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diff --git a/Documentation/PCI/MSI-HOWTO.txt b/Documentation/PCI/MSI-HOWTO.txt new file mode 100644 index 0000000..256defd --- /dev/null +++ b/Documentation/PCI/MSI-HOWTO.txt @@ -0,0 +1,509 @@ + 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 +supported. + +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 ------------------------ + | | <=============== | | + | MSI MODE | | PIN-IRQ ASSERTION MODE | + | | ===============> | | + ------------ 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) ------------------------ + | | <=============== | | + | MSI-X MODE | | PIN-IRQ ASSERTION MODE | + | | ===============> | | + ------------ 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 +re-assigned. + +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 +CONFIG_X86_LOCAL_APIC=y. + +In SMP environment, CONFIG_X86_LOCAL_APIC is automatically set; +however, in UP environment, users must manually set +CONFIG_X86_LOCAL_APIC. Once CONFIG_X86_LOCAL_APIC=y, setting +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. + + CPU0 CPU1 + 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). |
