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4736. FAQ 4736. MSI quirks
474
475Several PCI chipsets or devices are known to not support MSI.
476The PCI stack provides 3 possible levels of MSI disabling:
477* on a single device
478* on all devices behind a specific bridge
479* globally
480
4816.1. Disabling MSI on a single device
482
483Under some circumstances, it might be required to disable MSI on a
484single device, It may be achived by either not calling pci_enable_msi()
485or all, or setting the pci_dev->no_msi flag before (most of the time
486in a quirk).
487
4886.2. Disabling MSI below a bridge
489
490The vast majority of MSI quirks are required by PCI bridges not
491being able to route MSI between busses. In this case, MSI have to be
492disabled on all devices behind this bridge. It is achieves by setting
493the PCI_BUS_FLAGS_NO_MSI flag in the pci_bus->bus_flags of the bridge
494subordinate bus. There is no need to set the same flag on bridges that
495are below the broken brigde. When pci_enable_msi() is called to enable
496MSI on a device, pci_msi_supported() takes care of checking the NO_MSI
497flag in all parent busses of the device.
498
499Some bridges actually support dynamic MSI support enabling/disabling
500by changing some bits in their PCI configuration space (especially
501the Hypertransport chipsets such as the nVidia nForce and Serverworks
502HT2000). It may then be required to update the NO_MSI flag on the
503corresponding devices in the sysfs hierarchy. To enable MSI support
504on device "0000:00:0e", do:
505
506 echo 1 > /sys/bus/pci/devices/0000:00:0e/msi_bus
507
508To disable MSI support, echo 0 instead of 1. Note that it should be
509used with caution since changing this value might break interrupts.
510
5116.3. Disabling MSI globally
512
513Some extreme cases may require to disable MSI globally on the system.
514For now, the only known case is a Serverworks PCI-X chipsets (MSI are
515not supported on several busses that are not all connected to the
516chipset in the Linux PCI hierarchy). In the vast majority of other
517cases, disabling only behind a specific bridge is enough.
518
519For debugging purpose, the user may also pass pci=nomsi on the kernel
520command-line to explicitly disable MSI globally. But, once the appro-
521priate quirks are added to the kernel, this option should not be
522required anymore.
523
5246.4. Finding why MSI cannot be enabled on a device
525
526Assuming that MSI are not enabled on a device, you should look at
527dmesg to find messages that quirks may output when disabling MSI
528on some devices, some bridges or even globally.
529Then, lspci -t gives the list of bridges above a device. Reading
530/sys/bus/pci/devices/0000:00:0e/msi_bus will tell you whether MSI
531are enabled (1) or disabled (0). In 0 is found in a single bridge
532msi_bus file above the device, MSI cannot be enabled.
533
5347. FAQ
474 535
475Q1. Are there any limitations on using the MSI? 536Q1. Are there any limitations on using the MSI?
476 537