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diff --git a/Documentation/PCI/MSI-HOWTO.txt b/Documentation/PCI/MSI-HOWTO.txt deleted file mode 100644 index 618e13d5e276..000000000000 --- a/Documentation/PCI/MSI-HOWTO.txt +++ /dev/null @@ -1,270 +0,0 @@ - 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 - Revised Jul 9, 2008 by Matthew Wilcox <willy@linux.intel.com> - Copyright 2003, 2008 Intel Corporation - -1. About this guide - -This guide describes the basics of Message Signaled Interrupts (MSIs), -the advantages of using MSI over traditional interrupt mechanisms, how -to change your driver to use MSI or MSI-X and some basic diagnostics to -try if a device doesn't support MSIs. - - -2. What are MSIs? - -A Message Signaled Interrupt is a write from the device to a special -address which causes an interrupt to be received by the CPU. - -The MSI capability was first specified in PCI 2.2 and was later enhanced -in PCI 3.0 to allow each interrupt to be masked individually. The MSI-X -capability was also introduced with PCI 3.0. It supports more interrupts -per device than MSI and allows interrupts to be independently configured. - -Devices may support both MSI and MSI-X, but only one can be enabled at -a time. - - -3. Why use MSIs? - -There are three reasons why using MSIs can give an advantage over -traditional pin-based interrupts. - -Pin-based PCI interrupts are often shared amongst several devices. -To support this, the kernel must call each interrupt handler associated -with an interrupt, which leads to reduced performance for the system as -a whole. MSIs are never shared, so this problem cannot arise. - -When a device writes data to memory, then raises a pin-based interrupt, -it is possible that the interrupt may arrive before all the data has -arrived in memory (this becomes more likely with devices behind PCI-PCI -bridges). In order to ensure that all the data has arrived in memory, -the interrupt handler must read a register on the device which raised -the interrupt. PCI transaction ordering rules require that all the data -arrive in memory before the value may be returned from the register. -Using MSIs avoids this problem as the interrupt-generating write cannot -pass the data writes, so by the time the interrupt is raised, the driver -knows that all the data has arrived in memory. - -PCI devices can only support a single pin-based interrupt per function. -Often drivers have to query the device to find out what event has -occurred, slowing down interrupt handling for the common case. With -MSIs, a device can support more interrupts, allowing each interrupt -to be specialised to a different purpose. One possible design gives -infrequent conditions (such as errors) their own interrupt which allows -the driver to handle the normal interrupt handling path more efficiently. -Other possible designs include giving one interrupt to each packet queue -in a network card or each port in a storage controller. - - -4. How to use MSIs - -PCI devices are initialised to use pin-based interrupts. The device -driver has to set up the device to use MSI or MSI-X. Not all machines -support MSIs correctly, and for those machines, the APIs described below -will simply fail and the device will continue to use pin-based interrupts. - -4.1 Include kernel support for MSIs - -To support MSI or MSI-X, the kernel must be built with the CONFIG_PCI_MSI -option enabled. This option is only available on some architectures, -and it may depend on some other options also being set. For example, -on x86, you must also enable X86_UP_APIC or SMP in order to see the -CONFIG_PCI_MSI option. - -4.2 Using MSI - -Most of the hard work is done for the driver in the PCI layer. The driver -simply has to request that the PCI layer set up the MSI capability for this -device. - -To automatically use MSI or MSI-X interrupt vectors, use the following -function: - - int pci_alloc_irq_vectors(struct pci_dev *dev, unsigned int min_vecs, - unsigned int max_vecs, unsigned int flags); - -which allocates up to max_vecs interrupt vectors for a PCI device. It -returns the number of vectors allocated or a negative error. If the device -has a requirements for a minimum number of vectors the driver can pass a -min_vecs argument set to this limit, and the PCI core will return -ENOSPC -if it can't meet the minimum number of vectors. - -The flags argument is used to specify which type of interrupt can be used -by the device and the driver (PCI_IRQ_LEGACY, PCI_IRQ_MSI, PCI_IRQ_MSIX). -A convenient short-hand (PCI_IRQ_ALL_TYPES) is also available to ask for -any possible kind of interrupt. If the PCI_IRQ_AFFINITY flag is set, -pci_alloc_irq_vectors() will spread the interrupts around the available CPUs. - -To get the Linux IRQ numbers passed to request_irq() and free_irq() and the -vectors, use the following function: - - int pci_irq_vector(struct pci_dev *dev, unsigned int nr); - -Any allocated resources should be freed before removing the device using -the following function: - - void pci_free_irq_vectors(struct pci_dev *dev); - -If a device supports both MSI-X and MSI capabilities, this API will use the -MSI-X facilities in preference to the MSI facilities. MSI-X supports any -number of interrupts between 1 and 2048. In contrast, MSI is restricted to -a maximum of 32 interrupts (and must be a power of two). In addition, the -MSI interrupt vectors must be allocated consecutively, so the system might -not be able to allocate as many vectors for MSI as it could for MSI-X. On -some platforms, MSI interrupts must all be targeted at the same set of CPUs -whereas MSI-X interrupts can all be targeted at different CPUs. - -If a device supports neither MSI-X or MSI it will fall back to a single -legacy IRQ vector. - -The typical usage of MSI or MSI-X interrupts is to allocate as many vectors -as possible, likely up to the limit supported by the device. If nvec is -larger than the number supported by the device it will automatically be -capped to the supported limit, so there is no need to query the number of -vectors supported beforehand: - - nvec = pci_alloc_irq_vectors(pdev, 1, nvec, PCI_IRQ_ALL_TYPES) - if (nvec < 0) - goto out_err; - -If a driver is unable or unwilling to deal with a variable number of MSI -interrupts it can request a particular number of interrupts by passing that -number to pci_alloc_irq_vectors() function as both 'min_vecs' and -'max_vecs' parameters: - - ret = pci_alloc_irq_vectors(pdev, nvec, nvec, PCI_IRQ_ALL_TYPES); - if (ret < 0) - goto out_err; - -The most notorious example of the request type described above is enabling -the single MSI mode for a device. It could be done by passing two 1s as -'min_vecs' and 'max_vecs': - - ret = pci_alloc_irq_vectors(pdev, 1, 1, PCI_IRQ_ALL_TYPES); - if (ret < 0) - goto out_err; - -Some devices might not support using legacy line interrupts, in which case -the driver can specify that only MSI or MSI-X is acceptable: - - nvec = pci_alloc_irq_vectors(pdev, 1, nvec, PCI_IRQ_MSI | PCI_IRQ_MSIX); - if (nvec < 0) - goto out_err; - -4.3 Legacy APIs - -The following old APIs to enable and disable MSI or MSI-X interrupts should -not be used in new code: - - pci_enable_msi() /* deprecated */ - pci_disable_msi() /* deprecated */ - pci_enable_msix_range() /* deprecated */ - pci_enable_msix_exact() /* deprecated */ - pci_disable_msix() /* deprecated */ - -Additionally there are APIs to provide the number of supported MSI or MSI-X -vectors: pci_msi_vec_count() and pci_msix_vec_count(). In general these -should be avoided in favor of letting pci_alloc_irq_vectors() cap the -number of vectors. If you have a legitimate special use case for the count -of vectors we might have to revisit that decision and add a -pci_nr_irq_vectors() helper that handles MSI and MSI-X transparently. - -4.4 Considerations when using MSIs - -4.4.1 Spinlocks - -Most device drivers have a per-device spinlock which is taken in the -interrupt handler. With pin-based interrupts or a single MSI, it is not -necessary to disable interrupts (Linux guarantees the same interrupt will -not be re-entered). If a device uses multiple interrupts, the driver -must disable interrupts while the lock is held. If the device sends -a different interrupt, the driver will deadlock trying to recursively -acquire the spinlock. Such deadlocks can be avoided by using -spin_lock_irqsave() or spin_lock_irq() which disable local interrupts -and acquire the lock (see Documentation/kernel-hacking/locking.rst). - -4.5 How to tell whether MSI/MSI-X is enabled on a device - -Using 'lspci -v' (as root) may show some devices with "MSI", "Message -Signalled Interrupts" or "MSI-X" capabilities. Each of these capabilities -has an 'Enable' flag which is followed with either "+" (enabled) -or "-" (disabled). - - -5. MSI quirks - -Several PCI chipsets or devices are known not to support MSIs. -The PCI stack provides three ways to disable MSIs: - -1. globally -2. on all devices behind a specific bridge -3. on a single device - -5.1. Disabling MSIs globally - -Some host chipsets simply don't support MSIs properly. If we're -lucky, the manufacturer knows this and has indicated it in the ACPI -FADT table. In this case, Linux automatically disables MSIs. -Some boards don't include this information in the table and so we have -to detect them ourselves. The complete list of these is found near the -quirk_disable_all_msi() function in drivers/pci/quirks.c. - -If you have a board which has problems with MSIs, you can pass pci=nomsi -on the kernel command line to disable MSIs on all devices. It would be -in your best interests to report the problem to linux-pci@vger.kernel.org -including a full 'lspci -v' so we can add the quirks to the kernel. - -5.2. Disabling MSIs below a bridge - -Some PCI bridges are not able to route MSIs between busses properly. -In this case, MSIs must be disabled on all devices behind the bridge. - -Some bridges allow you to enable MSIs by changing some bits in their -PCI configuration space (especially the Hypertransport chipsets such -as the nVidia nForce and Serverworks HT2000). As with host chipsets, -Linux mostly knows about them and automatically enables MSIs if it can. -If you have a bridge unknown to Linux, you can enable -MSIs in configuration space using whatever method you know works, then -enable MSIs on that bridge by doing: - - echo 1 > /sys/bus/pci/devices/$bridge/msi_bus - -where $bridge is the PCI address of the bridge you've enabled (eg -0000:00:0e.0). - -To disable MSIs, echo 0 instead of 1. Changing this value should be -done with caution as it could break interrupt handling for all devices -below this bridge. - -Again, please notify linux-pci@vger.kernel.org of any bridges that need -special handling. - -5.3. Disabling MSIs on a single device - -Some devices are known to have faulty MSI implementations. Usually this -is handled in the individual device driver, but occasionally it's necessary -to handle this with a quirk. Some drivers have an option to disable use -of MSI. While this is a convenient workaround for the driver author, -it is not good practice, and should not be emulated. - -5.4. Finding why MSIs are disabled on a device - -From the above three sections, you can see that there are many reasons -why MSIs may not be enabled for a given device. Your first step should -be to examine your dmesg carefully to determine whether MSIs are enabled -for your machine. You should also check your .config to be sure you -have enabled CONFIG_PCI_MSI. - -Then, 'lspci -t' gives the list of bridges above a device. Reading -/sys/bus/pci/devices/*/msi_bus will tell you whether MSIs are enabled (1) -or disabled (0). If 0 is found in any of the msi_bus files belonging -to bridges between the PCI root and the device, MSIs are disabled. - -It is also worth checking the device driver to see whether it supports MSIs. -For example, it may contain calls to pci_irq_alloc_vectors() with the -PCI_IRQ_MSI or PCI_IRQ_MSIX flags. |