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+$Id: gameport-programming.txt,v 1.3 2001/04/24 13:51:37 vojtech Exp $
+
+Programming gameport drivers
+~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+
+1. A basic classic gameport
+~~~~~~~~~~~~~~~~~~~~~~~~~~~
+
+If the gameport doesn't provide more than the inb()/outb() functionality,
+the code needed to register it with the joystick drivers is simple:
+
+        struct gameport gameport;
+
+        gameport.io = MY_IO_ADDRESS;
+        gameport_register_port(&gameport);
+
+Make sure struct gameport is initialized to 0 in all other fields. The
+gameport generic code will take care of the rest.
+
+If your hardware supports more than one io address, and your driver can
+choose which one program the hardware to, starting from the more exotic
+addresses is preferred, because the likelyhood of clashing with the standard
+0x201 address is smaller.
+
+Eg. if your driver supports addresses 0x200, 0x208, 0x210 and 0x218, then
+0x218 would be the address of first choice.
+
+If your hardware supports a gameport address that is not mapped to ISA io
+space (is above 0x1000), use that one, and don't map the ISA mirror.
+
+Also, always request_region() on the whole io space occupied by the
+gameport. Although only one ioport is really used, the gameport usually
+occupies from one to sixteen addresses in the io space.
+
+Please also consider enabling the gameport on the card in the ->open()
+callback if the io is mapped to ISA space - this way it'll occupy the io
+space only when something really is using it. Disable it again in the
+->close() callback. You also can select the io address in the ->open()
+callback, so that it doesn't fail if some of the possible addresses are
+already occupied by other gameports.
+
+2. Memory mapped gameport
+~~~~~~~~~~~~~~~~~~~~~~~~~
+
+When a gameport can be accessed through MMIO, this way is preferred, because
+it is faster, allowing more reads per second. Registering such a gameport
+isn't as easy as a basic IO one, but not so much complex:
+
+        struct gameport gameport;
+
+        void my_trigger(struct gameport *gameport)
+        {
+                my_mmio = 0xff;
+        }
+
+        unsigned char my_read(struct gameport *gameport)
+        {
+                return my_mmio; 
+        }
+
+        gameport.read = my_read;
+        gameport.trigger = my_trigger;
+        gameport_register_port(&gameport);
+
+3. Cooked mode gameport
+~~~~~~~~~~~~~~~~~~~~~~~
+
+There are gameports that can report the axis values as numbers, that means
+the driver doesn't have to measure them the old way - an ADC is built into
+the gameport. To register a cooked gameport:
+
+        struct gameport gameport;
+
+        int my_cooked_read(struct gameport *gameport, int *axes, int *buttons)
+        {
+                int i;
+
+                for (i = 0; i < 4; i++)
+                        axes[i] = my_mmio[i];
+                buttons[i] = my_mmio[4];
+        }
+
+        int my_open(struct gameport *gameport, int mode)
+        {
+                return -(mode != GAMEPORT_MODE_COOKED);
+        }
+
+        gameport.cooked_read = my_cooked_read;
+        gameport.open = my_open;
+        gameport.fuzz = 8;
+        gameport_register_port(&gameport);
+
+The only confusing thing here is the fuzz value. Best determined by
+experimentation, it is the amount of noise in the ADC data. Perfect
+gameports can set this to zero, most common have fuzz between 8 and 32.
+See analog.c and input.c for handling of fuzz - the fuzz value determines
+the size of a gaussian filter window that is used to eliminate the noise
+in the data.
+
+4. More complex gameports
+~~~~~~~~~~~~~~~~~~~~~~~~~
+
+Gameports can support both raw and cooked modes. In that case combine either
+examples 1+2 or 1+3. Gameports can support internal calibration - see below,
+and also lightning.c and analog.c on how that works. If your driver supports
+more than one gameport instance simultaneously, use the ->private member of
+the gameport struct to point to your data.
+
+5. Unregistering a gameport
+~~~~~~~~~~~~~~~~~~~~~~~~~~~
+
+Simple:
+
+gameport_unregister_port(&gameport);
+
+6. The gameport structure
+~~~~~~~~~~~~~~~~~~~~~~~~~
+
+struct gameport {
+
+        void *private;
+
+A private pointer for free use in the gameport driver. (Not the joystick
+driver!)
+
+        int number;
+
+Number assigned to the gameport when registered. Informational purpose only.
+
+        int io;
+
+I/O address for use with raw mode. You have to either set this, or ->read()
+to some value if your gameport supports raw mode.
+
+        int speed;
+
+Raw mode speed of the gameport reads in thousands of reads per second.
+
+        int fuzz;
+
+If the gameport supports cooked mode, this should be set to a value that
+represents the amount of noise in the data. See section 3.
+
+        void (*trigger)(struct gameport *);
+
+Trigger. This function should trigger the ns558 oneshots. If set to NULL,
+outb(0xff, io) will be used.
+
+        unsigned char (*read)(struct gameport *);
+
+Read the buttons and ns558 oneshot bits. If set to NULL, inb(io) will be
+used instead.
+
+        int (*cooked_read)(struct gameport *, int *axes, int *buttons); 
+
+If the gameport supports cooked mode, it should point this to its cooked
+read function. It should fill axes[0..3] with four values of the joystick axes
+and buttons[0] with four bits representing the buttons.
+
+        int (*calibrate)(struct gameport *, int *axes, int *max); 
+
+Function for calibrating the ADC hardware. When called, axes[0..3] should be
+pre-filled by cooked data by the caller, max[0..3] should be pre-filled with
+expected maximums for each axis. The calibrate() function should set the
+sensitivity of the ADC hardware so that the maximums fit in its range and
+recompute the axes[] values to match the new sensitivity or re-read them from
+the hardware so that they give valid values. 
+
+        int (*open)(struct gameport *, int mode);
+
+Open() serves two purposes. First a driver either opens the port in raw or
+in cooked mode, the open() callback can decide which modes are supported.
+Second, resource allocation can happen here. The port can also be enabled
+here. Prior to this call, other fields of the gameport struct (namely the io
+member) need not to be valid.
+
+        void (*close)(struct gameport *);
+
+Close() should free the resources allocated by open, possibly disabling the
+gameport.
+
+        struct gameport_dev *dev;
+        struct gameport *next;
+
+For internal use by the gameport layer.
+
+};
+
+Enjoy!