Documentation: PCI: convert MSI-HOWTO.txt to reST
Convert plain text documentation to reStructuredText format and add it to Sphinx TOC tree. No essential content change. Signed-off-by: Changbin Du <changbin.du@gmail.com> Signed-off-by: Bjorn Helgaas <bhelgaas@google.com> Reviewed-by: Mauro Carvalho Chehab <mchehab+samsung@kernel.org>
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Changbin Du
Bjorn Helgaas
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4d2c729c62
commit
3b9bae029b
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@ -11,3 +11,4 @@ Linux PCI Bus Subsystem
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pci
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picebus-howto
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pci-iov-howto
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msi-howto
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@ -1,13 +1,16 @@
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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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.. SPDX-License-Identifier: GPL-2.0
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.. include:: <isonum.txt>
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1. About this guide
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==========================
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The MSI Driver Guide HOWTO
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==========================
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:Authors: Tom L Nguyen; Martine Silbermann; Matthew Wilcox
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:Copyright: 2003, 2008 Intel Corporation
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About this guide
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================
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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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@ -15,7 +18,8 @@ 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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What are MSIs?
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==============
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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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@ -29,7 +33,8 @@ 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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Why use MSIs?
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=============
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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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@ -61,14 +66,16 @@ 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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How to use MSIs
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===============
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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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Include kernel support for MSIs
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-------------------------------
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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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@ -76,14 +83,15 @@ 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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Using MSI
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---------
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Most of the hard work is done for the driver in the PCI layer. The driver
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simply has to request that the PCI layer set up the MSI capability for this
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device.
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To automatically use MSI or MSI-X interrupt vectors, use the following
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function:
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function::
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int pci_alloc_irq_vectors(struct pci_dev *dev, unsigned int min_vecs,
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unsigned int max_vecs, unsigned int flags);
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@ -101,12 +109,12 @@ any possible kind of interrupt. If the PCI_IRQ_AFFINITY flag is set,
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pci_alloc_irq_vectors() will spread the interrupts around the available CPUs.
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To get the Linux IRQ numbers passed to request_irq() and free_irq() and the
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vectors, use the following function:
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vectors, use the following function::
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int pci_irq_vector(struct pci_dev *dev, unsigned int nr);
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Any allocated resources should be freed before removing the device using
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the following function:
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the following function::
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void pci_free_irq_vectors(struct pci_dev *dev);
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@ -126,7 +134,7 @@ The typical usage of MSI or MSI-X interrupts is to allocate as many vectors
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as possible, likely up to the limit supported by the device. If nvec is
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larger than the number supported by the device it will automatically be
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capped to the supported limit, so there is no need to query the number of
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vectors supported beforehand:
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vectors supported beforehand::
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nvec = pci_alloc_irq_vectors(pdev, 1, nvec, PCI_IRQ_ALL_TYPES)
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if (nvec < 0)
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@ -135,7 +143,7 @@ vectors supported beforehand:
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If a driver is unable or unwilling to deal with a variable number of MSI
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interrupts it can request a particular number of interrupts by passing that
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number to pci_alloc_irq_vectors() function as both 'min_vecs' and
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'max_vecs' parameters:
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'max_vecs' parameters::
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ret = pci_alloc_irq_vectors(pdev, nvec, nvec, PCI_IRQ_ALL_TYPES);
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if (ret < 0)
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@ -143,23 +151,24 @@ number to pci_alloc_irq_vectors() function as both 'min_vecs' and
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The most notorious example of the request type described above is enabling
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the single MSI mode for a device. It could be done by passing two 1s as
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'min_vecs' and 'max_vecs':
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'min_vecs' and 'max_vecs'::
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ret = pci_alloc_irq_vectors(pdev, 1, 1, PCI_IRQ_ALL_TYPES);
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if (ret < 0)
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goto out_err;
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Some devices might not support using legacy line interrupts, in which case
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the driver can specify that only MSI or MSI-X is acceptable:
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the driver can specify that only MSI or MSI-X is acceptable::
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nvec = pci_alloc_irq_vectors(pdev, 1, nvec, PCI_IRQ_MSI | PCI_IRQ_MSIX);
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if (nvec < 0)
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goto out_err;
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4.3 Legacy APIs
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Legacy APIs
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-----------
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The following old APIs to enable and disable MSI or MSI-X interrupts should
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not be used in new code:
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not be used in new code::
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pci_enable_msi() /* deprecated */
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pci_disable_msi() /* deprecated */
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@ -174,9 +183,11 @@ number of vectors. If you have a legitimate special use case for the count
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of vectors we might have to revisit that decision and add a
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pci_nr_irq_vectors() helper that handles MSI and MSI-X transparently.
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4.4 Considerations when using MSIs
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Considerations when using MSIs
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------------------------------
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4.4.1 Spinlocks
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Spinlocks
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~~~~~~~~~
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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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@ -188,7 +199,8 @@ acquire the spinlock. Such deadlocks can be avoided by using
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spin_lock_irqsave() or spin_lock_irq() which disable local interrupts
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and acquire the lock (see Documentation/kernel-hacking/locking.rst).
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4.5 How to tell whether MSI/MSI-X is enabled on a device
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How to tell whether MSI/MSI-X is enabled on a device
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----------------------------------------------------
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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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@ -196,7 +208,8 @@ has an 'Enable' flag which is followed with either "+" (enabled)
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or "-" (disabled).
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5. MSI quirks
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MSI quirks
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==========
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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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@ -205,7 +218,8 @@ The PCI stack provides three ways to disable MSIs:
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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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Disabling MSIs globally
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-----------------------
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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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@ -219,7 +233,8 @@ 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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Disabling MSIs below a bridge
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-----------------------------
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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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@ -230,7 +245,7 @@ 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 unknown to Linux, 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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enable MSIs on that bridge by doing::
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echo 1 > /sys/bus/pci/devices/$bridge/msi_bus
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@ -244,7 +259,8 @@ 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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Disabling MSIs on a single device
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---------------------------------
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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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@ -252,7 +268,8 @@ 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 practice, and should not be emulated.
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5.4. Finding why MSIs are disabled on a device
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Finding why MSIs are disabled on a device
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-----------------------------------------
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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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@ -260,8 +277,8 @@ 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 MSIs are enabled (1)
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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 MSIs 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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