+++ /dev/null
- 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.
--- /dev/null
+.. SPDX-License-Identifier: GPL-2.0
+.. include:: <isonum.txt>
+
+==========================
+The MSI Driver Guide HOWTO
+==========================
+
+:Authors: Tom L Nguyen; Martine Silbermann; Matthew Wilcox
+
+:Copyright: 2003, 2008 Intel Corporation
+
+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.
+
+
+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.
+
+
+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.
+
+
+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.
+
+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.
+
+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;
+
+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.
+
+Considerations when using MSIs
+------------------------------
+
+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).
+
+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).
+
+
+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
+
+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.
+
+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.
+
+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.
+
+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.
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