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Encourage driver writers to think about supporting a variable number of MSI-X interrupts, and give an example of how to do such a request. Acked-by: Matthew Wilcox <willy@linux.intel.com> Signed-off-by: Michael Ellerman <michael@ellerman.id.au> Signed-off-by: Jesse Barnes <jbarnes@virtuousgeek.org>
360 lines
16 KiB
Plaintext
360 lines
16 KiB
Plaintext
The MSI Driver Guide HOWTO
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Tom L Nguyen tom.l.nguyen@intel.com
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10/03/2003
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Revised Feb 12, 2004 by Martine Silbermann
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email: Martine.Silbermann@hp.com
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Revised Jun 25, 2004 by Tom L Nguyen
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Revised Jul 9, 2008 by Matthew Wilcox <willy@linux.intel.com>
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Copyright 2003, 2008 Intel Corporation
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1. About this guide
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This guide describes the basics of Message Signaled Interrupts (MSIs),
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the advantages of using MSI over traditional interrupt mechanisms, how
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to change your driver to use MSI or MSI-X and some basic diagnostics to
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try if a device doesn't support MSIs.
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2. What are MSIs?
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A Message Signaled Interrupt is a write from the device to a special
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address which causes an interrupt to be received by the CPU.
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The MSI capability was first specified in PCI 2.2 and was later enhanced
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in PCI 3.0 to allow each interrupt to be masked individually. The MSI-X
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capability was also introduced with PCI 3.0. It supports more interrupts
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per device than MSI and allows interrupts to be independently configured.
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Devices may support both MSI and MSI-X, but only one can be enabled at
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a time.
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3. Why use MSIs?
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There are three reasons why using MSIs can give an advantage over
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traditional pin-based interrupts.
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Pin-based PCI interrupts are often shared amongst several devices.
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To support this, the kernel must call each interrupt handler associated
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with an interrupt, which leads to reduced performance for the system as
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a whole. MSIs are never shared, so this problem cannot arise.
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When a device writes data to memory, then raises a pin-based interrupt,
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it is possible that the interrupt may arrive before all the data has
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arrived in memory (this becomes more likely with devices behind PCI-PCI
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bridges). In order to ensure that all the data has arrived in memory,
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the interrupt handler must read a register on the device which raised
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the interrupt. PCI transaction ordering rules require that all the data
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arrives in memory before the value can be returned from the register.
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Using MSIs avoids this problem as the interrupt-generating write cannot
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pass the data writes, so by the time the interrupt is raised, the driver
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knows that all the data has arrived in memory.
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PCI devices can only support a single pin-based interrupt per function.
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Often drivers have to query the device to find out what event has
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occurred, slowing down interrupt handling for the common case. With
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MSIs, a device can support more interrupts, allowing each interrupt
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to be specialised to a different purpose. One possible design gives
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infrequent conditions (such as errors) their own interrupt which allows
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the driver to handle the normal interrupt handling path more efficiently.
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Other possible designs include giving one interrupt to each packet queue
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in a network card or each port in a storage controller.
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4. How to use MSIs
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PCI devices are initialised to use pin-based interrupts. The device
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driver has to set up the device to use MSI or MSI-X. Not all machines
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support MSIs correctly, and for those machines, the APIs described below
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will simply fail and the device will continue to use pin-based interrupts.
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4.1 Include kernel support for MSIs
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To support MSI or MSI-X, the kernel must be built with the CONFIG_PCI_MSI
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option enabled. This option is only available on some architectures,
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and it may depend on some other options also being set. For example,
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on x86, you must also enable X86_UP_APIC or SMP in order to see the
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CONFIG_PCI_MSI option.
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4.2 Using MSI
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Most of the hard work is done for the driver in the PCI layer. It simply
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has to request that the PCI layer set up the MSI capability for this
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device.
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4.2.1 pci_enable_msi
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int pci_enable_msi(struct pci_dev *dev)
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A successful call will allocate ONE interrupt to the device, regardless
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of how many MSIs the device supports. The device will be switched from
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pin-based interrupt mode to MSI mode. The dev->irq number is changed
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to a new number which represents the message signaled interrupt.
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This function should be called before the driver calls request_irq()
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since enabling MSIs disables the pin-based IRQ and the driver will not
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receive interrupts on the old interrupt.
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4.2.2 pci_enable_msi_block
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int pci_enable_msi_block(struct pci_dev *dev, int count)
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This variation on the above call allows a device driver to request multiple
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MSIs. The MSI specification only allows interrupts to be allocated in
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powers of two, up to a maximum of 2^5 (32).
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If this function returns 0, it has succeeded in allocating at least as many
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interrupts as the driver requested (it may have allocated more in order
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to satisfy the power-of-two requirement). In this case, the function
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enables MSI on this device and updates dev->irq to be the lowest of
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the new interrupts assigned to it. The other interrupts assigned to
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the device are in the range dev->irq to dev->irq + count - 1.
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If this function returns a negative number, it indicates an error and
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the driver should not attempt to request any more MSI interrupts for
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this device. If this function returns a positive number, it will be
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less than 'count' and indicate the number of interrupts that could have
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been allocated. In neither case will the irq value have been
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updated, nor will the device have been switched into MSI mode.
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The device driver must decide what action to take if
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pci_enable_msi_block() returns a value less than the number asked for.
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Some devices can make use of fewer interrupts than the maximum they
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request; in this case the driver should call pci_enable_msi_block()
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again. Note that it is not guaranteed to succeed, even when the
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'count' has been reduced to the value returned from a previous call to
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pci_enable_msi_block(). This is because there are multiple constraints
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on the number of vectors that can be allocated; pci_enable_msi_block()
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will return as soon as it finds any constraint that doesn't allow the
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call to succeed.
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4.2.3 pci_disable_msi
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void pci_disable_msi(struct pci_dev *dev)
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This function should be used to undo the effect of pci_enable_msi() or
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pci_enable_msi_block(). Calling it restores dev->irq to the pin-based
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interrupt number and frees the previously allocated message signaled
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interrupt(s). The interrupt may subsequently be assigned to another
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device, so drivers should not cache the value of dev->irq.
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A device driver must always call free_irq() on the interrupt(s)
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for which it has called request_irq() before calling this function.
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Failure to do so will result in a BUG_ON(), the device will be left with
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MSI enabled and will leak its vector.
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4.3 Using MSI-X
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The MSI-X capability is much more flexible than the MSI capability.
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It supports up to 2048 interrupts, each of which can be controlled
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independently. To support this flexibility, drivers must use an array of
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`struct msix_entry':
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struct msix_entry {
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u16 vector; /* kernel uses to write alloc vector */
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u16 entry; /* driver uses to specify entry */
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};
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This allows for the device to use these interrupts in a sparse fashion;
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for example it could use interrupts 3 and 1027 and allocate only a
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two-element array. The driver is expected to fill in the 'entry' value
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in each element of the array to indicate which entries it wants the kernel
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to assign interrupts for. It is invalid to fill in two entries with the
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same number.
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4.3.1 pci_enable_msix
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int pci_enable_msix(struct pci_dev *dev, struct msix_entry *entries, int nvec)
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Calling this function asks the PCI subsystem to allocate 'nvec' MSIs.
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The 'entries' argument is a pointer to an array of msix_entry structs
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which should be at least 'nvec' entries in size. On success, the
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function will return 0 and the device will have been switched into
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MSI-X interrupt mode. The 'vector' elements in each entry will have
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been filled in with the interrupt number. The driver should then call
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request_irq() for each 'vector' that it decides to use.
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If this function returns a negative number, it indicates an error and
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the driver should not attempt to allocate any more MSI-X interrupts for
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this device. If it returns a positive number, it indicates the maximum
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number of interrupt vectors that could have been allocated. See example
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below.
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This function, in contrast with pci_enable_msi(), does not adjust
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dev->irq. The device will not generate interrupts for this interrupt
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number once MSI-X is enabled. The device driver is responsible for
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keeping track of the interrupts assigned to the MSI-X vectors so it can
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free them again later.
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Device drivers should normally call this function once per device
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during the initialization phase.
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It is ideal if drivers can cope with a variable number of MSI-X interrupts,
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there are many reasons why the platform may not be able to provide the
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exact number a driver asks for.
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A request loop to achieve that might look like:
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static int foo_driver_enable_msix(struct foo_adapter *adapter, int nvec)
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{
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while (nvec >= FOO_DRIVER_MINIMUM_NVEC) {
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rc = pci_enable_msix(adapter->pdev,
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adapter->msix_entries, nvec);
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if (rc > 0)
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nvec = rc;
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else
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return rc;
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}
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return -ENOSPC;
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}
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4.3.2 pci_disable_msix
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void pci_disable_msix(struct pci_dev *dev)
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This API should be used to undo the effect of pci_enable_msix(). It frees
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the previously allocated message signaled interrupts. The interrupts may
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subsequently be assigned to another device, so drivers should not cache
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the value of the 'vector' elements over a call to pci_disable_msix().
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A device driver must always call free_irq() on the interrupt(s)
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for which it has called request_irq() before calling this function.
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Failure to do so will result in a BUG_ON(), the device will be left with
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MSI enabled and will leak its vector.
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4.3.3 The MSI-X Table
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The MSI-X capability specifies a BAR and offset within that BAR for the
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MSI-X Table. This address is mapped by the PCI subsystem, and should not
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be accessed directly by the device driver. If the driver wishes to
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mask or unmask an interrupt, it should call disable_irq() / enable_irq().
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4.4 Handling devices implementing both MSI and MSI-X capabilities
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If a device implements both MSI and MSI-X capabilities, it can
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run in either MSI mode or MSI-X mode but not both simultaneously.
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This is a requirement of the PCI spec, and it is enforced by the
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PCI layer. Calling pci_enable_msi() when MSI-X is already enabled or
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pci_enable_msix() when MSI is already enabled will result in an error.
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If a device driver wishes to switch between MSI and MSI-X at runtime,
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it must first quiesce the device, then switch it back to pin-interrupt
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mode, before calling pci_enable_msi() or pci_enable_msix() and resuming
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operation. This is not expected to be a common operation but may be
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useful for debugging or testing during development.
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4.5 Considerations when using MSIs
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4.5.1 Choosing between MSI-X and MSI
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If your device supports both MSI-X and MSI capabilities, you should use
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the MSI-X facilities in preference to the MSI facilities. As mentioned
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above, MSI-X supports any number of interrupts between 1 and 2048.
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In constrast, MSI is restricted to a maximum of 32 interrupts (and
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must be a power of two). In addition, the MSI interrupt vectors must
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be allocated consecutively, so the system may not be able to allocate
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as many vectors for MSI as it could for MSI-X. On some platforms, MSI
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interrupts must all be targetted at the same set of CPUs whereas MSI-X
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interrupts can all be targetted at different CPUs.
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4.5.2 Spinlocks
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Most device drivers have a per-device spinlock which is taken in the
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interrupt handler. With pin-based interrupts or a single MSI, it is not
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necessary to disable interrupts (Linux guarantees the same interrupt will
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not be re-entered). If a device uses multiple interrupts, the driver
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must disable interrupts while the lock is held. If the device sends
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a different interrupt, the driver will deadlock trying to recursively
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acquire the spinlock.
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There are two solutions. The first is to take the lock with
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spin_lock_irqsave() or spin_lock_irq() (see
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Documentation/DocBook/kernel-locking). The second is to specify
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IRQF_DISABLED to request_irq() so that the kernel runs the entire
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interrupt routine with interrupts disabled.
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If your MSI interrupt routine does not hold the lock for the whole time
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it is running, the first solution may be best. The second solution is
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normally preferred as it avoids making two transitions from interrupt
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disabled to enabled and back again.
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4.6 How to tell whether MSI/MSI-X is enabled on a device
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Using 'lspci -v' (as root) may show some devices with "MSI", "Message
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Signalled Interrupts" or "MSI-X" capabilities. Each of these capabilities
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has an 'Enable' flag which will be followed with either "+" (enabled)
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or "-" (disabled).
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5. MSI quirks
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Several PCI chipsets or devices are known not to support MSIs.
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The PCI stack provides three ways to disable MSIs:
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1. globally
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2. on all devices behind a specific bridge
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3. on a single device
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5.1. Disabling MSIs globally
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Some host chipsets simply don't support MSIs properly. If we're
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lucky, the manufacturer knows this and has indicated it in the ACPI
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FADT table. In this case, Linux will automatically disable MSIs.
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Some boards don't include this information in the table and so we have
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to detect them ourselves. The complete list of these is found near the
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quirk_disable_all_msi() function in drivers/pci/quirks.c.
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If you have a board which has problems with MSIs, you can pass pci=nomsi
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on the kernel command line to disable MSIs on all devices. It would be
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in your best interests to report the problem to linux-pci@vger.kernel.org
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including a full 'lspci -v' so we can add the quirks to the kernel.
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5.2. Disabling MSIs below a bridge
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Some PCI bridges are not able to route MSIs between busses properly.
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In this case, MSIs must be disabled on all devices behind the bridge.
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Some bridges allow you to enable MSIs by changing some bits in their
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PCI configuration space (especially the Hypertransport chipsets such
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as the nVidia nForce and Serverworks HT2000). As with host chipsets,
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Linux mostly knows about them and automatically enables MSIs if it can.
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If you have a bridge which Linux doesn't yet know about, you can enable
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MSIs in configuration space using whatever method you know works, then
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enable MSIs on that bridge by doing:
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echo 1 > /sys/bus/pci/devices/$bridge/msi_bus
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where $bridge is the PCI address of the bridge you've enabled (eg
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0000:00:0e.0).
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To disable MSIs, echo 0 instead of 1. Changing this value should be
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done with caution as it can break interrupt handling for all devices
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below this bridge.
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Again, please notify linux-pci@vger.kernel.org of any bridges that need
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special handling.
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5.3. Disabling MSIs on a single device
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Some devices are known to have faulty MSI implementations. Usually this
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is handled in the individual device driver but occasionally it's necessary
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to handle this with a quirk. Some drivers have an option to disable use
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of MSI. While this is a convenient workaround for the driver author,
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it is not good practise, and should not be emulated.
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5.4. Finding why MSIs are disabled on a device
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From the above three sections, you can see that there are many reasons
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why MSIs may not be enabled for a given device. Your first step should
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be to examine your dmesg carefully to determine whether MSIs are enabled
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for your machine. You should also check your .config to be sure you
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have enabled CONFIG_PCI_MSI.
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Then, 'lspci -t' gives the list of bridges above a device. Reading
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/sys/bus/pci/devices/*/msi_bus will tell you whether MSI are enabled (1)
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or disabled (0). If 0 is found in any of the msi_bus files belonging
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to bridges between the PCI root and the device, MSIs are disabled.
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It is also worth checking the device driver to see whether it supports MSIs.
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For example, it may contain calls to pci_enable_msi(), pci_enable_msix() or
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pci_enable_msi_block().
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