use those respective CS lines.
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If you are using OE, apply the board patch by copying it to the org.openembedded.dev/recipes/linux/linux-omap3-2.6.xx/ directory and then adding a line to include the patch in your kernel recipe org.openembedded.dev/recipes/linux/linux-omap3_2.6.xx.bb
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There are more detailed instructions for getting spidev working on an Overo at the 'Writing a Linux SPI driver' link below.
  
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Spidev is a stock linux module that implements a generic SPI protocol driver while exposing a char device interface to userland via sysfs.
 
  
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Spidev is not designed to dynamically register itself with the SPI subsystem. You will need to add code to do this in either a kernel module or more likely in the board file, board-overo.c, so registration takes place during boot.
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=== Additional Links ===
  
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Spidev implements its own synchronous interface to the SPI system.  
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[http://www.jumpnowtek.com/index.php?option=com_content&view=article&id=57:gumstix-spi-driver-part1&catid=35:gumstix&Itemid=62 Writing a Linux SPI driver] - A series of articles using Gumstix Overo in the examples
  
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(This is just from reading the code. I have never tried to use spidev. Maybe someone who has can elaborate.)
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[http://elinux.org/BeagleBoard/SPI BeagleBoard/SPI] - Beagleboard SPI section on eLinux.org
  
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Here's one Gumstix [http://www.nabble.com/SPI%2C-spidev%2C-et.-al-to24588398.html#a24594755 mailing list thread] posted on 7/21/09, where some users have successfully used spidev with the Overos.
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[http://en.wikipedia.org/wiki/Serial_Peripheral_Interface_Bus Serial Peripheral Interface Bus] - Wikipedia SPI article
  
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You can also search the [http://www.nabble.com/forum/Search.jtp?query=spidev&sort=date&local=y&forum=22543 Gumstix mailing list archives for spidev]
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[http://gumstix.8.x6.nabble.com/template/NamlServlet.jtp?macro=search_page&node=558772&query=spidev Gumstix mailing list archives for spidev]

Latest revision as of 13:01, 1 April 2016

Overo SPI

Kernel documentation for the SPI framework can be found in the Documentation directory of your linux source tree or here Documentation/spi/spi-summary.

Linux SPI drivers are broken into two parts. A controller driver that handles direct communication with the hardware and a protocol driver to handle the details of the data for a particular device. The interface defined in spi.c/spi.h is the bridge between the two.

For the OMAP3's, the controller driver is omap2_mcspi.c. The OMAP3's have 4 SPI busses which the omap2_mcspi driver handles. The kernel config option to include the omap2_mcspi driver is CONFIG_SPI_OMAP24XX and is already built into standard Gumstix Overo kernels.

The OMAP3 SPI controller has the ability to act as either a SPI master or SPI slave, but the current kernel code only supports the SPI master configuration.

Gumstix brings out SPI bus 1 chip select pins 0 and 1 on most of the Overo expansion boards.

SPI pins for the OMAP3 are multi-functional and need to be multiplexed for SPI use. The SPI bus 1 MOSI, SIMO, CLK, CS0 and CS1 brought out on the Overo expansion boards are correctly setup for SPI use. This configuration is done in the bootloader U-Boot.


Protocol Drivers

A protocol driver is what you need to use the SPI system on Linux.

There are a couple of steps that need to happen.

  1. Slave devices must be registered for each SPI bus and chip select position used.
  2. A protocol driver needs to be registered with the system using the same name as the devices it will be handling so the kernel can link the two.

It doesn't matter which order these steps are performed. Most existing SPI drivers do the device registration completely outside the driver code in a board file. If you are writing your own driver, it is probably more convenient to do it in the driver code itself. When both device and driver registration are completed, the probe() function for the protocol driver will be called and SPI communications can start.

SPI data communication happens using I/O queues. One or more SPI transfers are bundled up into SPI messages in the protocol driver. The protocol driver submits the SPI messages to the controller driver's I/O queue. These messages are then processed asynchronously by the controller driver in FIFO order. As each message finishes, the controller notifies the protocol driver via a callback function.

There are some synchronous helper functions exposed by the interface to simplify the asynchronous behaviour of the Linux SPI system for protocol driver writers.


Spidev

Spidev is a standard Linux module that implements a generic SPI protocol driver while exposing a char device interface to userland.

The documentation for spidev can be found here - Documentation/spi/spidev.

The spidev driver does not register devices so a board file patch like this one spidev-enable.patch is required to use spidev with the Overos. You should customize that patch for the particular device(s) you are using. Things you might want to change are the bus speed and the mode.

The spidev driver is already built into the standard Overo kernels.

Only one protocol driver at a time can manage a particular CS line on a given SPI bus.

As you can see from the above patch, the standard Overo kernels already have devices registering for SPI bus 1 CS0 (ADS7846 touchscreen controller) and SPI bus 1 CS1 (LGPHILIPS LB035Q02 display driver). You will have to disable one or both of these drivers in your kernel config if you want to use those respective CS lines.

If you are using OE, apply the board patch by copying it to the org.openembedded.dev/recipes/linux/linux-omap3-2.6.xx/ directory and then adding a line to include the patch in your kernel recipe org.openembedded.dev/recipes/linux/linux-omap3_2.6.xx.bb

There are more detailed instructions for getting spidev working on an Overo at the 'Writing a Linux SPI driver' link below.


Additional Links

Writing a Linux SPI driver - A series of articles using Gumstix Overo in the examples

BeagleBoard/SPI - Beagleboard SPI section on eLinux.org

Serial Peripheral Interface Bus - Wikipedia SPI article

Gumstix mailing list archives for spidev

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