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Diffstat (limited to 'Documentation/PCI/MSI-HOWTO.txt')
| -rw-r--r-- | Documentation/PCI/MSI-HOWTO.txt | 469 |
1 files changed, 79 insertions, 390 deletions
diff --git a/Documentation/PCI/MSI-HOWTO.txt b/Documentation/PCI/MSI-HOWTO.txt index 1179850..c55df29 100644 --- a/Documentation/PCI/MSI-HOWTO.txt +++ b/Documentation/PCI/MSI-HOWTO.txt @@ -78,422 +78,111 @@ 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 +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. -4.2.1 pci_enable_msi +To automatically use MSI or MSI-X interrupt vectors, use the following +function: -int pci_enable_msi(struct pci_dev *dev) + int pci_alloc_irq_vectors(struct pci_dev *dev, unsigned int min_vecs, + unsigned int max_vecs, unsigned int flags); -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. +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. -4.2.2 pci_enable_msi_range +The flags argument should normally be set to 0, but can be used to pass the +PCI_IRQ_NOMSI and PCI_IRQ_NOMSIX flag in case a device claims to support +MSI or MSI-X, but the support is broken, or to pass PCI_IRQ_NOLEGACY in +case the device does not support legacy interrupt lines. -int pci_enable_msi_range(struct pci_dev *dev, int minvec, int maxvec) +By default this function will spread the interrupts around the available +CPUs, but this feature can be disabled by passing the PCI_IRQ_NOAFFINITY +flag. -This function allows a device driver to request any number of MSI -interrupts within specified range from 'minvec' to 'maxvec'. +To get the Linux IRQ numbers passed to request_irq() and free_irq() and the +vectors, use the following function: -If this function returns a positive number it indicates the number of -MSI interrupts that have been successfully allocated. In this case -the device is switched from pin-based interrupt mode to MSI mode 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. Device driver can use the returned -number of successfully allocated MSI interrupts to further allocate -and initialize device resources. + int pci_irq_vector(struct pci_dev *dev, unsigned int nr); -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. +Any allocated resources should be freed before removing the device using +the following function: -This function should be called before the driver calls request_irq(), -because MSI interrupts are delivered via vectors that are different -from the vector of a pin-based interrupt. + void pci_free_irq_vectors(struct pci_dev *dev); -It is ideal if drivers can cope with a variable number of MSI interrupts; -there are many reasons why the platform may not be able to provide the -exact number that a driver asks for. +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. -There could be devices that can not operate with just any number of MSI -interrupts within a range. See chapter 4.3.1.3 to get the idea how to -handle such devices for MSI-X - the same logic applies to MSI. +If a device supports neither MSI-X or MSI it will fall back to a single +legacy IRQ vector. -4.2.1.1 Maximum possible number of MSI interrupts +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: -The typical usage of MSI interrupts is to allocate as many vectors as -possible, likely up to the limit returned by pci_msi_vec_count() function: - -static int foo_driver_enable_msi(struct pci_dev *pdev, int nvec) -{ - return pci_enable_msi_range(pdev, 1, nvec); -} - -Note the value of 'minvec' parameter is 1. As 'minvec' is inclusive, -the value of 0 would be meaningless and could result in error. - -Some devices have a minimal limit on number of MSI interrupts. -In this case the function could look like this: - -static int foo_driver_enable_msi(struct pci_dev *pdev, int nvec) -{ - return pci_enable_msi_range(pdev, FOO_DRIVER_MINIMUM_NVEC, nvec); -} - -4.2.1.2 Exact number of MSI interrupts + nvec = pci_alloc_irq_vectors(pdev, 1, nvec, 0); + if (nvec < 0) + goto out_err; If a driver is unable or unwilling to deal with a variable number of MSI -interrupts it could request a particular number of interrupts by passing -that number to pci_enable_msi_range() function as both 'minvec' and 'maxvec' -parameters: - -static int foo_driver_enable_msi(struct pci_dev *pdev, int nvec) -{ - return pci_enable_msi_range(pdev, nvec, nvec); -} - -Note, unlike pci_enable_msi_exact() function, which could be also used to -enable a particular number of MSI-X interrupts, pci_enable_msi_range() -returns either a negative errno or 'nvec' (not negative errno or 0 - as -pci_enable_msi_exact() does). - -4.2.1.3 Single MSI mode - -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 'minvec' and 'maxvec': - -static int foo_driver_enable_single_msi(struct pci_dev *pdev) -{ - return pci_enable_msi_range(pdev, 1, 1); -} - -Note, unlike pci_enable_msi() function, which could be also used to -enable the single MSI mode, pci_enable_msi_range() returns either a -negative errno or 1 (not negative errno or 0 - as pci_enable_msi() -does). - -4.2.3 pci_enable_msi_exact - -int pci_enable_msi_exact(struct pci_dev *dev, int nvec) - -This variation on pci_enable_msi_range() call allows a device driver to -request exactly 'nvec' MSIs. - -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. - -By contrast with pci_enable_msi_range() function, pci_enable_msi_exact() -returns zero in case of success, which indicates MSI interrupts have been -successfully allocated. - -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_range(). -Calling it restores dev->irq to the pin-based interrupt number and frees -the previously allocated MSIs. The interrupts 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. - -4.2.4 pci_msi_vec_count - -int pci_msi_vec_count(struct pci_dev *dev) - -This function could be used to retrieve the number of MSI vectors the -device requested (via the Multiple Message Capable register). The MSI -specification only allows the returned value to be a power of two, -up to a maximum of 2^5 (32). - -If this function returns a negative number, it indicates the device is -not capable of sending MSIs. - -If this function returns a positive number, it indicates the maximum -number of MSI interrupt vectors that could be allocated. - -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_range - -int pci_enable_msix_range(struct pci_dev *dev, struct msix_entry *entries, - int minvec, int maxvec) - -Calling this function asks the PCI subsystem to allocate any number of -MSI-X interrupts within specified range from 'minvec' to 'maxvec'. -The 'entries' argument is a pointer to an array of msix_entry structs -which should be at least 'maxvec' entries in size. - -On success, the device is switched into MSI-X mode and the function -returns the number of MSI-X interrupts that have been successfully -allocated. In this case the 'vector' member in entries numbered from -0 to the returned value - 1 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. -Device driver can use the returned number of successfully allocated MSI-X -interrupts to further allocate and initialize device resources. - -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. - -This function, in contrast with pci_enable_msi_range(), 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. - -There could be devices that can not operate with just any number of MSI-X -interrupts within a range. E.g., an network adapter might need let's say -four vectors per each queue it provides. Therefore, a number of MSI-X -interrupts allocated should be a multiple of four. In this case interface -pci_enable_msix_range() can not be used alone to request MSI-X interrupts -(since it can allocate any number within the range, without any notion of -the multiple of four) and the device driver should master a custom logic -to request the required number of MSI-X interrupts. - -4.3.1.1 Maximum possible number of MSI-X interrupts - -The typical usage of MSI-X interrupts is to allocate as many vectors as -possible, likely up to the limit returned by pci_msix_vec_count() function: - -static int foo_driver_enable_msix(struct foo_adapter *adapter, int nvec) -{ - return pci_enable_msix_range(adapter->pdev, adapter->msix_entries, - 1, nvec); -} - -Note the value of 'minvec' parameter is 1. As 'minvec' is inclusive, -the value of 0 would be meaningless and could result in error. - -Some devices have a minimal limit on number of MSI-X interrupts. -In this case the function could look like this: - -static int foo_driver_enable_msix(struct foo_adapter *adapter, int nvec) -{ - return pci_enable_msix_range(adapter->pdev, adapter->msix_entries, - FOO_DRIVER_MINIMUM_NVEC, nvec); -} - -4.3.1.2 Exact number of MSI-X interrupts - -If a driver is unable or unwilling to deal with a variable number of MSI-X -interrupts it could request a particular number of interrupts by passing -that number to pci_enable_msix_range() function as both 'minvec' and 'maxvec' -parameters: - -static int foo_driver_enable_msix(struct foo_adapter *adapter, int nvec) -{ - return pci_enable_msix_range(adapter->pdev, adapter->msix_entries, - nvec, nvec); -} - -Note, unlike pci_enable_msix_exact() function, which could be also used to -enable a particular number of MSI-X interrupts, pci_enable_msix_range() -returns either a negative errno or 'nvec' (not negative errno or 0 - as -pci_enable_msix_exact() does). - -4.3.1.3 Specific requirements to the number of MSI-X interrupts - -As noted above, there could be devices that can not operate with just any -number of MSI-X interrupts within a range. E.g., let's assume a device that -is only capable sending the number of MSI-X interrupts which is a power of -two. A routine that enables MSI-X mode for such device might look like this: - -/* - * Assume 'minvec' and 'maxvec' are non-zero - */ -static int foo_driver_enable_msix(struct foo_adapter *adapter, - int minvec, int maxvec) -{ - int rc; - - minvec = roundup_pow_of_two(minvec); - maxvec = rounddown_pow_of_two(maxvec); - - if (minvec > maxvec) - return -ERANGE; - -retry: - rc = pci_enable_msix_range(adapter->pdev, adapter->msix_entries, - maxvec, maxvec); - /* - * -ENOSPC is the only error code allowed to be analyzed - */ - if (rc == -ENOSPC) { - if (maxvec == 1) - return -ENOSPC; - - maxvec /= 2; - - if (minvec > maxvec) - return -ENOSPC; - - goto retry; - } - - return rc; -} - -Note how pci_enable_msix_range() return value is analyzed for a fallback - -any error code other than -ENOSPC indicates a fatal error and should not -be retried. - -4.3.2 pci_enable_msix_exact - -int pci_enable_msix_exact(struct pci_dev *dev, - struct msix_entry *entries, int nvec) - -This variation on pci_enable_msix_range() call allows a device driver to -request exactly 'nvec' MSI-Xs. - -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. - -By contrast with pci_enable_msix_range() function, pci_enable_msix_exact() -returns zero in case of success, which indicates MSI-X interrupts have been -successfully allocated. - -Another version of a routine that enables MSI-X mode for a device with -specific requirements described in chapter 4.3.1.3 might look like this: - -/* - * Assume 'minvec' and 'maxvec' are non-zero - */ -static int foo_driver_enable_msix(struct foo_adapter *adapter, - int minvec, int maxvec) -{ - int rc; - - minvec = roundup_pow_of_two(minvec); - maxvec = rounddown_pow_of_two(maxvec); - - if (minvec > maxvec) - return -ERANGE; - -retry: - rc = pci_enable_msix_exact(adapter->pdev, - adapter->msix_entries, maxvec); - - /* - * -ENOSPC is the only error code allowed to be analyzed - */ - if (rc == -ENOSPC) { - if (maxvec == 1) - return -ENOSPC; - - maxvec /= 2; - - if (minvec > maxvec) - return -ENOSPC; - - goto retry; - } else if (rc < 0) { - return rc; - } - - return maxvec; -} - -4.3.3 pci_disable_msix - -void pci_disable_msix(struct pci_dev *dev) - -This function should be used to undo the effect of pci_enable_msix_range(). -It frees the previously allocated MSI-X 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(). +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: -4.3.4 pci_msix_vec_count + ret = pci_alloc_irq_vectors(pdev, nvec, nvec, 0); + if (ret < 0) + goto out_err; -int pci_msix_vec_count(struct pci_dev *dev) +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': -This function could be used to retrieve number of entries in the device -MSI-X table. + ret = pci_alloc_irq_vectors(pdev, 1, 1, 0); + if (ret < 0) + goto out_err; -If this function returns a negative number, it indicates the device is -not capable of sending MSI-Xs. +Some devices might not support using legacy line interrupts, in which case +the PCI_IRQ_NOLEGACY flag can be used to fail the request if the platform +can't provide MSI or MSI-X interrupts: -If this function returns a positive number, it indicates the maximum -number of MSI-X interrupt vectors that could be allocated. + nvec = pci_alloc_irq_vectors(pdev, 1, nvec, PCI_IRQ_NOLEGACY); + if (nvec < 0) + goto out_err; -4.4 Handling devices implementing both MSI and MSI-X capabilities +4.3 Legacy APIs -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_range() when MSI-X is already -enabled or pci_enable_msix_range() 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_range() -or pci_enable_msix_range() and resuming operation. This is not expected -to be a common operation but may be useful for debugging or testing -during development. +The following old APIs to enable and disable MSI or MSI-X interrupts should +not be used in new code: -4.5 Considerations when using MSIs + pci_enable_msi() /* deprecated */ + pci_enable_msi_range() /* deprecated */ + pci_enable_msi_exact() /* deprecated */ + pci_disable_msi() /* deprecated */ + pci_enable_msix_range() /* deprecated */ + pci_enable_msix_exact() /* deprecated */ + pci_disable_msix() /* deprecated */ -4.5.1 Choosing between MSI-X and MSI +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. -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 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. +4.4 Considerations when using MSIs -4.5.2 Spinlocks +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 @@ -505,7 +194,7 @@ 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/DocBook/kernel-locking). -4.6 How to tell whether MSI/MSI-X is enabled on a device +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 |