path: root/Documentation/PCI/MSI-HOWTO.txt
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authorRandy Dunlap <randy.dunlap@oracle.com>2008-11-13 21:33:24 +0000
committerRandy Dunlap <randy.dunlap@oracle.com>2008-11-14 17:28:53 +0000
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parent3edac25f2e8ac8c2a84904c140e1aeb434e73e75 (diff)
Create/use more directory structure in the Documentation/ tree.
Create Documentation/blockdev/ sub-directory and populate it. Populate the Documentation/serial/ sub-directory. Move MSI-HOWTO.txt to Documentation/PCI/. Move ioctl-number.txt to Documentation/ioctl/. Update all relevant 00-INDEX files. Update all relevant Kconfig files and source files. Signed-off-by: Randy Dunlap <randy.dunlap@oracle.com>
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+ The MSI Driver Guide HOWTO
+ Tom L Nguyen tom.l.nguyen@intel.com
+ 10/03/2003
+ Revised Feb 12, 2004 by Martine Silbermann
+ email: Martine.Silbermann@hp.com
+ Revised Jun 25, 2004 by Tom L Nguyen
+1. About this guide
+This guide describes the basics of Message Signaled Interrupts (MSI),
+the advantages of using MSI over traditional interrupt mechanisms,
+and how to enable your driver to use MSI or MSI-X. Also included is
+a Frequently Asked Questions (FAQ) section.
+1.1 Terminology
+PCI devices can be single-function or multi-function. In either case,
+when this text talks about enabling or disabling MSI on a "device
+function," it is referring to one specific PCI device and function and
+not to all functions on a PCI device (unless the PCI device has only
+one function).
+2. Copyright 2003 Intel Corporation
+3. What is MSI/MSI-X?
+Message Signaled Interrupt (MSI), as described in the PCI Local Bus
+Specification Revision 2.3 or later, is an optional feature, and a
+required feature for PCI Express devices. MSI enables a device function
+to request service by sending an Inbound Memory Write on its PCI bus to
+the FSB as a Message Signal Interrupt transaction. Because MSI is
+generated in the form of a Memory Write, all transaction conditions,
+such as a Retry, Master-Abort, Target-Abort or normal completion, are
+A PCI device that supports MSI must also support pin IRQ assertion
+interrupt mechanism to provide backward compatibility for systems that
+do not support MSI. In systems which support MSI, the bus driver is
+responsible for initializing the message address and message data of
+the device function's MSI/MSI-X capability structure during device
+initial configuration.
+An MSI capable device function indicates MSI support by implementing
+the MSI/MSI-X capability structure in its PCI capability list. The
+device function may implement both the MSI capability structure and
+the MSI-X capability structure; however, the bus driver should not
+enable both.
+The MSI capability structure contains Message Control register,
+Message Address register and Message Data register. These registers
+provide the bus driver control over MSI. The Message Control register
+indicates the MSI capability supported by the device. The Message
+Address register specifies the target address and the Message Data
+register specifies the characteristics of the message. To request
+service, the device function writes the content of the Message Data
+register to the target address. The device and its software driver
+are prohibited from writing to these registers.
+The MSI-X capability structure is an optional extension to MSI. It
+uses an independent and separate capability structure. There are
+some key advantages to implementing the MSI-X capability structure
+over the MSI capability structure as described below.
+ - Support a larger maximum number of vectors per function.
+ - Provide the ability for system software to configure
+ each vector with an independent message address and message
+ data, specified by a table that resides in Memory Space.
+ - MSI and MSI-X both support per-vector masking. Per-vector
+ masking is an optional extension of MSI but a required
+ feature for MSI-X. Per-vector masking provides the kernel the
+ ability to mask/unmask a single MSI while running its
+ interrupt service routine. If per-vector masking is
+ not supported, then the device driver should provide the
+ hardware/software synchronization to ensure that the device
+ generates MSI when the driver wants it to do so.
+4. Why use MSI?
+As a benefit to the simplification of board design, MSI allows board
+designers to remove out-of-band interrupt routing. MSI is another
+step towards a legacy-free environment.
+Due to increasing pressure on chipset and processor packages to
+reduce pin count, the need for interrupt pins is expected to
+diminish over time. Devices, due to pin constraints, may implement
+messages to increase performance.
+PCI Express endpoints uses INTx emulation (in-band messages) instead
+of IRQ pin assertion. Using INTx emulation requires interrupt
+sharing among devices connected to the same node (PCI bridge) while
+MSI is unique (non-shared) and does not require BIOS configuration
+support. As a result, the PCI Express technology requires MSI
+support for better interrupt performance.
+Using MSI enables the device functions to support two or more
+vectors, which can be configured to target different CPUs to
+increase scalability.
+5. Configuring a driver to use MSI/MSI-X
+By default, the kernel will not enable MSI/MSI-X on all devices that
+support this capability. The CONFIG_PCI_MSI kernel option
+must be selected to enable MSI/MSI-X support.
+5.1 Including MSI/MSI-X support into the kernel
+To allow MSI/MSI-X capable device drivers to selectively enable
+MSI/MSI-X (using pci_enable_msi()/pci_enable_msix() as described
+below), the VECTOR based scheme needs to be enabled by setting
+CONFIG_PCI_MSI during kernel config.
+Since the target of the inbound message is the local APIC, providing
+CONFIG_X86_LOCAL_APIC must be enabled as well as CONFIG_PCI_MSI.
+5.2 Configuring for MSI support
+Due to the non-contiguous fashion in vector assignment of the
+existing Linux kernel, this version does not support multiple
+messages regardless of a device function is capable of supporting
+more than one vector. To enable MSI on a device function's MSI
+capability structure requires a device driver to call the function
+pci_enable_msi() explicitly.
+5.2.1 API pci_enable_msi
+int pci_enable_msi(struct pci_dev *dev)
+With this new API, a device driver that wants to have MSI
+enabled on its device function must call this API to enable MSI.
+A successful call will initialize the MSI capability structure
+with ONE vector, regardless of whether a device function is
+capable of supporting multiple messages. This vector replaces the
+pre-assigned dev->irq with a new MSI vector. To avoid a conflict
+of the new assigned vector with existing pre-assigned vector requires
+a device driver to call this API before calling request_irq().
+5.2.2 API pci_disable_msi
+void pci_disable_msi(struct pci_dev *dev)
+This API should always be used to undo the effect of pci_enable_msi()
+when a device driver is unloading. This API restores dev->irq with
+the pre-assigned IOAPIC vector and switches a device's interrupt
+mode to PCI pin-irq assertion/INTx emulation mode.
+Note that a device driver should always call free_irq() on the MSI vector
+that it has done request_irq() on before calling this API. Failure to do
+so results in a BUG_ON() and a device will be left with MSI enabled and
+leaks its vector.
+5.2.3 MSI mode vs. legacy mode diagram
+The below diagram shows the events which switch the interrupt
+mode on the MSI-capable device function between MSI mode and
+PIN-IRQ assertion mode.
+ ------------ pci_enable_msi ------------------------
+ | | <=============== | |
+ | | ===============> | |
+ ------------ pci_disable_msi ------------------------
+Figure 1. MSI Mode vs. Legacy Mode
+In Figure 1, a device operates by default in legacy mode. Legacy
+in this context means PCI pin-irq assertion or PCI-Express INTx
+emulation. A successful MSI request (using pci_enable_msi()) switches
+a device's interrupt mode to MSI mode. A pre-assigned IOAPIC vector
+stored in dev->irq will be saved by the PCI subsystem and a new
+assigned MSI vector will replace dev->irq.
+To return back to its default mode, a device driver should always call
+pci_disable_msi() to undo the effect of pci_enable_msi(). Note that a
+device driver should always call free_irq() on the MSI vector it has
+done request_irq() on before calling pci_disable_msi(). Failure to do
+so results in a BUG_ON() and a device will be left with MSI enabled and
+leaks its vector. Otherwise, the PCI subsystem restores a device's
+dev->irq with a pre-assigned IOAPIC vector and marks the released
+MSI vector as unused.
+Once being marked as unused, there is no guarantee that the PCI
+subsystem will reserve this MSI vector for a device. Depending on
+the availability of current PCI vector resources and the number of
+MSI/MSI-X requests from other drivers, this MSI may be re-assigned.
+For the case where the PCI subsystem re-assigns this MSI vector to
+another driver, a request to switch back to MSI mode may result
+in being assigned a different MSI vector or a failure if no more
+vectors are available.
+5.3 Configuring for MSI-X support
+Due to the ability of the system software to configure each vector of
+the MSI-X capability structure with an independent message address
+and message data, the non-contiguous fashion in vector assignment of
+the existing Linux kernel has no impact on supporting multiple
+messages on an MSI-X capable device functions. To enable MSI-X on
+a device function's MSI-X capability structure requires its device
+driver to call the function pci_enable_msix() explicitly.
+The function pci_enable_msix(), once invoked, enables either
+all or nothing, depending on the current availability of PCI vector
+resources. If the PCI vector resources are available for the number
+of vectors requested by a device driver, this function will configure
+the MSI-X table of the MSI-X capability structure of a device with
+requested messages. To emphasize this reason, for example, a device
+may be capable for supporting the maximum of 32 vectors while its
+software driver usually may request 4 vectors. It is recommended
+that the device driver should call this function once during the
+initialization phase of the device driver.
+Unlike the function pci_enable_msi(), the function pci_enable_msix()
+does not replace the pre-assigned IOAPIC dev->irq with a new MSI
+vector because the PCI subsystem writes the 1:1 vector-to-entry mapping
+into the field vector of each element contained in a second argument.
+Note that the pre-assigned IOAPIC dev->irq is valid only if the device
+operates in PIN-IRQ assertion mode. In MSI-X mode, any attempt at
+using dev->irq by the device driver to request for interrupt service
+may result in unpredictable behavior.
+For each MSI-X vector granted, a device driver is responsible for calling
+other functions like request_irq(), enable_irq(), etc. to enable
+this vector with its corresponding interrupt service handler. It is
+a device driver's choice to assign all vectors with the same
+interrupt service handler or each vector with a unique interrupt
+service handler.
+5.3.1 Handling MMIO address space of MSI-X Table
+The PCI 3.0 specification has implementation notes that MMIO address
+space for a device's MSI-X structure should be isolated so that the
+software system can set different pages for controlling accesses to the
+MSI-X structure. The implementation of MSI support requires the PCI
+subsystem, not a device driver, to maintain full control of the MSI-X
+table/MSI-X PBA (Pending Bit Array) and MMIO address space of the MSI-X
+table/MSI-X PBA. A device driver should not access the MMIO address
+space of the MSI-X table/MSI-X PBA.
+5.3.2 API pci_enable_msix
+int pci_enable_msix(struct pci_dev *dev, struct msix_entry *entries, int nvec)
+This API enables a device driver to request the PCI subsystem
+to enable MSI-X messages on its hardware device. Depending on
+the availability of PCI vectors resources, the PCI subsystem enables
+either all or none of the requested vectors.
+Argument 'dev' points to the device (pci_dev) structure.
+Argument 'entries' is a pointer to an array of msix_entry structs.
+The number of entries is indicated in argument 'nvec'.
+struct msix_entry is defined in /driver/pci/msi.h:
+struct msix_entry {
+ u16 vector; /* kernel uses to write alloc vector */
+ u16 entry; /* driver uses to specify entry */
+A device driver is responsible for initializing the field 'entry' of
+each element with a unique entry supported by MSI-X table. Otherwise,
+-EINVAL will be returned as a result. A successful return of zero
+indicates the PCI subsystem completed initializing each of the requested
+entries of the MSI-X table with message address and message data.
+Last but not least, the PCI subsystem will write the 1:1
+vector-to-entry mapping into the field 'vector' of each element. A
+device driver is responsible for keeping track of allocated MSI-X
+vectors in its internal data structure.
+A return of zero indicates that the number of MSI-X vectors was
+successfully allocated. A return of greater than zero indicates
+MSI-X vector shortage. Or a return of less than zero indicates
+a failure. This failure may be a result of duplicate entries
+specified in second argument, or a result of no available vector,
+or a result of failing to initialize MSI-X table entries.
+5.3.3 API pci_disable_msix
+void pci_disable_msix(struct pci_dev *dev)
+This API should always be used to undo the effect of pci_enable_msix()
+when a device driver is unloading. Note that a device driver should
+always call free_irq() on all MSI-X vectors it has done request_irq()
+on before calling this API. Failure to do so results in a BUG_ON() and
+a device will be left with MSI-X enabled and leaks its vectors.
+5.3.4 MSI-X mode vs. legacy mode diagram
+The below diagram shows the events which switch the interrupt
+mode on the MSI-X capable device function between MSI-X mode and
+PIN-IRQ assertion mode (legacy).
+ ------------ pci_enable_msix(,,n) ------------------------
+ | | <=============== | |
+ | | ===============> | |
+ ------------ pci_disable_msix ------------------------
+Figure 2. MSI-X Mode vs. Legacy Mode
+In Figure 2, a device operates by default in legacy mode. A
+successful MSI-X request (using pci_enable_msix()) switches a
+device's interrupt mode to MSI-X mode. A pre-assigned IOAPIC vector
+stored in dev->irq will be saved by the PCI subsystem; however,
+unlike MSI mode, the PCI subsystem will not replace dev->irq with
+assigned MSI-X vector because the PCI subsystem already writes the 1:1
+vector-to-entry mapping into the field 'vector' of each element
+specified in second argument.
+To return back to its default mode, a device driver should always call
+pci_disable_msix() to undo the effect of pci_enable_msix(). Note that
+a device driver should always call free_irq() on all MSI-X vectors it
+has done request_irq() on before calling pci_disable_msix(). Failure
+to do so results in a BUG_ON() and a device will be left with MSI-X
+enabled and leaks its vectors. Otherwise, the PCI subsystem switches a
+device function's interrupt mode from MSI-X mode to legacy mode and
+marks all allocated MSI-X vectors as unused.
+Once being marked as unused, there is no guarantee that the PCI
+subsystem will reserve these MSI-X vectors for a device. Depending on
+the availability of current PCI vector resources and the number of
+MSI/MSI-X requests from other drivers, these MSI-X vectors may be
+For the case where the PCI subsystem re-assigned these MSI-X vectors
+to other drivers, a request to switch back to MSI-X mode may result
+being assigned with another set of MSI-X vectors or a failure if no
+more vectors are available.
+5.4 Handling function implementing both MSI and MSI-X capabilities
+For the case where a function implements both MSI and MSI-X
+capabilities, the PCI subsystem enables a device to run either in MSI
+mode or MSI-X mode but not both. A device driver determines whether it
+wants MSI or MSI-X enabled on its hardware device. Once a device
+driver requests for MSI, for example, it is prohibited from requesting
+MSI-X; in other words, a device driver is not permitted to ping-pong
+between MSI mod MSI-X mode during a run-time.
+5.5 Hardware requirements for MSI/MSI-X support
+MSI/MSI-X support requires support from both system hardware and
+individual hardware device functions.
+5.5.1 Required x86 hardware support
+Since the target of MSI address is the local APIC CPU, enabling
+MSI/MSI-X support in the Linux kernel is dependent on whether existing
+system hardware supports local APIC. Users should verify that their
+system supports local APIC operation by testing that it runs when
+In SMP environment, CONFIG_X86_LOCAL_APIC is automatically set;
+however, in UP environment, users must manually set
+CONFIG_PCI_MSI enables the VECTOR based scheme and the option for
+MSI-capable device drivers to selectively enable MSI/MSI-X.
+Note that CONFIG_X86_IO_APIC setting is irrelevant because MSI/MSI-X
+vector is allocated new during runtime and MSI/MSI-X support does not
+depend on BIOS support. This key independency enables MSI/MSI-X
+support on future IOxAPIC free platforms.
+5.5.2 Device hardware support
+The hardware device function supports MSI by indicating the
+MSI/MSI-X capability structure on its PCI capability list. By
+default, this capability structure will not be initialized by
+the kernel to enable MSI during the system boot. In other words,
+the device function is running on its default pin assertion mode.
+Note that in many cases the hardware supporting MSI have bugs,
+which may result in system hangs. The software driver of specific
+MSI-capable hardware is responsible for deciding whether to call
+pci_enable_msi or not. A return of zero indicates the kernel
+successfully initialized the MSI/MSI-X capability structure of the
+device function. The device function is now running on MSI/MSI-X mode.
+5.6 How to tell whether MSI/MSI-X is enabled on device function
+At the driver level, a return of zero from the function call of
+pci_enable_msi()/pci_enable_msix() indicates to a device driver that
+its device function is initialized successfully and ready to run in
+MSI/MSI-X mode.
+At the user level, users can use the command 'cat /proc/interrupts'
+to display the vectors allocated for devices and their interrupt
+MSI/MSI-X modes ("PCI-MSI"/"PCI-MSI-X"). Below shows MSI mode is
+enabled on a SCSI Adaptec 39320D Ultra320 controller.
+ 0: 324639 0 IO-APIC-edge timer
+ 1: 1186 0 IO-APIC-edge i8042
+ 2: 0 0 XT-PIC cascade
+ 12: 2797 0 IO-APIC-edge i8042
+ 14: 6543 0 IO-APIC-edge ide0
+ 15: 1 0 IO-APIC-edge ide1
+169: 0 0 IO-APIC-level uhci-hcd
+185: 0 0 IO-APIC-level uhci-hcd
+193: 138 10 PCI-MSI aic79xx
+201: 30 0 PCI-MSI aic79xx
+225: 30 0 IO-APIC-level aic7xxx
+233: 30 0 IO-APIC-level aic7xxx
+NMI: 0 0
+LOC: 324553 325068
+ERR: 0
+MIS: 0
+6. MSI quirks
+Several PCI chipsets or devices are known to not support MSI.
+The PCI stack provides 3 possible levels of MSI disabling:
+* on a single device
+* on all devices behind a specific bridge
+* globally
+6.1. Disabling MSI on a single device
+Under some circumstances it might be required to disable MSI on a
+single device. This may be achieved by either not calling pci_enable_msi()
+or all, or setting the pci_dev->no_msi flag before (most of the time
+in a quirk).
+6.2. Disabling MSI below a bridge
+The vast majority of MSI quirks are required by PCI bridges not
+being able to route MSI between busses. In this case, MSI have to be
+disabled on all devices behind this bridge. It is achieves by setting
+the PCI_BUS_FLAGS_NO_MSI flag in the pci_bus->bus_flags of the bridge
+subordinate bus. There is no need to set the same flag on bridges that
+are below the broken bridge. When pci_enable_msi() is called to enable
+MSI on a device, pci_msi_supported() takes care of checking the NO_MSI
+flag in all parent busses of the device.
+Some bridges actually support dynamic MSI support enabling/disabling
+by changing some bits in their PCI configuration space (especially
+the Hypertransport chipsets such as the nVidia nForce and Serverworks
+HT2000). It may then be required to update the NO_MSI flag on the
+corresponding devices in the sysfs hierarchy. To enable MSI support
+on device "0000:00:0e", do:
+ echo 1 > /sys/bus/pci/devices/0000:00:0e/msi_bus
+To disable MSI support, echo 0 instead of 1. Note that it should be
+used with caution since changing this value might break interrupts.
+6.3. Disabling MSI globally
+Some extreme cases may require to disable MSI globally on the system.
+For now, the only known case is a Serverworks PCI-X chipsets (MSI are
+not supported on several busses that are not all connected to the
+chipset in the Linux PCI hierarchy). In the vast majority of other
+cases, disabling only behind a specific bridge is enough.
+For debugging purpose, the user may also pass pci=nomsi on the kernel
+command-line to explicitly disable MSI globally. But, once the appro-
+priate quirks are added to the kernel, this option should not be
+required anymore.
+6.4. Finding why MSI cannot be enabled on a device
+Assuming that MSI are not enabled on a device, you should look at
+dmesg to find messages that quirks may output when disabling MSI
+on some devices, some bridges or even globally.
+Then, lspci -t gives the list of bridges above a device. Reading
+/sys/bus/pci/devices/0000:00:0e/msi_bus will tell you whether MSI
+are enabled (1) or disabled (0). In 0 is found in a single bridge
+msi_bus file above the device, MSI cannot be enabled.
+7. FAQ
+Q1. Are there any limitations on using the MSI?
+A1. If the PCI device supports MSI and conforms to the
+specification and the platform supports the APIC local bus,
+then using MSI should work.
+Q2. Will it work on all the Pentium processors (P3, P4, Xeon,
+AMD processors)? In P3 IPI's are transmitted on the APIC local
+bus and in P4 and Xeon they are transmitted on the system
+bus. Are there any implications with this?
+A2. MSI support enables a PCI device sending an inbound
+memory write (0xfeexxxxx as target address) on its PCI bus
+directly to the FSB. Since the message address has a
+redirection hint bit cleared, it should work.
+Q3. The target address 0xfeexxxxx will be translated by the
+Host Bridge into an interrupt message. Are there any
+limitations on the chipsets such as Intel 8xx, Intel e7xxx,
+or VIA?
+A3. If these chipsets support an inbound memory write with
+target address set as 0xfeexxxxx, as conformed to PCI
+specification 2.3 or latest, then it should work.
+Q4. From the driver point of view, if the MSI is lost because
+of errors occurring during inbound memory write, then it may
+wait forever. Is there a mechanism for it to recover?
+A4. Since the target of the transaction is an inbound memory
+write, all transaction termination conditions (Retry,
+Master-Abort, Target-Abort, or normal completion) are
+supported. A device sending an MSI must abide by all the PCI
+rules and conditions regarding that inbound memory write. So,
+if a retry is signaled it must retry, etc... We believe that
+the recommendation for Abort is also a retry (refer to PCI
+specification 2.3 or latest).

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