4. The MSI Driver Guide HOWTO — The Linux Kernel 6.14.0-gnu-rt3-parabola1-1-rt documentation

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# 4. The MSI Driver Guide HOWTO

Authors:

  Tom L Nguyen; Martine Silbermann; Matthew Wilcox

Copyright:

  2003, 2008 Intel Corporation

## 4.1. 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.

## 4.2. 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.

## 4.3. 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.

## 4.4. 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.

### 4.4.1. 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.

### 4.4.2. 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_INTX, 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;

### 4.4.3. 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.

### 4.4.4. Considerations when using MSIs

#### 4.4.4.1. 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 Unreliable Guide To Locking).

### 4.4.5. 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).

## 4.5. 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

### 4.5.1. 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.

### 4.5.2. Disabling MSIs below a bridge

Some PCI bridges are not able to route MSIs between buses 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.

### 4.5.3. 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 practice, and should not be emulated.

### 4.5.4. 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_alloc_irq_vectors()` with the PCI_IRQ_MSI or PCI_IRQ_MSIX flags.

## 4.6. List of device drivers MSI(-X) APIs

The PCI/MSI subsystem has a dedicated C file for its exported device driver APIs — drivers/pci/msi/api.c. The following functions are exported:

int pci_enable_msi(struct pci_dev *dev)

  Enable MSI interrupt mode on device

Parameters

`struct pci_dev *dev`

  the PCI device to operate on

Description

Legacy device driver API to enable MSI interrupts mode on device and allocate a single interrupt vector. On success, the allocated vector Linux IRQ will be saved at dev->irq. The driver must invoke `pci_disable_msi()` on cleanup.

NOTE

The newer `pci_alloc_irq_vectors()` / `pci_free_irq_vectors()` API pair should, in general, be used instead.

Return

0 on success, errno otherwise

void pci_disable_msi(struct pci_dev *dev)

  Disable MSI interrupt mode on device

Parameters

`struct pci_dev *dev`

  the PCI device to operate on

Description

Legacy device driver API to disable MSI interrupt mode on device, free earlier allocated interrupt vectors, and restore INTx emulation. The PCI device Linux IRQ (dev->irq) is restored to its default pin-assertion IRQ. This is the cleanup pair of `pci_enable_msi()`.

NOTE

The newer `pci_alloc_irq_vectors()` / `pci_free_irq_vectors()` API pair should, in general, be used instead.

int pci_msix_vec_count(struct pci_dev *dev)

  Get number of MSI-X interrupt vectors on device

Parameters

`struct pci_dev *dev`

  the PCI device to operate on

Return

number of MSI-X interrupt vectors available on this device (i.e., the device’s MSI-X capability structure “table size”), -EINVAL if the device is not MSI-X capable, other errnos otherwise.

int pci_enable_msix_range(struct pci_dev *dev, struct msix_entry *entries, int minvec, int maxvec)

  Enable MSI-X interrupt mode on device

Parameters

`struct pci_dev *dev`

  the PCI device to operate on

`struct msix_entry *entries`

  input/output parameter, array of MSI-X configuration entries

`int minvec`

  minimum required number of MSI-X vectors

`int maxvec`

  maximum desired number of MSI-X vectors

Description

Legacy device driver API to enable MSI-X interrupt mode on device and configure its MSI-X capability structure as appropriate. The passed entries array must have each of its members “entry” field set to a desired (valid) MSI-X vector number, where the range of valid MSI-X vector numbers can be queried through `pci_msix_vec_count()`. If successful, the driver must invoke `pci_disable_msix()` on cleanup.

NOTE

The newer `pci_alloc_irq_vectors()` / `pci_free_irq_vectors()` API pair should, in general, be used instead.

Return

number of MSI-X vectors allocated (which might be smaller than maxvecs), where Linux IRQ numbers for such allocated vectors are saved back in the entries array elements’ “vector” field. Return -ENOSPC if less than minvecs interrupt vectors are available. Return -EINVAL if one of the passed entries members “entry” field was invalid or a duplicate, or if plain MSI interrupts mode was earlier enabled on device. Return other errnos otherwise.

bool pci_msix_can_alloc_dyn(struct pci_dev *dev)

  Query whether dynamic allocation after enabling MSI-X is supported

Parameters

`struct pci_dev *dev`

  PCI device to operate on

Return

True if supported, false otherwise

struct msi_map pci_msix_alloc_irq_at(struct pci_dev *dev, unsigned int index, const struct irq_affinity_desc *affdesc)

  Allocate an MSI-X interrupt after enabling MSI-X at a given MSI-X vector index or any free vector index

Parameters

`struct pci_dev *dev`

  PCI device to operate on

`unsigned int index`

  Index to allocate. If index == MSI_ANY_INDEX this allocates the next free index in the MSI-X table

`const struct irq_affinity_desc *affdesc`

  Optional pointer to an affinity descriptor structure. NULL otherwise

Return

A struct msi_map

> On success msi_map::index contains the allocated index (>= 0) and msi_map::virq contains the allocated Linux interrupt number (> 0).

> On fail msi_map::index contains the error code and msi_map::virq is set to 0.

void pci_msix_free_irq(struct pci_dev *dev, struct msi_map map)

  Free an interrupt on a PCI/MSIX interrupt domain

Parameters

`struct pci_dev *dev`

  The PCI device to operate on

`struct msi_map map`

  A struct msi_map describing the interrupt to free

Description

Undo an interrupt vector allocation. Does not disable MSI-X.

void pci_disable_msix(struct pci_dev *dev)

  Disable MSI-X interrupt mode on device

Parameters

`struct pci_dev *dev`

  the PCI device to operate on

Description

Legacy device driver API to disable MSI-X interrupt mode on device, free earlier-allocated interrupt vectors, and restore INTx. The PCI device Linux IRQ (dev->irq) is restored to its default pin assertion IRQ. This is the cleanup pair of `pci_enable_msix_range()`.

NOTE

The newer `pci_alloc_irq_vectors()` / `pci_free_irq_vectors()` API pair should, in general, be used instead.

int pci_alloc_irq_vectors(struct pci_dev *dev, unsigned int min_vecs, unsigned int max_vecs, unsigned int flags)

  Allocate multiple device interrupt vectors

Parameters

`struct pci_dev *dev`

  the PCI device to operate on

`unsigned int min_vecs`

  minimum required number of vectors (must be >= 1)

`unsigned int max_vecs`

  maximum desired number of vectors

`unsigned int flags`

  One or more of:

  - `PCI_IRQ_MSIX` Allow trying MSI-X vector allocations

  - `PCI_IRQ_MSI` Allow trying MSI vector allocations

  - `PCI_IRQ_INTX` Allow trying INTx interrupts, if and only if min_vecs == 1

  - `PCI_IRQ_AFFINITY` Auto-manage IRQs affinity by spreading the vectors around available CPUs

Description

Allocate up to max_vecs interrupt vectors on device. MSI-X irq vector allocation has a higher precedence over plain MSI, which has a higher precedence over legacy INTx emulation.

Upon a successful allocation, the caller should use `pci_irq_vector()` to get the Linux IRQ number to be passed to `request_threaded_irq()`. The driver must call `pci_free_irq_vectors()` on cleanup.

Return

number of allocated vectors (which might be smaller than max_vecs), -ENOSPC if less than min_vecs interrupt vectors are available, other errnos otherwise.

int pci_alloc_irq_vectors_affinity(struct pci_dev *dev, unsigned int min_vecs, unsigned int max_vecs, unsigned int flags, struct irq_affinity *affd)

  Allocate multiple device interrupt vectors with affinity requirements

Parameters

`struct pci_dev *dev`

  the PCI device to operate on

`unsigned int min_vecs`

  minimum required number of vectors (must be >= 1)

`unsigned int max_vecs`

  maximum desired number of vectors

`unsigned int flags`

  allocation flags, as in `pci_alloc_irq_vectors()`

`struct irq_affinity *affd`

  affinity requirements (can be `NULL`).

Description

Same as `pci_alloc_irq_vectors()`, but with the extra affd parameter. Check that function docs, and `struct irq_affinity`, for more details.

int pci_irq_vector(struct pci_dev *dev, unsigned int nr)

  Get Linux IRQ number of a device interrupt vector

Parameters

`struct pci_dev *dev`

  the PCI device to operate on

`unsigned int nr`

  device-relative interrupt vector index (0-based); has different meanings, depending on interrupt mode:

  > - MSI-X the index in the MSI-X vector table

  > - MSI the index of the enabled MSI vectors

  > - INTx must be 0

Return

the Linux IRQ number, or -EINVAL if nr is out of range

const struct cpumask *pci_irq_get_affinity(struct pci_dev *dev, int nr)

  Get a device interrupt vector affinity

Parameters

`struct pci_dev *dev`

  the PCI device to operate on

`int nr`

  device-relative interrupt vector index (0-based); has different meanings, depending on interrupt mode:

  > - MSI-X the index in the MSI-X vector table

  > - MSI the index of the enabled MSI vectors

  > - INTx must be 0

Return

MSI/MSI-X vector affinity, NULL if nr is out of range or if the MSI(-X) vector was allocated without explicit affinity requirements (e.g., by `pci_enable_msi()`, `pci_enable_msix_range()`, or `pci_alloc_irq_vectors()` without the `PCI_IRQ_AFFINITY` flag). Return a generic set of CPU IDs representing all possible CPUs available during system boot if the device is in legacy INTx mode.

void pci_free_irq_vectors(struct pci_dev *dev)

  Free previously allocated IRQs for a device

Parameters

`struct pci_dev *dev`

  the PCI device to operate on

Description

Undo the interrupt vector allocations and possible device MSI/MSI-X enablement earlier done through `pci_alloc_irq_vectors_affinity()` or `pci_alloc_irq_vectors()`.

void pci_restore_msi_state(struct pci_dev *dev)

  Restore cached MSI(-X) state on device

Parameters

`struct pci_dev *dev`

  the PCI device to operate on

Description

Write the Linux-cached MSI(-X) state back on device. This is typically useful upon system resume, or after an error-recovery PCI adapter reset.

int pci_msi_enabled(void)

  Are MSI(-X) interrupts enabled system-wide?

Parameters

`void`

  no arguments

Return

true if MSI has not been globally disabled through ACPI FADT, PCI bridge quirks, or the “pci=nomsi” kernel command-line option.