303 lines
10 KiB
Plaintext
Executable File
303 lines
10 KiB
Plaintext
Executable File
Programming input drivers
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~~~~~~~~~~~~~~~~~~~~~~~~~
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1. Creating an input device driver
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~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
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1.0 The simplest example
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~~~~~~~~~~~~~~~~~~~~~~~~
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Here comes a very simple example of an input device driver. The device has
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just one button and the button is accessible at i/o port BUTTON_PORT. When
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pressed or released a BUTTON_IRQ happens. The driver could look like:
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#include <linux/input.h>
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#include <linux/module.h>
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#include <linux/init.h>
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#include <asm/irq.h>
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#include <asm/io.h>
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static struct input_dev *button_dev;
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static irqreturn_t button_interrupt(int irq, void *dummy)
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{
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input_report_key(button_dev, BTN_0, inb(BUTTON_PORT) & 1);
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input_sync(button_dev);
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return IRQ_HANDLED;
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}
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static int __init button_init(void)
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{
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int error;
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if (request_irq(BUTTON_IRQ, button_interrupt, 0, "button", NULL)) {
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printk(KERN_ERR "button.c: Can't allocate irq %d\n", button_irq);
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return -EBUSY;
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}
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button_dev = input_allocate_device();
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if (!button_dev) {
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printk(KERN_ERR "button.c: Not enough memory\n");
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error = -ENOMEM;
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goto err_free_irq;
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}
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button_dev->evbit[0] = BIT_MASK(EV_KEY);
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button_dev->keybit[BIT_WORD(BTN_0)] = BIT_MASK(BTN_0);
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error = input_register_device(button_dev);
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if (error) {
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printk(KERN_ERR "button.c: Failed to register device\n");
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goto err_free_dev;
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}
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return 0;
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err_free_dev:
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input_free_device(button_dev);
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err_free_irq:
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free_irq(BUTTON_IRQ, button_interrupt);
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return error;
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}
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static void __exit button_exit(void)
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{
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input_unregister_device(button_dev);
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free_irq(BUTTON_IRQ, button_interrupt);
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}
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module_init(button_init);
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module_exit(button_exit);
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1.1 What the example does
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~~~~~~~~~~~~~~~~~~~~~~~~~
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First it has to include the <linux/input.h> file, which interfaces to the
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input subsystem. This provides all the definitions needed.
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In the _init function, which is called either upon module load or when
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booting the kernel, it grabs the required resources (it should also check
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for the presence of the device).
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Then it allocates a new input device structure with input_allocate_device()
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and sets up input bitfields. This way the device driver tells the other
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parts of the input systems what it is - what events can be generated or
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accepted by this input device. Our example device can only generate EV_KEY
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type events, and from those only BTN_0 event code. Thus we only set these
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two bits. We could have used
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set_bit(EV_KEY, button_dev.evbit);
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set_bit(BTN_0, button_dev.keybit);
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as well, but with more than single bits the first approach tends to be
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shorter.
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Then the example driver registers the input device structure by calling
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input_register_device(&button_dev);
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This adds the button_dev structure to linked lists of the input driver and
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calls device handler modules _connect functions to tell them a new input
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device has appeared. input_register_device() may sleep and therefore must
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not be called from an interrupt or with a spinlock held.
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While in use, the only used function of the driver is
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button_interrupt()
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which upon every interrupt from the button checks its state and reports it
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via the
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input_report_key()
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call to the input system. There is no need to check whether the interrupt
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routine isn't reporting two same value events (press, press for example) to
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the input system, because the input_report_* functions check that
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themselves.
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Then there is the
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input_sync()
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call to tell those who receive the events that we've sent a complete report.
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This doesn't seem important in the one button case, but is quite important
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for for example mouse movement, where you don't want the X and Y values
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to be interpreted separately, because that'd result in a different movement.
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1.2 dev->open() and dev->close()
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~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
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In case the driver has to repeatedly poll the device, because it doesn't
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have an interrupt coming from it and the polling is too expensive to be done
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all the time, or if the device uses a valuable resource (eg. interrupt), it
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can use the open and close callback to know when it can stop polling or
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release the interrupt and when it must resume polling or grab the interrupt
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again. To do that, we would add this to our example driver:
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static int button_open(struct input_dev *dev)
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{
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if (request_irq(BUTTON_IRQ, button_interrupt, 0, "button", NULL)) {
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printk(KERN_ERR "button.c: Can't allocate irq %d\n", button_irq);
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return -EBUSY;
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}
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return 0;
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}
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static void button_close(struct input_dev *dev)
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{
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free_irq(IRQ_AMIGA_VERTB, button_interrupt);
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}
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static int __init button_init(void)
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{
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...
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button_dev->open = button_open;
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button_dev->close = button_close;
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...
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}
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Note that input core keeps track of number of users for the device and
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makes sure that dev->open() is called only when the first user connects
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to the device and that dev->close() is called when the very last user
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disconnects. Calls to both callbacks are serialized.
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The open() callback should return a 0 in case of success or any nonzero value
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in case of failure. The close() callback (which is void) must always succeed.
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1.3 Basic event types
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~~~~~~~~~~~~~~~~~~~~~
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The most simple event type is EV_KEY, which is used for keys and buttons.
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It's reported to the input system via:
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input_report_key(struct input_dev *dev, int code, int value)
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See linux/input.h for the allowable values of code (from 0 to KEY_MAX).
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Value is interpreted as a truth value, ie any nonzero value means key
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pressed, zero value means key released. The input code generates events only
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in case the value is different from before.
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In addition to EV_KEY, there are two more basic event types: EV_REL and
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EV_ABS. They are used for relative and absolute values supplied by the
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device. A relative value may be for example a mouse movement in the X axis.
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The mouse reports it as a relative difference from the last position,
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because it doesn't have any absolute coordinate system to work in. Absolute
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events are namely for joysticks and digitizers - devices that do work in an
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absolute coordinate systems.
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Having the device report EV_REL buttons is as simple as with EV_KEY, simply
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set the corresponding bits and call the
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input_report_rel(struct input_dev *dev, int code, int value)
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function. Events are generated only for nonzero value.
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However EV_ABS requires a little special care. Before calling
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input_register_device, you have to fill additional fields in the input_dev
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struct for each absolute axis your device has. If our button device had also
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the ABS_X axis:
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button_dev.absmin[ABS_X] = 0;
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button_dev.absmax[ABS_X] = 255;
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button_dev.absfuzz[ABS_X] = 4;
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button_dev.absflat[ABS_X] = 8;
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Or, you can just say:
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input_set_abs_params(button_dev, ABS_X, 0, 255, 4, 8);
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This setting would be appropriate for a joystick X axis, with the minimum of
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0, maximum of 255 (which the joystick *must* be able to reach, no problem if
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it sometimes reports more, but it must be able to always reach the min and
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max values), with noise in the data up to +- 4, and with a center flat
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position of size 8.
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If you don't need absfuzz and absflat, you can set them to zero, which mean
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that the thing is precise and always returns to exactly the center position
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(if it has any).
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1.4 BITS_TO_LONGS(), BIT_WORD(), BIT_MASK()
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~~~~~~~~~~~~~~~~~~~~~~~~~~
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These three macros from bitops.h help some bitfield computations:
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BITS_TO_LONGS(x) - returns the length of a bitfield array in longs for
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x bits
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BIT_WORD(x) - returns the index in the array in longs for bit x
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BIT_MASK(x) - returns the index in a long for bit x
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1.5 The id* and name fields
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~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
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The dev->name should be set before registering the input device by the input
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device driver. It's a string like 'Generic button device' containing a
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user friendly name of the device.
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The id* fields contain the bus ID (PCI, USB, ...), vendor ID and device ID
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of the device. The bus IDs are defined in input.h. The vendor and device ids
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are defined in pci_ids.h, usb_ids.h and similar include files. These fields
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should be set by the input device driver before registering it.
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The idtype field can be used for specific information for the input device
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driver.
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The id and name fields can be passed to userland via the evdev interface.
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1.6 The keycode, keycodemax, keycodesize fields
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~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
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These three fields should be used by input devices that have dense keymaps.
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The keycode is an array used to map from scancodes to input system keycodes.
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The keycode max should contain the size of the array and keycodesize the
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size of each entry in it (in bytes).
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Userspace can query and alter current scancode to keycode mappings using
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EVIOCGKEYCODE and EVIOCSKEYCODE ioctls on corresponding evdev interface.
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When a device has all 3 aforementioned fields filled in, the driver may
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rely on kernel's default implementation of setting and querying keycode
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mappings.
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1.7 dev->getkeycode() and dev->setkeycode()
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~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
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getkeycode() and setkeycode() callbacks allow drivers to override default
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keycode/keycodesize/keycodemax mapping mechanism provided by input core
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and implement sparse keycode maps.
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1.8 Key autorepeat
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~~~~~~~~~~~~~~~~~~
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... is simple. It is handled by the input.c module. Hardware autorepeat is
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not used, because it's not present in many devices and even where it is
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present, it is broken sometimes (at keyboards: Toshiba notebooks). To enable
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autorepeat for your device, just set EV_REP in dev->evbit. All will be
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handled by the input system.
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1.9 Other event types, handling output events
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~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
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The other event types up to now are:
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EV_LED - used for the keyboard LEDs.
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EV_SND - used for keyboard beeps.
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They are very similar to for example key events, but they go in the other
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direction - from the system to the input device driver. If your input device
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driver can handle these events, it has to set the respective bits in evbit,
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*and* also the callback routine:
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button_dev->event = button_event;
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int button_event(struct input_dev *dev, unsigned int type, unsigned int code, int value);
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{
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if (type == EV_SND && code == SND_BELL) {
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outb(value, BUTTON_BELL);
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return 0;
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}
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return -1;
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}
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This callback routine can be called from an interrupt or a BH (although that
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isn't a rule), and thus must not sleep, and must not take too long to finish.
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