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diff --git a/Documentation/PCI/pci.txt b/Documentation/PCI/pci.txt new file mode 100644 index 0000000..8d4dc62 --- /dev/null +++ b/Documentation/PCI/pci.txt @@ -0,0 +1,646 @@ + + How To Write Linux PCI Drivers + + by Martin Mares <mj@ucw.cz> on 07-Feb-2000 + updated by Grant Grundler <grundler@parisc-linux.org> on 23-Dec-2006 + +~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ +The world of PCI is vast and full of (mostly unpleasant) surprises. +Since each CPU architecture implements different chip-sets and PCI devices +have different requirements (erm, "features"), the result is the PCI support +in the Linux kernel is not as trivial as one would wish. This short paper +tries to introduce all potential driver authors to Linux APIs for +PCI device drivers. + +A more complete resource is the third edition of "Linux Device Drivers" +by Jonathan Corbet, Alessandro Rubini, and Greg Kroah-Hartman. +LDD3 is available for free (under Creative Commons License) from: + + http://lwn.net/Kernel/LDD3/ + +However, keep in mind that all documents are subject to "bit rot". +Refer to the source code if things are not working as described here. + +Please send questions/comments/patches about Linux PCI API to the +"Linux PCI" <linux-pci@atrey.karlin.mff.cuni.cz> mailing list. + + + +0. Structure of PCI drivers +~~~~~~~~~~~~~~~~~~~~~~~~~~~ +PCI drivers "discover" PCI devices in a system via pci_register_driver(). +Actually, it's the other way around. When the PCI generic code discovers +a new device, the driver with a matching "description" will be notified. +Details on this below. + +pci_register_driver() leaves most of the probing for devices to +the PCI layer and supports online insertion/removal of devices [thus +supporting hot-pluggable PCI, CardBus, and Express-Card in a single driver]. +pci_register_driver() call requires passing in a table of function +pointers and thus dictates the high level structure of a driver. + +Once the driver knows about a PCI device and takes ownership, the +driver generally needs to perform the following initialization: + + Enable the device + Request MMIO/IOP resources + Set the DMA mask size (for both coherent and streaming DMA) + Allocate and initialize shared control data (pci_allocate_coherent()) + Access device configuration space (if needed) + Register IRQ handler (request_irq()) + Initialize non-PCI (i.e. LAN/SCSI/etc parts of the chip) + Enable DMA/processing engines + +When done using the device, and perhaps the module needs to be unloaded, +the driver needs to take the follow steps: + Disable the device from generating IRQs + Release the IRQ (free_irq()) + Stop all DMA activity + Release DMA buffers (both streaming and coherent) + Unregister from other subsystems (e.g. scsi or netdev) + Release MMIO/IOP resources + Disable the device + +Most of these topics are covered in the following sections. +For the rest look at LDD3 or <linux/pci.h> . + +If the PCI subsystem is not configured (CONFIG_PCI is not set), most of +the PCI 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. pci_register_driver() call +~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ + +PCI device drivers call pci_register_driver() during their +initialization with a pointer to a structure describing the driver +(struct pci_driver): + + field name Description + ---------- ------------------------------------------------------ + 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 This probing function 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 + "owned" by the other drivers yet. This function gets + passed a "struct pci_dev *" for each device whose + entry in the ID table matches the device. The probe + function returns zero when the driver chooses to + take "ownership" of the device or an error code + (negative number) otherwise. + The probe function always gets called from process + context, so it can sleep. + + remove The remove() function 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). + The remove function always gets called from process + context, so it can sleep. + + suspend Put device into low power state. + suspend_late Put device into low power state. + + resume_early Wake device from low power state. + resume Wake device from low power state. + + (Please see Documentation/power/pci.txt for descriptions + of PCI Power Management and the related functions.) + + shutdown Hook into reboot_notifier_list (kernel/sys.c). + Intended to stop any idling DMA operations. + Useful for enabling wake-on-lan (NIC) or changing + the power state of a device before reboot. + e.g. drivers/net/e100.c. + + err_handler See Documentation/PCI/pci-error-recovery.txt + + +The ID table is an array of struct pci_device_id entries ending with an +all-zero entry; use of the macro DEFINE_PCI_DEVICE_TABLE is the preferred +method of declaring the table. 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, subclass, and "interface" to match. + See Appendix D of the PCI Local Bus Spec or + include/linux/pci_ids.h for a full list of classes. + Most drivers do not need to specify class/class_mask + as vendor/device is normally sufficient. + + class_mask limit which sub-fields of the class field are compared. + See drivers/scsi/sym53c8xx_2/ for example of usage. + + driver_data Data private to the driver. + Most drivers don't need to use driver_data field. + Best practice is to use driver_data as an index + into a static list of equivalent device types, + instead of using it as a pointer. + + +Most drivers only need PCI_DEVICE() or PCI_DEVICE_CLASS() to set up +a pci_device_id table. + +New PCI IDs may be added to a device driver pci_ids table at runtime +as shown below: + +echo "vendor device subvendor subdevice class class_mask driver_data" > \ +/sys/bus/pci/drivers/{driver}/new_id + +All fields are passed in as hexadecimal values (no leading 0x). +The vendor and device fields are mandatory, the others are optional. Users +need pass only as many optional fields as necessary: + o subvendor and subdevice fields default to PCI_ANY_ID (FFFFFFFF) + o class and classmask fields default to 0 + o driver_data defaults to 0UL. + +Once added, the driver probe routine will be invoked for any unclaimed +PCI devices listed in its (newly updated) pci_ids list. + +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. + + +1.1 "Attributes" for driver functions/data + +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 on when/where to use the above attributes: + o The module_init()/module_exit() functions (and all + initialization functions called _only_ from these) + should be marked __init/__exit. + + o Do not mark the struct pci_driver. + + o The ID table array should be marked __devinitconst; this is done + automatically if the table is declared with DEFINE_PCI_DEVICE_TABLE(). + + o The probe() and remove() functions should be marked __devinit + and __devexit respectively. All initialization functions + exclusively called by the probe() routine, can be marked __devinit. + Ditto for remove() and __devexit. + + o If mydriver_remove() is marked with __devexit(), then all address + references to mydriver_remove must use __devexit_p(mydriver_remove) + (in the struct pci_driver declaration for example). + __devexit_p() will generate the function name _or_ NULL if the + function will be discarded. For an example, see drivers/net/tg3.c. + + o Do NOT mark a function if you are not sure which mark to use. + Better to not mark the function than mark the function wrong. + + + +2. How to find PCI devices manually +~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ + +PCI drivers should have a really good reason for not using the +pci_register_driver() interface to search for PCI devices. +The main reason PCI devices are controlled by multiple drivers +is because one PCI device implements several different HW services. +E.g. combined serial/parallel port/floppy controller. + +A manual search may be performed 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. Device Initialization Steps +~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ + +As noted in the introduction, most PCI drivers need the following steps +for device initialization: + + Enable the device + Request MMIO/IOP resources + Set the DMA mask size (for both coherent and streaming DMA) + Allocate and initialize shared control data (pci_allocate_coherent()) + Access device configuration space (if needed) + Register IRQ handler (request_irq()) + Initialize non-PCI (i.e. LAN/SCSI/etc parts of the chip) + Enable DMA/processing engines. + +The driver can access PCI config space registers at any time. +(Well, almost. When running BIST, config space can go away...but +that will just result in a PCI Bus Master Abort and config reads +will return garbage). + + +3.1 Enable the PCI device +~~~~~~~~~~~~~~~~~~~~~~~~~ +Before touching any device registers, the driver needs to enable +the PCI device by calling pci_enable_device(). This will: + o wake up the device if it was in suspended state, + o allocate I/O and memory regions of the device (if BIOS did not), + o allocate an IRQ (if BIOS did not). + +NOTE: pci_enable_device() can fail! Check the return value. + +[ OS BUG: we don't check resource allocations before enabling those + resources. The sequence would make more sense if we called + pci_request_resources() before calling pci_enable_device(). + Currently, the device drivers can't detect the bug when when two + devices have been allocated the same range. This is not a common + problem and unlikely to get fixed soon. + + This has been discussed before but not changed as of 2.6.19: + http://lkml.org/lkml/2006/3/2/194 +] + +pci_set_master() will enable DMA by setting the bus master bit +in the PCI_COMMAND register. It also fixes the latency timer value if +it's set to something bogus by the BIOS. + +If the PCI device can 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. +Check the return value of pci_set_mwi() as not all architectures +or chip-sets may support Memory-Write-Invalidate. Alternatively, +if Mem-Wr-Inval would be nice to have but is not required, call +pci_try_set_mwi() to have the system do its best effort at enabling +Mem-Wr-Inval. + + +3.2 Request MMIO/IOP resources +~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ +Memory (MMIO), and I/O port addresses should NOT be read directly +from the PCI device config space. Use the values in the pci_dev structure +as the PCI "bus address" might have been remapped to a "host physical" +address by the arch/chip-set specific kernel support. + +See Documentation/IO-mapping.txt for how to access device registers +or device memory. + +The device driver needs to call pci_request_region() to verify +no other device is already using the same address resource. +Conversely, drivers should call pci_release_region() AFTER +calling pci_disable_device(). +The idea is to prevent two devices colliding on the same address range. + +[ See OS BUG comment above. Currently (2.6.19), The driver can only + determine MMIO and IO Port resource availability _after_ calling + pci_enable_device(). ] + +Generic flavors of pci_request_region() are request_mem_region() +(for MMIO ranges) and request_region() (for IO Port ranges). +Use these for address resources that are not described by "normal" PCI +BARs. + +Also see pci_request_selected_regions() below. + + +3.3 Set the DMA mask size +~~~~~~~~~~~~~~~~~~~~~~~~~ +[ If anything below doesn't make sense, please refer to + Documentation/DMA-API.txt. This section is just a reminder that + drivers need to indicate DMA capabilities of the device and is not + an authoritative source for DMA interfaces. ] + +While all drivers should explicitly indicate the DMA capability +(e.g. 32 or 64 bit) of the PCI bus master, devices with more than +32-bit bus master capability for streaming data need the driver +to "register" this capability by calling pci_set_dma_mask() with +appropriate parameters. In general this allows more efficient DMA +on systems where System RAM exists above 4G _physical_ address. + +Drivers for all PCI-X and PCIe compliant devices must call +pci_set_dma_mask() as they are 64-bit DMA devices. + +Similarly, drivers must also "register" this capability if the device +can directly address "consistent memory" in System RAM above 4G physical +address by calling pci_set_consistent_dma_mask(). +Again, this includes drivers for all PCI-X and PCIe compliant devices. +Many 64-bit "PCI" devices (before PCI-X) and some PCI-X devices are +64-bit DMA capable for payload ("streaming") data but not control +("consistent") data. + + +3.4 Setup shared control data +~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ +Once the DMA masks are set, the driver can allocate "consistent" (a.k.a. shared) +memory. See Documentation/DMA-API.txt for a full description of +the DMA APIs. This section is just a reminder that it needs to be done +before enabling DMA on the device. + + +3.5 Initialize device registers +~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ +Some drivers will need specific "capability" fields programmed +or other "vendor specific" register initialized or reset. +E.g. clearing pending interrupts. + + +3.6 Register IRQ handler +~~~~~~~~~~~~~~~~~~~~~~~~ +While calling request_irq() is the last step described here, +this is often just another intermediate step to initialize a device. +This step can often be deferred until the device is opened for use. + +All interrupt handlers for IRQ lines should be registered with IRQF_SHARED +and use the devid to map IRQs to devices (remember that all PCI IRQ lines +can be shared). + +request_irq() will associate an interrupt handler and device handle +with an interrupt number. Historically interrupt numbers represent +IRQ lines which run from the PCI device to the Interrupt controller. +With MSI and MSI-X (more below) the interrupt number is a CPU "vector". + +request_irq() also enables the interrupt. Make sure the device is +quiesced and does not have any interrupts pending before registering +the interrupt handler. + +MSI and MSI-X are PCI capabilities. Both are "Message Signaled Interrupts" +which deliver interrupts to the CPU via a DMA write to a Local APIC. +The fundamental difference between MSI and MSI-X is how multiple +"vectors" get allocated. MSI requires contiguous blocks of vectors +while MSI-X can allocate several individual ones. + +MSI capability can be enabled by calling pci_enable_msi() or +pci_enable_msix() before calling request_irq(). This causes +the PCI support to program CPU vector data into the PCI device +capability registers. + +If your PCI device supports both, try to enable MSI-X first. +Only one can be enabled at a time. Many architectures, chip-sets, +or BIOSes do NOT support MSI or MSI-X and the call to pci_enable_msi/msix +will fail. This is important to note since many drivers have +two (or more) interrupt handlers: one for MSI/MSI-X and another for IRQs. +They choose which handler to register with request_irq() based on the +return value from pci_enable_msi/msix(). + +There are (at least) two really good reasons for using MSI: +1) MSI is an exclusive interrupt vector by definition. + This means the interrupt handler doesn't have to verify + its device caused the interrupt. + +2) MSI avoids DMA/IRQ race conditions. DMA to host memory is guaranteed + to be visible to the host CPU(s) when the MSI is delivered. This + is important for both data coherency and avoiding stale control data. + This guarantee allows the driver to omit MMIO reads to flush + the DMA stream. + +See drivers/infiniband/hw/mthca/ or drivers/net/tg3.c for examples +of MSI/MSI-X usage. + + + +4. PCI device shutdown +~~~~~~~~~~~~~~~~~~~~~~~ + +When a PCI device driver is being unloaded, most of the following +steps need to be performed: + + Disable the device from generating IRQs + Release the IRQ (free_irq()) + Stop all DMA activity + Release DMA buffers (both streaming and consistent) + Unregister from other subsystems (e.g. scsi or netdev) + Disable device from responding to MMIO/IO Port addresses + Release MMIO/IO Port resource(s) + + +4.1 Stop IRQs on the device +~~~~~~~~~~~~~~~~~~~~~~~~~~~ +How to do this is chip/device specific. If it's not done, it opens +the possibility of a "screaming interrupt" if (and only if) +the IRQ is shared with another device. + +When the shared IRQ handler is "unhooked", the remaining devices +using the same IRQ line will still need the IRQ enabled. Thus if the +"unhooked" device asserts IRQ line, the system will respond assuming +it was one of the remaining devices asserted the IRQ line. Since none +of the other devices will handle the IRQ, the system will "hang" until +it decides the IRQ isn't going to get handled and masks the IRQ (100,000 +iterations later). Once the shared IRQ is masked, the remaining devices +will stop functioning properly. Not a nice situation. + +This is another reason to use MSI or MSI-X if it's available. +MSI and MSI-X are defined to be exclusive interrupts and thus +are not susceptible to the "screaming interrupt" problem. + + +4.2 Release the IRQ +~~~~~~~~~~~~~~~~~~~ +Once the device is quiesced (no more IRQs), one can call free_irq(). +This function will return control once any pending IRQs are handled, +"unhook" the drivers IRQ handler from that IRQ, and finally release +the IRQ if no one else is using it. + + +4.3 Stop all DMA activity +~~~~~~~~~~~~~~~~~~~~~~~~~ +It's extremely important to stop all DMA operations BEFORE attempting +to deallocate DMA control data. Failure to do so can result in memory +corruption, hangs, and on some chip-sets a hard crash. + +Stopping DMA after stopping the IRQs can avoid races where the +IRQ handler might restart DMA engines. + +While this step sounds obvious and trivial, several "mature" drivers +didn't get this step right in the past. + + +4.4 Release DMA buffers +~~~~~~~~~~~~~~~~~~~~~~~ +Once DMA is stopped, clean up streaming DMA first. +I.e. unmap data buffers and return buffers to "upstream" +owners if there is one. + +Then clean up "consistent" buffers which contain the control data. + +See Documentation/DMA-API.txt for details on unmapping interfaces. + + +4.5 Unregister from other subsystems +~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ +Most low level PCI device drivers support some other subsystem +like USB, ALSA, SCSI, NetDev, Infiniband, etc. Make sure your +driver isn't losing resources from that other subsystem. +If this happens, typically the symptom is an Oops (panic) when +the subsystem attempts to call into a driver that has been unloaded. + + +4.6 Disable Device from responding to MMIO/IO Port addresses +~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ +io_unmap() MMIO or IO Port resources and then call pci_disable_device(). +This is the symmetric opposite of pci_enable_device(). +Do not access device registers after calling pci_disable_device(). + + +4.7 Release MMIO/IO Port Resource(s) +~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ +Call pci_release_region() to mark the MMIO or IO Port range as available. +Failure to do so usually results in the inability to reload the driver. + + + +5. 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. + + + +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_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 device 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). + +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. + +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. Vendor and device identifications +~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ + +One is not not required to add new device ids to include/linux/pci_ids.h. +Please add PCI_VENDOR_ID_xxx for vendors and a hex constant for device ids. + +PCI_VENDOR_ID_xxx constants are re-used. The device ids are arbitrary +hex numbers (vendor controlled) and normally used only in a single +location, the pci_device_id table. + +Please DO submit new vendor/device ids to pciids.sourceforge.net project. + + + +9. 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. + +pci_find_device() Superseded by pci_get_device() +pci_find_subsys() Superseded by pci_get_subsys() +pci_find_slot() Superseded by pci_get_slot() + + +The alternative is the traditional PCI device driver that walks PCI +device lists. This is still possible but discouraged. + + + +10. MMIO Space and "Write Posting" +~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ + +Converting a driver from using I/O Port space to using MMIO space +often requires some additional changes. Specifically, "write posting" +needs to be handled. Many drivers (e.g. tg3, acenic, sym53c8xx_2) +already do this. I/O Port space guarantees write transactions reach the PCI +device before the CPU can continue. Writes to MMIO space allow the CPU +to continue before the transaction reaches the PCI device. HW weenies +call this "Write Posting" because the write completion is "posted" to +the CPU before the transaction has reached its destination. + +Thus, timing sensitive code should add readl() where the CPU is +expected to wait before doing other work. The classic "bit banging" +sequence works fine for I/O Port space: + + for (i = 8; --i; val >>= 1) { + outb(val & 1, ioport_reg); /* write bit */ + udelay(10); + } + +The same sequence for MMIO space should be: + + for (i = 8; --i; val >>= 1) { + writeb(val & 1, mmio_reg); /* write bit */ + readb(safe_mmio_reg); /* flush posted write */ + udelay(10); + } + +It is important that "safe_mmio_reg" not have any side effects that +interferes with the correct operation of the device. + +Another case to watch out for is when resetting a PCI device. Use PCI +Configuration space reads to flush the writel(). This will gracefully +handle the PCI master abort on all platforms if the PCI device is +expected to not respond to a readl(). Most x86 platforms will allow +MMIO reads to master abort (a.k.a. "Soft Fail") and return garbage +(e.g. ~0). But many RISC platforms will crash (a.k.a."Hard Fail"). + |
