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>

Documentation/PCI/msi-howto.rst

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+.. 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.