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134 lines
5.5 KiB
ReStructuredText
134 lines
5.5 KiB
ReStructuredText
Port I/O
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========
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86Box handles the x86 port I/O space through **I/O handlers**. These handlers can be added with the ``io_sethandler`` function and removed with the ``io_removehandler`` function, both provided by ``86box/io.h``.
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.. flat-table:: io_sethandler / io_removehandler
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:header-rows: 1
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:widths: 1 999
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* - Parameter
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- Description
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* - base
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- First I/O port (0x0000-0xFFFF) covered by this handler.
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* - size
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- Amount of I/O ports (1-65536) covered by this handler, starting at ``base``.
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* - inb
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- :rspan:`2` I/O read operation callback functions. Can be ``NULL``. Each callback takes the form of:
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``TYPE callback(uint16_t addr, void *priv)``
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* ``TYPE``: operation width: ``uint8_t`` for ``inb``, ``uint16_t`` for ``inw``, ``uint32_t`` for ``inl``;
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* ``addr``: exact I/O port being read;
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* ``priv``: opaque pointer (see ``priv`` below);
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* Return value: 8- (``inb``), 16- (``inw``) or 32-bit (``inl``) value read from this port.
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* - inw
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* - inl
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* - outb
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- :rspan:`2` I/O write operation callback functions. Can be ``NULL``. Each callback takes the form of:
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``void callback(uint16_t addr, TYPE val, void *priv)``
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* ``addr``: exact I/O port being written;
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* ``TYPE``: operation width: ``uint8_t`` for ``outb``, ``uint16_t`` for ``outw``, ``uint32_t`` for ``outl``;
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* ``val``: 8- (``outb``), 16- (``outw``) or 32-bit (``outl``) value being written to this port;
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* ``priv``: opaque pointer (see ``priv`` below).
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* - outw
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* - outl
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* - priv
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- Opaque pointer passed to this handler's read/write operation callbacks.
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Usually a pointer to a device's :ref:`state structure <dev/api/device:State structure>`.
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I/O handlers can be added or removed at any time, although ``io_removehandler`` must be called with the **exact same** parameters that ``io_sethandler`` was originally called with. For non-Plug and Play devices, you might want to add handlers in the ``init`` callback; for ISA Plug and Play devices, you'd add and/or remove handlers on the ``config_changed`` callback; for PCI devices, you'd do the same whenever the Command register or Base Address (BAR) registers are written to; and so on.
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.. note:: There is no need to call ``io_removehandler`` on the device's ``close`` callback, since a hard reset already removes all I/O handlers.
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Callback fallbacks
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------------------
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When an I/O handler receives an operation with a width for which it has no callback, the operation will automatically **fall back** to a lower width for which there is a callback. For example, if an ``inl`` operation falls on a handler which has no ``inl`` callback, 86Box will break the operation down to ``inw`` or ``inb`` callbacks on successive port numbers, then combine their return values:
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* ``inl`` callback present::
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uint32_t val = inl(port);
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* ``inl`` callback not present, but ``inw`` callback present::
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uint32_t val = inw(port);
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val |= (inw(port + 2) << 16);
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* ``inl`` and ``inw`` callbacks not present, but ``inb`` callback present::
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uint32_t val = inb(port);
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val |= (inb(port + 1) << 8);
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val |= (inb(port + 2) << 16);
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val |= (inb(port + 3) << 24);
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* ``inl``, ``inw`` and ``inb`` callbacks not present::
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uint32_t val = 0xffffffff; /* don't care */
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The same rule applies to write callbacks:
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* ``outl`` callback present::
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uint32_t val = /* ... */;
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outl(port, val);
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* ``outl`` callback not present, but ``outw`` callback present::
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uint32_t val = /* ... */;
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outw(port, val & 0xffff);
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outw(port + 2, (val >> 16) & 0xffff);
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* ``outl`` and ``outw`` callbacks not present, but ``outb`` callback present::
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uint32_t val = /* ... */;
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outb(port, val & 0xff);
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outb(port + 1, (val >> 8) & 0xff);
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outb(port + 2, (val >> 16) & 0xff);
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outb(port + 3, (val >> 24) & 0xff);
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* ``outl``, ``outw`` and ``outb`` callbacks not present:
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Don't care, no operation performed.
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.. note:: Each broken-down operation triggers the I/O handlers for its respective port number, no matter which handlers are responsible for the starting port number. A handler will **never** receive callbacks for ports outside its ``base`` and ``size`` boundaries.
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This feature's main use cases are devices which store registers that are 8-bit wide but may be accessed with 16- or 32-bit operations::
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typedef struct {
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uint8_t regs[256];
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} foo_t;
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static uint8_t
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foo_inb(uint16_t addr, void *priv)
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{
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foo_t *dev = (foo_t *) priv;
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return dev->regs[addr & 0xff]; /* example: register index = I/O port's least significant byte */
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}
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/* No foo_inw, so a 16-bit read will read two 8-bit registers in succession.
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No foo_inl, so a 32-bit read will read four 8-bit registers in succession. */
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Multiple I/O handlers
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---------------------
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Any given I/O port can have an **unlimited** amount of I/O handlers, such that:
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* when a **read** operation occurs, all read callbacks will be called, and their return values will be logically **AND**\ ed together;
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* when a **write** operation occurs, all write callbacks will be called with the same written value.
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Read callbacks can effectively return "don't care" (without interfering with other handlers) by returning a value with all bits set: ``0xff`` with ``inb``, ``0xffff`` with ``inw`` or ``0xffffffff`` with ``inl``.
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.. note:: The same callback fallback rules specified above also apply with multiple handlers. Handlers without valid callbacks for the operation's type and width are automatically skipped.
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