How To Write Linux PCI Drivers

                   by Martin Mares <mj@ucw.cz> on 07-Feb-2000

~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
The world of PCI is vast and it's full of (mostly unpleasant) surprises.
Different PCI devices have different requirements and different bugs --
because of this, the PCI support layer in Linux kernel is not as trivial
as one would wish. This short pamphlet tries to help all potential driver
authors find their way through the deep forests of PCI handling.


0. Structure of PCI drivers
~~~~~~~~~~~~~~~~~~~~~~~~~~~
There exist two kinds of PCI drivers: new-style ones (which leave most of
probing for devices to the PCI layer and support online insertion and removal
of devices [thus supporting PCI, hot-pluggable PCI and CardBus in a single
driver]) and old-style ones which just do all the probing themselves. Unless
you have a very good reason to do so, please don't use the old way of probing
in any new code. After the driver finds the devices it wishes to operate
on (either the old or the new way), it needs to perform the following steps:

        Enable the device
        Access device configuration space
        Discover resources (addresses and IRQ numbers) provided by the device
        Allocate these resources
        Communicate with the device
        Disable the device

Most of these topics are covered by the following sections, for the rest
look at <linux/pci.h>, it's hopefully well commented.

If the PCI subsystem is not configured (CONFIG_PCI is not set), most of
the functions described below are defined as inline functions either completely
empty or just returning an appropriate error codes to avoid lots of ifdefs
in the drivers.


1. New-style drivers
~~~~~~~~~~~~~~~~~~~~
The new-style drivers just call pci_register_driver during their initialization
with a pointer to a structure describing the driver (struct pci_driver) which
contains:

        name            Name of the driver
        id_table        Pointer to table of device ID's the driver is
                        interested in.  Most drivers should export this
                        table using MODULE_DEVICE_TABLE(pci,...).
        probe           Pointer to a probing function which gets called (during
                        execution of pci_register_driver for already existing
                        devices or later if a new device gets inserted) for all
                        PCI devices which match the ID table and are not handled
                        by the other drivers yet. This function gets passed a
                        pointer to the pci_dev structure representing the device
                        and also which entry in the ID table did the device
                        match. It returns zero when the driver has accepted the
                        device or an error code (negative number) otherwise.
                        This function always gets called from process context,
                        so it can sleep.
        remove          Pointer to a function which gets called whenever a
                        device being handled by this driver is removed (either
                        during deregistration of the driver or when it's
                        manually pulled out of a hot-pluggable slot). This
                        function always gets called from process context, so it
                        can sleep.
        save_state      Save a device's state before it's suspend.
        suspend         Put device into low power state.
        resume          Wake device from low power state.
        enable_wake     Enable device to generate wake events from a low power
                        state.

                        (Please see Documentation/power/pci.txt for descriptions
                        of PCI Power Management and the related functions)

The ID table is an array of struct pci_device_id ending with a all-zero entry.
Each entry consists of:

        vendor, device  Vendor and device ID to match (or PCI_ANY_ID)
        subvendor,      Subsystem vendor and device ID to match (or PCI_ANY_ID)
        subdevice
        class,          Device class to match. The class_mask tells which bits
        class_mask      of the class are honored during the comparison.
        driver_data     Data private to the driver.

Most drivers don't need to use the driver_data field.  Best practice
for use of driver_data is to use it as an index into a static list of
equivalant device types, not to use it as a pointer.

Have a table entry {PCI_ANY_ID, PCI_ANY_ID, PCI_ANY_ID, PCI_ANY_ID}
to have probe() called for every PCI device known to the system.

New PCI IDs may be added to a device driver at runtime by writing
to the file /sys/bus/pci/drivers/{driver}/new_id.  When added, the
driver will probe for all devices it can support.

echo "vendor device subvendor subdevice class class_mask driver_data" > \
 /sys/bus/pci/drivers/{driver}/new_id
where all fields are passed in as hexadecimal values (no leading 0x).
Users need pass only as many fields as necessary; vendor, device,
subvendor, and subdevice fields default to PCI_ANY_ID (FFFFFFFF),
class and classmask fields default to 0, and driver_data defaults to
0UL.  Device drivers must call
   pci_dynids_set_use_driver_data(pci_driver *, 1)
in order for the driver_data field to get passed to the driver.
Otherwise, only a 0 is passed in that field.

When the driver exits, it just calls pci_unregister_driver() and the PCI layer
automatically calls the remove hook for all devices handled by the driver.

Please mark the initialization and cleanup functions where appropriate
(the corresponding macros are defined in <linux/init.h>):

        __init          Initialization code. Thrown away after the driver
                        initializes.
        __exit          Exit code. Ignored for non-modular drivers.
        __devinit       Device initialization code. Identical to __init if
                        the kernel is not compiled with CONFIG_HOTPLUG, normal
                        function otherwise.
        __devexit       The same for __exit.

Tips:
        The module_init()/module_exit() functions (and all initialization
        functions called only from these) should be marked __init/exit.
        The struct pci_driver shouldn't be marked with any of these tags.
        The ID table array should be marked __devinitdata.
        The probe() and remove() functions (and all initialization
        functions called only from these) should be marked __devinit/exit.
        If you are sure the driver is not a hotplug driver then use only 
        __init/exit __initdata/exitdata.

        Pointers to functions marked as __devexit must be created using
        __devexit_p(function_name).  That will generate the function
        name or NULL if the __devexit function will be discarded.


2. How to find PCI devices manually (the old style)
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
PCI drivers not using the pci_register_driver() interface search
for PCI devices manually using the following constructs:

Searching by vendor and device ID:

        struct pci_dev *dev = NULL;
        while (dev = pci_get_device(VENDOR_ID, DEVICE_ID, dev))
                configure_device(dev);

Searching by class ID (iterate in a similar way):

        pci_get_class(CLASS_ID, dev)

Searching by both vendor/device and subsystem vendor/device ID:

        pci_get_subsys(VENDOR_ID, DEVICE_ID, SUBSYS_VENDOR_ID, SUBSYS_DEVICE_ID, dev).

   You can use the constant PCI_ANY_ID as a wildcard replacement for
VENDOR_ID or DEVICE_ID.  This allows searching for any device from a
specific vendor, for example.

   These functions are hotplug-safe. They increment the reference count on
the pci_dev that they return. You must eventually (possibly at module unload)
decrement the reference count on these devices by calling pci_dev_put().


3. Enabling and disabling devices
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
   Before you do anything with the device you've found, you need to enable
it by calling pci_enable_device() which enables I/O and memory regions of
the device, allocates an IRQ if necessary, assigns missing resources if
needed and wakes up the device if it was in suspended state. Please note
that this function can fail.

   If you want to use the device in bus mastering mode, call pci_set_master()
which enables the bus master bit in PCI_COMMAND register and also fixes
the latency timer value if it's set to something bogus by the BIOS.

   If you want to use the PCI Memory-Write-Invalidate transaction,
call pci_set_mwi().  This enables the PCI_COMMAND bit for Mem-Wr-Inval
and also ensures that the cache line size register is set correctly.
Make sure to check the return value of pci_set_mwi(), not all architectures
may support Memory-Write-Invalidate.

   If your driver decides to stop using the device (e.g., there was an
error while setting it up or the driver module is being unloaded), it
should call pci_disable_device() to deallocate any IRQ resources, disable
PCI bus-mastering, etc.  You should not do anything with the device after
calling pci_disable_device().

4. How to access PCI config space
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
   You can use pci_(read|write)_config_(byte|word|dword) to access the config
space of a device represented by struct pci_dev *. All these functions return 0
when successful or an error code (PCIBIOS_...) which can be translated to a text
string by pcibios_strerror. Most drivers expect that accesses to valid PCI
devices don't fail.

   If you don't have a struct pci_dev available, you can call
pci_bus_(read|write)_config_(byte|word|dword) to access a given device
and function on that bus.

   If you access fields in the standard portion of the config header, please
use symbolic names of locations and bits declared in <linux/pci.h>.

   If you need to access Extended PCI Capability registers, just call
pci_find_capability() for the particular capability and it will find the
corresponding register block for you.


5. Addresses and interrupts
~~~~~~~~~~~~~~~~~~~~~~~~~~~
   Memory and port addresses and interrupt numbers should NOT be read from the
config space. You should use the values in the pci_dev structure as they might
have been remapped by the kernel.

   See Documentation/IO-mapping.txt for how to access device memory.

   You still need to call request_region() for I/O regions and
request_mem_region() for memory regions to make sure nobody else is using the
same device.

   All interrupt handlers should be registered with SA_SHIRQ and use the devid
to map IRQs to devices (remember that all PCI interrupts are shared).


6. Other interesting functions
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
pci_find_slot()                 Find pci_dev corresponding to given bus and
                                slot numbers.
pci_set_power_state()           Set PCI Power Management state (0=D0 ... 3=D3)
pci_find_capability()           Find specified capability in device's capability
                                list.
pci_module_init()               Inline helper function for ensuring correct
                                pci_driver initialization and error handling.
pci_resource_start()            Returns bus start address for a given PCI region
pci_resource_end()              Returns bus end address for a given PCI region
pci_resource_len()              Returns the byte length of a PCI region
pci_set_drvdata()               Set private driver data pointer for a pci_dev
pci_get_drvdata()               Return private driver data pointer for a pci_dev
pci_set_mwi()                   Enable Memory-Write-Invalidate transactions.
pci_clear_mwi()                 Disable Memory-Write-Invalidate transactions.


7. Miscellaneous hints
~~~~~~~~~~~~~~~~~~~~~~
When displaying PCI slot names to the user (for example when a driver wants
to tell the user what card has it found), please use pci_name(pci_dev)
for this purpose.

Always refer to the PCI devices by a pointer to the pci_dev structure.
All PCI layer functions use this identification and it's the only
reasonable one. Don't use bus/slot/function numbers except for very
special purposes -- on systems with multiple primary buses their semantics
can be pretty complex.

If you're going to use PCI bus mastering DMA, take a look at
Documentation/DMA-mapping.txt.

Don't try to turn on Fast Back to Back writes in your driver.  All devices
on the bus need to be capable of doing it, so this is something which needs
to be handled by platform and generic code, not individual drivers.


8. Obsolete functions
~~~~~~~~~~~~~~~~~~~~~
There are several functions which you might come across when trying to
port an old driver to the new PCI interface.  They are no longer present
in the kernel as they aren't compatible with hotplug or PCI domains or
having sane locking.

pcibios_present() and           Since ages, you don't need to test presence
pci_present()                   of PCI subsystem when trying to talk to it.
                                If it's not there, the list of PCI devices
                                is empty and all functions for searching for
                                devices just return NULL.
pcibios_(read|write)_*          Superseded by their pci_(read|write)_*
                                counterparts.
pcibios_find_*                  Superseded by their pci_get_* counterparts.
pci_for_each_dev()              Superseded by pci_get_device()
pci_for_each_dev_reverse()      Superseded by pci_find_device_reverse()
pci_for_each_bus()              Superseded by pci_find_next_bus()
pci_find_device()               Superseded by pci_get_device()
pci_find_subsys()               Superseded by pci_get_subsys()
pcibios_find_class()            Superseded by pci_get_class()
pci_find_class()                Superseded by pci_get_class()
pci_(read|write)_*_nodev()      Superseded by pci_bus_(read|write)_*()