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Based on kernel version 3.9. Page generated on 2013-05-02 23:12 EST.

			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.  It simply
	has to request that the PCI layer set up the MSI capability for this
	device.
	
	4.2.1 pci_enable_msi
	
	int pci_enable_msi(struct pci_dev *dev)
	
	A successful call allocates ONE interrupt to the device, regardless
	of how many MSIs the device supports.  The device is switched from
	pin-based interrupt mode to MSI mode.  The dev->irq number is changed
	to a new number which represents the message signaled interrupt;
	consequently, this function should be called before the driver calls
	request_irq(), because an MSI is delivered via a vector that is
	different from the vector of a pin-based interrupt.
	
	4.2.2 pci_enable_msi_block
	
	int pci_enable_msi_block(struct pci_dev *dev, int count)
	
	This variation on the above call allows a device driver to request multiple
	MSIs.  The MSI specification only allows interrupts to be allocated in
	powers of two, up to a maximum of 2^5 (32).
	
	If this function returns 0, it has succeeded in allocating at least as many
	interrupts as the driver requested (it may have allocated more in order
	to satisfy the power-of-two requirement).  In this case, the function
	enables MSI on this device and updates dev->irq to be the lowest of
	the new interrupts assigned to it.  The other interrupts assigned to
	the device are in the range dev->irq to dev->irq + count - 1.
	
	If this function returns a negative number, it indicates an error and
	the driver should not attempt to request any more MSI interrupts for
	this device.  If this function returns a positive number, it is
	less than 'count' and indicates the number of interrupts that could have
	been allocated.  In neither case is the irq value updated or the device
	switched into MSI mode.
	
	The device driver must decide what action to take if
	pci_enable_msi_block() returns a value less than the number requested.
	For instance, the driver could still make use of fewer interrupts;
	in this case the driver should call pci_enable_msi_block()
	again.  Note that it is not guaranteed to succeed, even when the
	'count' has been reduced to the value returned from a previous call to
	pci_enable_msi_block().  This is because there are multiple constraints
	on the number of vectors that can be allocated; pci_enable_msi_block()
	returns as soon as it finds any constraint that doesn't allow the
	call to succeed.
	
	4.2.3 pci_enable_msi_block_auto
	
	int pci_enable_msi_block_auto(struct pci_dev *dev, unsigned int *count)
	
	This variation on pci_enable_msi() call allows a device driver to request
	the maximum possible number of MSIs.  The MSI specification only allows
	interrupts to be allocated in powers of two, up to a maximum of 2^5 (32).
	
	If this function returns a positive number, it indicates that it has
	succeeded and the returned value is the number of allocated interrupts. In
	this case, the function enables MSI on this device and updates dev->irq to
	be the lowest of the new interrupts assigned to it.  The other interrupts
	assigned to the device are in the range dev->irq to dev->irq + returned
	value - 1.
	
	If this function returns a negative number, it indicates an error and
	the driver should not attempt to request any more MSI interrupts for
	this device.
	
	If the device driver needs to know the number of interrupts the device
	supports it can pass the pointer count where that number is stored. The
	device driver must decide what action to take if pci_enable_msi_block_auto()
	succeeds, but returns a value less than the number of interrupts supported.
	If the device driver does not need to know the number of interrupts
	supported, it can set the pointer count to NULL.
	
	4.2.4 pci_disable_msi
	
	void pci_disable_msi(struct pci_dev *dev)
	
	This function should be used to undo the effect of pci_enable_msi() or
	pci_enable_msi_block() or pci_enable_msi_block_auto().  Calling it restores
	dev->irq to the pin-based interrupt number and frees the previously
	allocated message signaled interrupt(s).  The interrupt may subsequently be
	assigned to another device, so drivers should not cache the value of
	dev->irq.
	
	Before calling this function, a device driver must always call free_irq()
	on any interrupt for which it previously called request_irq().
	Failure to do so results in a BUG_ON(), leaving the device with
	MSI enabled and thus leaking its vector.
	
	4.3 Using MSI-X
	
	The MSI-X capability is much more flexible than the MSI capability.
	It supports up to 2048 interrupts, each of which can be controlled
	independently.  To support this flexibility, drivers must use an array of
	`struct msix_entry':
	
	struct msix_entry {
		u16 	vector; /* kernel uses to write alloc vector */
		u16	entry; /* driver uses to specify entry */
	};
	
	This allows for the device to use these interrupts in a sparse fashion;
	for example, it could use interrupts 3 and 1027 and yet allocate only a
	two-element array.  The driver is expected to fill in the 'entry' value
	in each element of the array to indicate for which entries the kernel
	should assign interrupts; it is invalid to fill in two entries with the
	same number.
	
	4.3.1 pci_enable_msix
	
	int pci_enable_msix(struct pci_dev *dev, struct msix_entry *entries, int nvec)
	
	Calling this function asks the PCI subsystem to allocate 'nvec' MSIs.
	The 'entries' argument is a pointer to an array of msix_entry structs
	which should be at least 'nvec' entries in size.  On success, the
	device is switched into MSI-X mode and the function returns 0.
	The 'vector' member in each entry is populated with the interrupt number;
	the driver should then call request_irq() for each 'vector' that it
	decides to use.  The device driver is responsible for keeping track of the
	interrupts assigned to the MSI-X vectors so it can free them again later.
	
	If this function returns a negative number, it indicates an error and
	the driver should not attempt to allocate any more MSI-X interrupts for
	this device.  If it returns a positive number, it indicates the maximum
	number of interrupt vectors that could have been allocated. See example
	below.
	
	This function, in contrast with pci_enable_msi(), does not adjust
	dev->irq.  The device will not generate interrupts for this interrupt
	number once MSI-X is enabled.
	
	Device drivers should normally call this function once per device
	during the initialization phase.
	
	It is ideal if drivers can cope with a variable number of MSI-X interrupts;
	there are many reasons why the platform may not be able to provide the
	exact number that a driver asks for.
	
	A request loop to achieve that might look like:
	
	static int foo_driver_enable_msix(struct foo_adapter *adapter, int nvec)
	{
		while (nvec >= FOO_DRIVER_MINIMUM_NVEC) {
			rc = pci_enable_msix(adapter->pdev,
					     adapter->msix_entries, nvec);
			if (rc > 0)
				nvec = rc;
			else
				return rc;
		}
	
		return -ENOSPC;
	}
	
	4.3.2 pci_disable_msix
	
	void pci_disable_msix(struct pci_dev *dev)
	
	This function should be used to undo the effect of pci_enable_msix().  It frees
	the previously allocated message signaled interrupts.  The interrupts may
	subsequently be assigned to another device, so drivers should not cache
	the value of the 'vector' elements over a call to pci_disable_msix().
	
	Before calling this function, a device driver must always call free_irq()
	on any interrupt for which it previously called request_irq().
	Failure to do so results in a BUG_ON(), leaving the device with
	MSI-X enabled and thus leaking its vector.
	
	4.3.3 The MSI-X Table
	
	The MSI-X capability specifies a BAR and offset within that BAR for the
	MSI-X Table.  This address is mapped by the PCI subsystem, and should not
	be accessed directly by the device driver.  If the driver wishes to
	mask or unmask an interrupt, it should call disable_irq() / enable_irq().
	
	4.4 Handling devices implementing both MSI and MSI-X capabilities
	
	If a device implements both MSI and MSI-X capabilities, it can
	run in either MSI mode or MSI-X mode, but not both simultaneously.
	This is a requirement of the PCI spec, and it is enforced by the
	PCI layer.  Calling pci_enable_msi() when MSI-X is already enabled or
	pci_enable_msix() when MSI is already enabled results in an error.
	If a device driver wishes to switch between MSI and MSI-X at runtime,
	it must first quiesce the device, then switch it back to pin-interrupt
	mode, before calling pci_enable_msi() or pci_enable_msix() and resuming
	operation.  This is not expected to be a common operation but may be
	useful for debugging or testing during development.
	
	4.5 Considerations when using MSIs
	
	4.5.1 Choosing between MSI-X and MSI
	
	If your device supports both MSI-X and MSI capabilities, you should use
	the MSI-X facilities in preference to the MSI facilities.  As mentioned
	above, MSI-X supports any number of interrupts between 1 and 2048.
	In constrast, 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.
	
	4.5.2 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.
	
	There are two solutions.  The first is to take the lock with
	spin_lock_irqsave() or spin_lock_irq() (see
	Documentation/DocBook/kernel-locking).  The second is to specify
	IRQF_DISABLED to request_irq() so that the kernel runs the entire
	interrupt routine with interrupts disabled.
	
	If your MSI interrupt routine does not hold the lock for the whole time
	it is running, the first solution may be best.  The second solution is
	normally preferred as it avoids making two transitions from interrupt
	disabled to enabled and back again.
	
	4.6 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 practise, 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_enable_msi(), pci_enable_msix() or
	pci_enable_msi_block().

• [ PCI ]
• 00-INDEX
• MSI-HOWTO.txt
• pci-error-recovery.txt
• pci-iov-howto.txt
• pci.txt
• pcieaer-howto.txt
• PCIEBUS-HOWTO.txt
•