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Linus Torvalds1da177e2005-04-16 15:20:36 -07001 The MSI Driver Guide HOWTO
2 Tom L Nguyen tom.l.nguyen@intel.com
3 10/03/2003
4 Revised Feb 12, 2004 by Martine Silbermann
5 email: Martine.Silbermann@hp.com
6 Revised Jun 25, 2004 by Tom L Nguyen
Matthew Wilcoxc41ade22009-03-17 08:54:05 -04007 Revised Jul 9, 2008 by Matthew Wilcox <willy@linux.intel.com>
8 Copyright 2003, 2008 Intel Corporation
Linus Torvalds1da177e2005-04-16 15:20:36 -07009
101. About this guide
11
Matthew Wilcoxc41ade22009-03-17 08:54:05 -040012This guide describes the basics of Message Signaled Interrupts (MSIs),
13the advantages of using MSI over traditional interrupt mechanisms, how
14to change your driver to use MSI or MSI-X and some basic diagnostics to
15try if a device doesn't support MSIs.
Randy Dunlap2500e7a2005-11-07 01:01:03 -080016
Randy Dunlap2500e7a2005-11-07 01:01:03 -080017
Matthew Wilcoxc41ade22009-03-17 08:54:05 -0400182. What are MSIs?
Linus Torvalds1da177e2005-04-16 15:20:36 -070019
Matthew Wilcoxc41ade22009-03-17 08:54:05 -040020A Message Signaled Interrupt is a write from the device to a special
21address which causes an interrupt to be received by the CPU.
Linus Torvalds1da177e2005-04-16 15:20:36 -070022
Matthew Wilcoxc41ade22009-03-17 08:54:05 -040023The MSI capability was first specified in PCI 2.2 and was later enhanced
24in PCI 3.0 to allow each interrupt to be masked individually. The MSI-X
25capability was also introduced with PCI 3.0. It supports more interrupts
26per device than MSI and allows interrupts to be independently configured.
Linus Torvalds1da177e2005-04-16 15:20:36 -070027
Matthew Wilcoxc41ade22009-03-17 08:54:05 -040028Devices may support both MSI and MSI-X, but only one can be enabled at
29a time.
Linus Torvalds1da177e2005-04-16 15:20:36 -070030
Linus Torvalds1da177e2005-04-16 15:20:36 -070031
Matthew Wilcoxc41ade22009-03-17 08:54:05 -0400323. Why use MSIs?
Linus Torvalds1da177e2005-04-16 15:20:36 -070033
Matthew Wilcoxc41ade22009-03-17 08:54:05 -040034There are three reasons why using MSIs can give an advantage over
35traditional pin-based interrupts.
Linus Torvalds1da177e2005-04-16 15:20:36 -070036
Matthew Wilcoxc41ade22009-03-17 08:54:05 -040037Pin-based PCI interrupts are often shared amongst several devices.
38To support this, the kernel must call each interrupt handler associated
39with an interrupt, which leads to reduced performance for the system as
40a whole. MSIs are never shared, so this problem cannot arise.
Linus Torvalds1da177e2005-04-16 15:20:36 -070041
Matthew Wilcoxc41ade22009-03-17 08:54:05 -040042When a device writes data to memory, then raises a pin-based interrupt,
43it is possible that the interrupt may arrive before all the data has
44arrived in memory (this becomes more likely with devices behind PCI-PCI
45bridges). In order to ensure that all the data has arrived in memory,
46the interrupt handler must read a register on the device which raised
47the interrupt. PCI transaction ordering rules require that all the data
Michael Witten891f6922011-07-14 17:53:54 +000048arrive in memory before the value may be returned from the register.
Matthew Wilcoxc41ade22009-03-17 08:54:05 -040049Using MSIs avoids this problem as the interrupt-generating write cannot
50pass the data writes, so by the time the interrupt is raised, the driver
51knows that all the data has arrived in memory.
Linus Torvalds1da177e2005-04-16 15:20:36 -070052
Matthew Wilcoxc41ade22009-03-17 08:54:05 -040053PCI devices can only support a single pin-based interrupt per function.
54Often drivers have to query the device to find out what event has
55occurred, slowing down interrupt handling for the common case. With
56MSIs, a device can support more interrupts, allowing each interrupt
57to be specialised to a different purpose. One possible design gives
58infrequent conditions (such as errors) their own interrupt which allows
59the driver to handle the normal interrupt handling path more efficiently.
60Other possible designs include giving one interrupt to each packet queue
61in a network card or each port in a storage controller.
Linus Torvalds1da177e2005-04-16 15:20:36 -070062
Linus Torvalds1da177e2005-04-16 15:20:36 -070063
Matthew Wilcoxc41ade22009-03-17 08:54:05 -0400644. How to use MSIs
Linus Torvalds1da177e2005-04-16 15:20:36 -070065
Matthew Wilcoxc41ade22009-03-17 08:54:05 -040066PCI devices are initialised to use pin-based interrupts. The device
67driver has to set up the device to use MSI or MSI-X. Not all machines
68support MSIs correctly, and for those machines, the APIs described below
69will simply fail and the device will continue to use pin-based interrupts.
Linus Torvalds1da177e2005-04-16 15:20:36 -070070
Matthew Wilcoxc41ade22009-03-17 08:54:05 -0400714.1 Include kernel support for MSIs
Linus Torvalds1da177e2005-04-16 15:20:36 -070072
Matthew Wilcoxc41ade22009-03-17 08:54:05 -040073To support MSI or MSI-X, the kernel must be built with the CONFIG_PCI_MSI
74option enabled. This option is only available on some architectures,
75and it may depend on some other options also being set. For example,
76on x86, you must also enable X86_UP_APIC or SMP in order to see the
77CONFIG_PCI_MSI option.
Linus Torvalds1da177e2005-04-16 15:20:36 -070078
Matthew Wilcoxc41ade22009-03-17 08:54:05 -0400794.2 Using MSI
Linus Torvalds1da177e2005-04-16 15:20:36 -070080
Matthew Wilcoxc41ade22009-03-17 08:54:05 -040081Most of the hard work is done for the driver in the PCI layer. It simply
82has to request that the PCI layer set up the MSI capability for this
83device.
Linus Torvalds1da177e2005-04-16 15:20:36 -070084
Matthew Wilcoxc41ade22009-03-17 08:54:05 -0400854.2.1 pci_enable_msi
Linus Torvalds1da177e2005-04-16 15:20:36 -070086
87int pci_enable_msi(struct pci_dev *dev)
88
Michael Witten4979de62011-07-14 19:52:56 +000089A successful call allocates ONE interrupt to the device, regardless
90of how many MSIs the device supports. The device is switched from
Matthew Wilcoxc41ade22009-03-17 08:54:05 -040091pin-based interrupt mode to MSI mode. The dev->irq number is changed
Michael Witten4979de62011-07-14 19:52:56 +000092to a new number which represents the message signaled interrupt;
93consequently, this function should be called before the driver calls
94request_irq(), because an MSI is delivered via a vector that is
95different from the vector of a pin-based interrupt.
Linus Torvalds1da177e2005-04-16 15:20:36 -070096
Matthew Wilcox1c8d7b02009-03-17 08:54:10 -0400974.2.2 pci_enable_msi_block
98
99int pci_enable_msi_block(struct pci_dev *dev, int count)
100
101This variation on the above call allows a device driver to request multiple
102MSIs. The MSI specification only allows interrupts to be allocated in
103powers of two, up to a maximum of 2^5 (32).
104
105If this function returns 0, it has succeeded in allocating at least as many
106interrupts as the driver requested (it may have allocated more in order
107to satisfy the power-of-two requirement). In this case, the function
108enables MSI on this device and updates dev->irq to be the lowest of
109the new interrupts assigned to it. The other interrupts assigned to
110the device are in the range dev->irq to dev->irq + count - 1.
111
112If this function returns a negative number, it indicates an error and
113the driver should not attempt to request any more MSI interrupts for
Michael Witten4979de62011-07-14 19:52:56 +0000114this device. If this function returns a positive number, it is
115less than 'count' and indicates the number of interrupts that could have
116been allocated. In neither case is the irq value updated or the device
117switched into MSI mode.
Matthew Wilcox1c8d7b02009-03-17 08:54:10 -0400118
119The device driver must decide what action to take if
Michael Wittena2d4d502011-07-14 20:03:28 +0000120pci_enable_msi_block() returns a value less than the number requested.
Michael Witten1d15afc2011-07-14 20:05:01 +0000121For instance, the driver could still make use of fewer interrupts;
122in this case the driver should call pci_enable_msi_block()
Matthew Wilcox1c8d7b02009-03-17 08:54:10 -0400123again. Note that it is not guaranteed to succeed, even when the
124'count' has been reduced to the value returned from a previous call to
125pci_enable_msi_block(). This is because there are multiple constraints
126on the number of vectors that can be allocated; pci_enable_msi_block()
Michael Witten4979de62011-07-14 19:52:56 +0000127returns as soon as it finds any constraint that doesn't allow the
Matthew Wilcox1c8d7b02009-03-17 08:54:10 -0400128call to succeed.
129
1304.2.3 pci_disable_msi
Linus Torvalds1da177e2005-04-16 15:20:36 -0700131
132void pci_disable_msi(struct pci_dev *dev)
133
Matthew Wilcox1c8d7b02009-03-17 08:54:10 -0400134This function should be used to undo the effect of pci_enable_msi() or
135pci_enable_msi_block(). Calling it restores dev->irq to the pin-based
136interrupt number and frees the previously allocated message signaled
137interrupt(s). The interrupt may subsequently be assigned to another
138device, so drivers should not cache the value of dev->irq.
Linus Torvalds1da177e2005-04-16 15:20:36 -0700139
Michael Witten263d8d52011-07-14 21:28:00 +0000140Before calling this function, a device driver must always call free_irq()
141on any interrupt for which it previously called request_irq().
Michael Witten4979de62011-07-14 19:52:56 +0000142Failure to do so results in a BUG_ON(), leaving the device with
143MSI enabled and thus leaking its vector.
Linus Torvalds1da177e2005-04-16 15:20:36 -0700144
Matthew Wilcoxc41ade22009-03-17 08:54:05 -04001454.3 Using MSI-X
Linus Torvalds1da177e2005-04-16 15:20:36 -0700146
Matthew Wilcoxc41ade22009-03-17 08:54:05 -0400147The MSI-X capability is much more flexible than the MSI capability.
148It supports up to 2048 interrupts, each of which can be controlled
149independently. To support this flexibility, drivers must use an array of
150`struct msix_entry':
Linus Torvalds1da177e2005-04-16 15:20:36 -0700151
152struct msix_entry {
153 u16 vector; /* kernel uses to write alloc vector */
154 u16 entry; /* driver uses to specify entry */
155};
156
Matthew Wilcoxc41ade22009-03-17 08:54:05 -0400157This allows for the device to use these interrupts in a sparse fashion;
Michael Wittene4439232011-07-14 21:30:18 +0000158for example, it could use interrupts 3 and 1027 and yet allocate only a
Matthew Wilcoxc41ade22009-03-17 08:54:05 -0400159two-element array. The driver is expected to fill in the 'entry' value
Michael Wittene4439232011-07-14 21:30:18 +0000160in each element of the array to indicate for which entries the kernel
161should assign interrupts; it is invalid to fill in two entries with the
Matthew Wilcoxc41ade22009-03-17 08:54:05 -0400162same number.
Linus Torvalds1da177e2005-04-16 15:20:36 -0700163
Matthew Wilcoxc41ade22009-03-17 08:54:05 -04001644.3.1 pci_enable_msix
Linus Torvalds1da177e2005-04-16 15:20:36 -0700165
Matthew Wilcoxc41ade22009-03-17 08:54:05 -0400166int pci_enable_msix(struct pci_dev *dev, struct msix_entry *entries, int nvec)
167
168Calling this function asks the PCI subsystem to allocate 'nvec' MSIs.
169The 'entries' argument is a pointer to an array of msix_entry structs
170which should be at least 'nvec' entries in size. On success, the
Michael Witten4979de62011-07-14 19:52:56 +0000171device is switched into MSI-X mode and the function returns 0.
172The 'vector' member in each entry is populated with the interrupt number;
173the driver should then call request_irq() for each 'vector' that it
Michael Witten6457d9b2011-07-14 21:54:18 +0000174decides to use. The device driver is responsible for keeping track of the
175interrupts assigned to the MSI-X vectors so it can free them again later.
Matthew Wilcoxc41ade22009-03-17 08:54:05 -0400176
177If this function returns a negative number, it indicates an error and
178the driver should not attempt to allocate any more MSI-X interrupts for
179this device. If it returns a positive number, it indicates the maximum
Michael Ellermanfafad5b2009-03-20 15:22:12 +1100180number of interrupt vectors that could have been allocated. See example
181below.
Matthew Wilcoxc41ade22009-03-17 08:54:05 -0400182
183This function, in contrast with pci_enable_msi(), does not adjust
184dev->irq. The device will not generate interrupts for this interrupt
Michael Witten6457d9b2011-07-14 21:54:18 +0000185number once MSI-X is enabled.
Matthew Wilcoxc41ade22009-03-17 08:54:05 -0400186
187Device drivers should normally call this function once per device
188during the initialization phase.
189
Michael Witten5a84fc32011-07-14 21:55:05 +0000190It is ideal if drivers can cope with a variable number of MSI-X interrupts;
Michael Ellermanfafad5b2009-03-20 15:22:12 +1100191there are many reasons why the platform may not be able to provide the
Michael Wittened737c12011-07-18 16:15:00 +0000192exact number that a driver asks for.
Michael Ellermanfafad5b2009-03-20 15:22:12 +1100193
194A request loop to achieve that might look like:
195
196static int foo_driver_enable_msix(struct foo_adapter *adapter, int nvec)
197{
198 while (nvec >= FOO_DRIVER_MINIMUM_NVEC) {
199 rc = pci_enable_msix(adapter->pdev,
200 adapter->msix_entries, nvec);
201 if (rc > 0)
202 nvec = rc;
203 else
204 return rc;
205 }
206
207 return -ENOSPC;
208}
209
Matthew Wilcoxc41ade22009-03-17 08:54:05 -04002104.3.2 pci_disable_msix
Linus Torvalds1da177e2005-04-16 15:20:36 -0700211
212void pci_disable_msix(struct pci_dev *dev)
213
Michael Wittene6ffceb2011-07-14 23:30:47 +0000214This function should be used to undo the effect of pci_enable_msix(). It frees
Matthew Wilcoxc41ade22009-03-17 08:54:05 -0400215the previously allocated message signaled interrupts. The interrupts may
216subsequently be assigned to another device, so drivers should not cache
217the value of the 'vector' elements over a call to pci_disable_msix().
Linus Torvalds1da177e2005-04-16 15:20:36 -0700218
Michael Witten263d8d52011-07-14 21:28:00 +0000219Before calling this function, a device driver must always call free_irq()
220on any interrupt for which it previously called request_irq().
Michael Witten4979de62011-07-14 19:52:56 +0000221Failure to do so results in a BUG_ON(), leaving the device with
222MSI-X enabled and thus leaking its vector.
Linus Torvalds1da177e2005-04-16 15:20:36 -0700223
Matthew Wilcoxc41ade22009-03-17 08:54:05 -04002244.3.3 The MSI-X Table
Linus Torvalds1da177e2005-04-16 15:20:36 -0700225
Matthew Wilcoxc41ade22009-03-17 08:54:05 -0400226The MSI-X capability specifies a BAR and offset within that BAR for the
227MSI-X Table. This address is mapped by the PCI subsystem, and should not
228be accessed directly by the device driver. If the driver wishes to
229mask or unmask an interrupt, it should call disable_irq() / enable_irq().
Linus Torvalds1da177e2005-04-16 15:20:36 -0700230
Matthew Wilcoxc41ade22009-03-17 08:54:05 -04002314.4 Handling devices implementing both MSI and MSI-X capabilities
Linus Torvalds1da177e2005-04-16 15:20:36 -0700232
Matthew Wilcoxc41ade22009-03-17 08:54:05 -0400233If a device implements both MSI and MSI-X capabilities, it can
Michael Wittene14bd7e2011-07-15 03:12:13 +0000234run in either MSI mode or MSI-X mode, but not both simultaneously.
Matthew Wilcoxc41ade22009-03-17 08:54:05 -0400235This is a requirement of the PCI spec, and it is enforced by the
236PCI layer. Calling pci_enable_msi() when MSI-X is already enabled or
Michael Witten4979de62011-07-14 19:52:56 +0000237pci_enable_msix() when MSI is already enabled results in an error.
Matthew Wilcoxc41ade22009-03-17 08:54:05 -0400238If a device driver wishes to switch between MSI and MSI-X at runtime,
239it must first quiesce the device, then switch it back to pin-interrupt
240mode, before calling pci_enable_msi() or pci_enable_msix() and resuming
241operation. This is not expected to be a common operation but may be
242useful for debugging or testing during development.
Linus Torvalds1da177e2005-04-16 15:20:36 -0700243
Matthew Wilcoxc41ade22009-03-17 08:54:05 -04002444.5 Considerations when using MSIs
Linus Torvalds1da177e2005-04-16 15:20:36 -0700245
Matthew Wilcoxc41ade22009-03-17 08:54:05 -04002464.5.1 Choosing between MSI-X and MSI
Linus Torvalds1da177e2005-04-16 15:20:36 -0700247
Matthew Wilcoxc41ade22009-03-17 08:54:05 -0400248If your device supports both MSI-X and MSI capabilities, you should use
249the MSI-X facilities in preference to the MSI facilities. As mentioned
250above, MSI-X supports any number of interrupts between 1 and 2048.
251In constrast, MSI is restricted to a maximum of 32 interrupts (and
252must be a power of two). In addition, the MSI interrupt vectors must
Michael Witten952df552011-07-15 03:15:10 +0000253be allocated consecutively, so the system might not be able to allocate
Matthew Wilcoxc41ade22009-03-17 08:54:05 -0400254as many vectors for MSI as it could for MSI-X. On some platforms, MSI
Lucas De Marchi25985ed2011-03-30 22:57:33 -0300255interrupts must all be targeted at the same set of CPUs whereas MSI-X
256interrupts can all be targeted at different CPUs.
Linus Torvalds1da177e2005-04-16 15:20:36 -0700257
Matthew Wilcoxc41ade22009-03-17 08:54:05 -04002584.5.2 Spinlocks
Linus Torvalds1da177e2005-04-16 15:20:36 -0700259
Matthew Wilcoxc41ade22009-03-17 08:54:05 -0400260Most device drivers have a per-device spinlock which is taken in the
261interrupt handler. With pin-based interrupts or a single MSI, it is not
262necessary to disable interrupts (Linux guarantees the same interrupt will
263not be re-entered). If a device uses multiple interrupts, the driver
264must disable interrupts while the lock is held. If the device sends
265a different interrupt, the driver will deadlock trying to recursively
266acquire the spinlock.
Linus Torvalds1da177e2005-04-16 15:20:36 -0700267
Matthew Wilcoxc41ade22009-03-17 08:54:05 -0400268There are two solutions. The first is to take the lock with
269spin_lock_irqsave() or spin_lock_irq() (see
270Documentation/DocBook/kernel-locking). The second is to specify
271IRQF_DISABLED to request_irq() so that the kernel runs the entire
272interrupt routine with interrupts disabled.
Randy Dunlap2500e7a2005-11-07 01:01:03 -0800273
Matthew Wilcoxc41ade22009-03-17 08:54:05 -0400274If your MSI interrupt routine does not hold the lock for the whole time
275it is running, the first solution may be best. The second solution is
276normally preferred as it avoids making two transitions from interrupt
277disabled to enabled and back again.
Linus Torvalds1da177e2005-04-16 15:20:36 -0700278
Matthew Wilcoxc41ade22009-03-17 08:54:05 -04002794.6 How to tell whether MSI/MSI-X is enabled on a device
Randy Dunlap2500e7a2005-11-07 01:01:03 -0800280
Matthew Wilcoxc41ade22009-03-17 08:54:05 -0400281Using 'lspci -v' (as root) may show some devices with "MSI", "Message
282Signalled Interrupts" or "MSI-X" capabilities. Each of these capabilities
Michael Witten4979de62011-07-14 19:52:56 +0000283has an 'Enable' flag which is followed with either "+" (enabled)
Matthew Wilcoxc41ade22009-03-17 08:54:05 -0400284or "-" (disabled).
Linus Torvalds1da177e2005-04-16 15:20:36 -0700285
Linus Torvalds1da177e2005-04-16 15:20:36 -0700286
Matthew Wilcoxc41ade22009-03-17 08:54:05 -04002875. MSI quirks
Linus Torvalds1da177e2005-04-16 15:20:36 -0700288
Matthew Wilcoxc41ade22009-03-17 08:54:05 -0400289Several PCI chipsets or devices are known not to support MSIs.
290The PCI stack provides three ways to disable MSIs:
Randy Dunlap2500e7a2005-11-07 01:01:03 -0800291
Matthew Wilcoxc41ade22009-03-17 08:54:05 -04002921. globally
2932. on all devices behind a specific bridge
2943. on a single device
Linus Torvalds1da177e2005-04-16 15:20:36 -0700295
Matthew Wilcoxc41ade22009-03-17 08:54:05 -04002965.1. Disabling MSIs globally
Linus Torvalds1da177e2005-04-16 15:20:36 -0700297
Matthew Wilcoxc41ade22009-03-17 08:54:05 -0400298Some host chipsets simply don't support MSIs properly. If we're
299lucky, the manufacturer knows this and has indicated it in the ACPI
Michael Witten4979de62011-07-14 19:52:56 +0000300FADT table. In this case, Linux automatically disables MSIs.
Matthew Wilcoxc41ade22009-03-17 08:54:05 -0400301Some boards don't include this information in the table and so we have
302to detect them ourselves. The complete list of these is found near the
303quirk_disable_all_msi() function in drivers/pci/quirks.c.
Linus Torvalds1da177e2005-04-16 15:20:36 -0700304
Matthew Wilcoxc41ade22009-03-17 08:54:05 -0400305If you have a board which has problems with MSIs, you can pass pci=nomsi
306on the kernel command line to disable MSIs on all devices. It would be
307in your best interests to report the problem to linux-pci@vger.kernel.org
308including a full 'lspci -v' so we can add the quirks to the kernel.
Linus Torvalds1da177e2005-04-16 15:20:36 -0700309
Matthew Wilcoxc41ade22009-03-17 08:54:05 -04003105.2. Disabling MSIs below a bridge
Linus Torvalds1da177e2005-04-16 15:20:36 -0700311
Matthew Wilcoxc41ade22009-03-17 08:54:05 -0400312Some PCI bridges are not able to route MSIs between busses properly.
313In this case, MSIs must be disabled on all devices behind the bridge.
Brice Goglin0cc2b372006-10-05 10:24:42 +0200314
Matthew Wilcoxc41ade22009-03-17 08:54:05 -0400315Some bridges allow you to enable MSIs by changing some bits in their
316PCI configuration space (especially the Hypertransport chipsets such
317as the nVidia nForce and Serverworks HT2000). As with host chipsets,
318Linux mostly knows about them and automatically enables MSIs if it can.
Michael Wittene6b85a12011-07-15 03:25:44 +0000319If you have a bridge unknown to Linux, you can enable
Matthew Wilcoxc41ade22009-03-17 08:54:05 -0400320MSIs in configuration space using whatever method you know works, then
321enable MSIs on that bridge by doing:
Brice Goglin0cc2b372006-10-05 10:24:42 +0200322
Matthew Wilcoxc41ade22009-03-17 08:54:05 -0400323 echo 1 > /sys/bus/pci/devices/$bridge/msi_bus
Brice Goglin0cc2b372006-10-05 10:24:42 +0200324
Matthew Wilcoxc41ade22009-03-17 08:54:05 -0400325where $bridge is the PCI address of the bridge you've enabled (eg
3260000:00:0e.0).
Brice Goglin0cc2b372006-10-05 10:24:42 +0200327
Matthew Wilcoxc41ade22009-03-17 08:54:05 -0400328To disable MSIs, echo 0 instead of 1. Changing this value should be
Michael Witten1b8386f2011-07-15 03:26:37 +0000329done with caution as it could break interrupt handling for all devices
Matthew Wilcoxc41ade22009-03-17 08:54:05 -0400330below this bridge.
Brice Goglin0cc2b372006-10-05 10:24:42 +0200331
Matthew Wilcoxc41ade22009-03-17 08:54:05 -0400332Again, please notify linux-pci@vger.kernel.org of any bridges that need
333special handling.
Brice Goglin0cc2b372006-10-05 10:24:42 +0200334
Matthew Wilcoxc41ade22009-03-17 08:54:05 -04003355.3. Disabling MSIs on a single device
Brice Goglin0cc2b372006-10-05 10:24:42 +0200336
Matthew Wilcoxc41ade22009-03-17 08:54:05 -0400337Some devices are known to have faulty MSI implementations. Usually this
Michael Wittenc2b65e12011-07-15 03:27:22 +0000338is handled in the individual device driver, but occasionally it's necessary
Matthew Wilcoxc41ade22009-03-17 08:54:05 -0400339to handle this with a quirk. Some drivers have an option to disable use
340of MSI. While this is a convenient workaround for the driver author,
341it is not good practise, and should not be emulated.
Brice Goglin0cc2b372006-10-05 10:24:42 +0200342
Matthew Wilcoxc41ade22009-03-17 08:54:05 -04003435.4. Finding why MSIs are disabled on a device
Brice Goglin0cc2b372006-10-05 10:24:42 +0200344
Matthew Wilcoxc41ade22009-03-17 08:54:05 -0400345From the above three sections, you can see that there are many reasons
346why MSIs may not be enabled for a given device. Your first step should
347be to examine your dmesg carefully to determine whether MSIs are enabled
348for your machine. You should also check your .config to be sure you
349have enabled CONFIG_PCI_MSI.
Brice Goglin0cc2b372006-10-05 10:24:42 +0200350
Matthew Wilcoxc41ade22009-03-17 08:54:05 -0400351Then, 'lspci -t' gives the list of bridges above a device. Reading
Michael Witten798c7942011-07-15 03:29:04 +0000352/sys/bus/pci/devices/*/msi_bus will tell you whether MSIs are enabled (1)
Matthew Wilcoxc41ade22009-03-17 08:54:05 -0400353or disabled (0). If 0 is found in any of the msi_bus files belonging
354to bridges between the PCI root and the device, MSIs are disabled.
Brice Goglin0cc2b372006-10-05 10:24:42 +0200355
Matthew Wilcoxc41ade22009-03-17 08:54:05 -0400356It is also worth checking the device driver to see whether it supports MSIs.
357For example, it may contain calls to pci_enable_msi(), pci_enable_msix() or
358pci_enable_msi_block().