linux/Documentation/PCI/msi-howto.rst
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   1.. SPDX-License-Identifier: GPL-2.0
   2.. include:: <isonum.txt>
   3
   4==========================
   5The MSI Driver Guide HOWTO
   6==========================
   7
   8:Authors: Tom L Nguyen; Martine Silbermann; Matthew Wilcox
   9
  10:Copyright: 2003, 2008 Intel Corporation
  11
  12About this guide
  13================
  14
  15This guide describes the basics of Message Signaled Interrupts (MSIs),
  16the advantages of using MSI over traditional interrupt mechanisms, how
  17to change your driver to use MSI or MSI-X and some basic diagnostics to
  18try if a device doesn't support MSIs.
  19
  20
  21What are MSIs?
  22==============
  23
  24A Message Signaled Interrupt is a write from the device to a special
  25address which causes an interrupt to be received by the CPU.
  26
  27The MSI capability was first specified in PCI 2.2 and was later enhanced
  28in PCI 3.0 to allow each interrupt to be masked individually.  The MSI-X
  29capability was also introduced with PCI 3.0.  It supports more interrupts
  30per device than MSI and allows interrupts to be independently configured.
  31
  32Devices may support both MSI and MSI-X, but only one can be enabled at
  33a time.
  34
  35
  36Why use MSIs?
  37=============
  38
  39There are three reasons why using MSIs can give an advantage over
  40traditional pin-based interrupts.
  41
  42Pin-based PCI interrupts are often shared amongst several devices.
  43To support this, the kernel must call each interrupt handler associated
  44with an interrupt, which leads to reduced performance for the system as
  45a whole.  MSIs are never shared, so this problem cannot arise.
  46
  47When a device writes data to memory, then raises a pin-based interrupt,
  48it is possible that the interrupt may arrive before all the data has
  49arrived in memory (this becomes more likely with devices behind PCI-PCI
  50bridges).  In order to ensure that all the data has arrived in memory,
  51the interrupt handler must read a register on the device which raised
  52the interrupt.  PCI transaction ordering rules require that all the data
  53arrive in memory before the value may be returned from the register.
  54Using MSIs avoids this problem as the interrupt-generating write cannot
  55pass the data writes, so by the time the interrupt is raised, the driver
  56knows that all the data has arrived in memory.
  57
  58PCI devices can only support a single pin-based interrupt per function.
  59Often drivers have to query the device to find out what event has
  60occurred, slowing down interrupt handling for the common case.  With
  61MSIs, a device can support more interrupts, allowing each interrupt
  62to be specialised to a different purpose.  One possible design gives
  63infrequent conditions (such as errors) their own interrupt which allows
  64the driver to handle the normal interrupt handling path more efficiently.
  65Other possible designs include giving one interrupt to each packet queue
  66in a network card or each port in a storage controller.
  67
  68
  69How to use MSIs
  70===============
  71
  72PCI devices are initialised to use pin-based interrupts.  The device
  73driver has to set up the device to use MSI or MSI-X.  Not all machines
  74support MSIs correctly, and for those machines, the APIs described below
  75will simply fail and the device will continue to use pin-based interrupts.
  76
  77Include kernel support for MSIs
  78-------------------------------
  79
  80To support MSI or MSI-X, the kernel must be built with the CONFIG_PCI_MSI
  81option enabled.  This option is only available on some architectures,
  82and it may depend on some other options also being set.  For example,
  83on x86, you must also enable X86_UP_APIC or SMP in order to see the
  84CONFIG_PCI_MSI option.
  85
  86Using MSI
  87---------
  88
  89Most of the hard work is done for the driver in the PCI layer.  The driver
  90simply has to request that the PCI layer set up the MSI capability for this
  91device.
  92
  93To automatically use MSI or MSI-X interrupt vectors, use the following
  94function::
  95
  96  int pci_alloc_irq_vectors(struct pci_dev *dev, unsigned int min_vecs,
  97                unsigned int max_vecs, unsigned int flags);
  98
  99which allocates up to max_vecs interrupt vectors for a PCI device.  It
 100returns the number of vectors allocated or a negative error.  If the device
 101has a requirements for a minimum number of vectors the driver can pass a
 102min_vecs argument set to this limit, and the PCI core will return -ENOSPC
 103if it can't meet the minimum number of vectors.
 104
 105The flags argument is used to specify which type of interrupt can be used
 106by the device and the driver (PCI_IRQ_LEGACY, PCI_IRQ_MSI, PCI_IRQ_MSIX).
 107A convenient short-hand (PCI_IRQ_ALL_TYPES) is also available to ask for
 108any possible kind of interrupt.  If the PCI_IRQ_AFFINITY flag is set,
 109pci_alloc_irq_vectors() will spread the interrupts around the available CPUs.
 110
 111To get the Linux IRQ numbers passed to request_irq() and free_irq() and the
 112vectors, use the following function::
 113
 114  int pci_irq_vector(struct pci_dev *dev, unsigned int nr);
 115
 116Any allocated resources should be freed before removing the device using
 117the following function::
 118
 119  void pci_free_irq_vectors(struct pci_dev *dev);
 120
 121If a device supports both MSI-X and MSI capabilities, this API will use the
 122MSI-X facilities in preference to the MSI facilities.  MSI-X supports any
 123number of interrupts between 1 and 2048.  In contrast, MSI is restricted to
 124a maximum of 32 interrupts (and must be a power of two).  In addition, the
 125MSI interrupt vectors must be allocated consecutively, so the system might
 126not be able to allocate as many vectors for MSI as it could for MSI-X.  On
 127some platforms, MSI interrupts must all be targeted at the same set of CPUs
 128whereas MSI-X interrupts can all be targeted at different CPUs.
 129
 130If a device supports neither MSI-X or MSI it will fall back to a single
 131legacy IRQ vector.
 132
 133The typical usage of MSI or MSI-X interrupts is to allocate as many vectors
 134as possible, likely up to the limit supported by the device.  If nvec is
 135larger than the number supported by the device it will automatically be
 136capped to the supported limit, so there is no need to query the number of
 137vectors supported beforehand::
 138
 139        nvec = pci_alloc_irq_vectors(pdev, 1, nvec, PCI_IRQ_ALL_TYPES)
 140        if (nvec < 0)
 141                goto out_err;
 142
 143If a driver is unable or unwilling to deal with a variable number of MSI
 144interrupts it can request a particular number of interrupts by passing that
 145number to pci_alloc_irq_vectors() function as both 'min_vecs' and
 146'max_vecs' parameters::
 147
 148        ret = pci_alloc_irq_vectors(pdev, nvec, nvec, PCI_IRQ_ALL_TYPES);
 149        if (ret < 0)
 150                goto out_err;
 151
 152The most notorious example of the request type described above is enabling
 153the single MSI mode for a device.  It could be done by passing two 1s as
 154'min_vecs' and 'max_vecs'::
 155
 156        ret = pci_alloc_irq_vectors(pdev, 1, 1, PCI_IRQ_ALL_TYPES);
 157        if (ret < 0)
 158                goto out_err;
 159
 160Some devices might not support using legacy line interrupts, in which case
 161the driver can specify that only MSI or MSI-X is acceptable::
 162
 163        nvec = pci_alloc_irq_vectors(pdev, 1, nvec, PCI_IRQ_MSI | PCI_IRQ_MSIX);
 164        if (nvec < 0)
 165                goto out_err;
 166
 167Legacy APIs
 168-----------
 169
 170The following old APIs to enable and disable MSI or MSI-X interrupts should
 171not be used in new code::
 172
 173  pci_enable_msi()              /* deprecated */
 174  pci_disable_msi()             /* deprecated */
 175  pci_enable_msix_range()       /* deprecated */
 176  pci_enable_msix_exact()       /* deprecated */
 177  pci_disable_msix()            /* deprecated */
 178
 179Additionally there are APIs to provide the number of supported MSI or MSI-X
 180vectors: pci_msi_vec_count() and pci_msix_vec_count().  In general these
 181should be avoided in favor of letting pci_alloc_irq_vectors() cap the
 182number of vectors.  If you have a legitimate special use case for the count
 183of vectors we might have to revisit that decision and add a
 184pci_nr_irq_vectors() helper that handles MSI and MSI-X transparently.
 185
 186Considerations when using MSIs
 187------------------------------
 188
 189Spinlocks
 190~~~~~~~~~
 191
 192Most device drivers have a per-device spinlock which is taken in the
 193interrupt handler.  With pin-based interrupts or a single MSI, it is not
 194necessary to disable interrupts (Linux guarantees the same interrupt will
 195not be re-entered).  If a device uses multiple interrupts, the driver
 196must disable interrupts while the lock is held.  If the device sends
 197a different interrupt, the driver will deadlock trying to recursively
 198acquire the spinlock.  Such deadlocks can be avoided by using
 199spin_lock_irqsave() or spin_lock_irq() which disable local interrupts
 200and acquire the lock (see Documentation/kernel-hacking/locking.rst).
 201
 202How to tell whether MSI/MSI-X is enabled on a device
 203----------------------------------------------------
 204
 205Using 'lspci -v' (as root) may show some devices with "MSI", "Message
 206Signalled Interrupts" or "MSI-X" capabilities.  Each of these capabilities
 207has an 'Enable' flag which is followed with either "+" (enabled)
 208or "-" (disabled).
 209
 210
 211MSI quirks
 212==========
 213
 214Several PCI chipsets or devices are known not to support MSIs.
 215The PCI stack provides three ways to disable MSIs:
 216
 2171. globally
 2182. on all devices behind a specific bridge
 2193. on a single device
 220
 221Disabling MSIs globally
 222-----------------------
 223
 224Some host chipsets simply don't support MSIs properly.  If we're
 225lucky, the manufacturer knows this and has indicated it in the ACPI
 226FADT table.  In this case, Linux automatically disables MSIs.
 227Some boards don't include this information in the table and so we have
 228to detect them ourselves.  The complete list of these is found near the
 229quirk_disable_all_msi() function in drivers/pci/quirks.c.
 230
 231If you have a board which has problems with MSIs, you can pass pci=nomsi
 232on the kernel command line to disable MSIs on all devices.  It would be
 233in your best interests to report the problem to linux-pci@vger.kernel.org
 234including a full 'lspci -v' so we can add the quirks to the kernel.
 235
 236Disabling MSIs below a bridge
 237-----------------------------
 238
 239Some PCI bridges are not able to route MSIs between busses properly.
 240In this case, MSIs must be disabled on all devices behind the bridge.
 241
 242Some bridges allow you to enable MSIs by changing some bits in their
 243PCI configuration space (especially the Hypertransport chipsets such
 244as the nVidia nForce and Serverworks HT2000).  As with host chipsets,
 245Linux mostly knows about them and automatically enables MSIs if it can.
 246If you have a bridge unknown to Linux, you can enable
 247MSIs in configuration space using whatever method you know works, then
 248enable MSIs on that bridge by doing::
 249
 250       echo 1 > /sys/bus/pci/devices/$bridge/msi_bus
 251
 252where $bridge is the PCI address of the bridge you've enabled (eg
 2530000:00:0e.0).
 254
 255To disable MSIs, echo 0 instead of 1.  Changing this value should be
 256done with caution as it could break interrupt handling for all devices
 257below this bridge.
 258
 259Again, please notify linux-pci@vger.kernel.org of any bridges that need
 260special handling.
 261
 262Disabling MSIs on a single device
 263---------------------------------
 264
 265Some devices are known to have faulty MSI implementations.  Usually this
 266is handled in the individual device driver, but occasionally it's necessary
 267to handle this with a quirk.  Some drivers have an option to disable use
 268of MSI.  While this is a convenient workaround for the driver author,
 269it is not good practice, and should not be emulated.
 270
 271Finding why MSIs are disabled on a device
 272-----------------------------------------
 273
 274From the above three sections, you can see that there are many reasons
 275why MSIs may not be enabled for a given device.  Your first step should
 276be to examine your dmesg carefully to determine whether MSIs are enabled
 277for your machine.  You should also check your .config to be sure you
 278have enabled CONFIG_PCI_MSI.
 279
 280Then, 'lspci -t' gives the list of bridges above a device. Reading
 281`/sys/bus/pci/devices/*/msi_bus` will tell you whether MSIs are enabled (1)
 282or disabled (0).  If 0 is found in any of the msi_bus files belonging
 283to bridges between the PCI root and the device, MSIs are disabled.
 284
 285It is also worth checking the device driver to see whether it supports MSIs.
 286For example, it may contain calls to pci_alloc_irq_vectors() with the
 287PCI_IRQ_MSI or PCI_IRQ_MSIX flags.
 288