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diff --git a/Documentation/MSI-HOWTO.txt b/Documentation/MSI-HOWTO.txt index c70306abb7b..5c34910665d 100644 --- a/Documentation/MSI-HOWTO.txt +++ b/Documentation/MSI-HOWTO.txt @@ -470,7 +470,68 @@ LOC: 324553 325068 ERR: 0 MIS: 0 -6. FAQ +6. MSI quirks + +Several PCI chipsets or devices are known to not support MSI. +The PCI stack provides 3 possible levels of MSI disabling: +* on a single device +* on all devices behind a specific bridge +* globally + +6.1. Disabling MSI on a single device + +Under some circumstances, it might be required to disable MSI on a +single device, It may be achived by either not calling pci_enable_msi() +or all, or setting the pci_dev->no_msi flag before (most of the time +in a quirk). + +6.2. Disabling MSI below a bridge + +The vast majority of MSI quirks are required by PCI bridges not +being able to route MSI between busses. In this case, MSI have to be +disabled on all devices behind this bridge. It is achieves by setting +the PCI_BUS_FLAGS_NO_MSI flag in the pci_bus->bus_flags of the bridge +subordinate bus. There is no need to set the same flag on bridges that +are below the broken brigde. When pci_enable_msi() is called to enable +MSI on a device, pci_msi_supported() takes care of checking the NO_MSI +flag in all parent busses of the device. + +Some bridges actually support dynamic MSI support enabling/disabling +by changing some bits in their PCI configuration space (especially +the Hypertransport chipsets such as the nVidia nForce and Serverworks +HT2000). It may then be required to update the NO_MSI flag on the +corresponding devices in the sysfs hierarchy. To enable MSI support +on device "0000:00:0e", do: + + echo 1 > /sys/bus/pci/devices/0000:00:0e/msi_bus + +To disable MSI support, echo 0 instead of 1. Note that it should be +used with caution since changing this value might break interrupts. + +6.3. Disabling MSI globally + +Some extreme cases may require to disable MSI globally on the system. +For now, the only known case is a Serverworks PCI-X chipsets (MSI are +not supported on several busses that are not all connected to the +chipset in the Linux PCI hierarchy). In the vast majority of other +cases, disabling only behind a specific bridge is enough. + +For debugging purpose, the user may also pass pci=nomsi on the kernel +command-line to explicitly disable MSI globally. But, once the appro- +priate quirks are added to the kernel, this option should not be +required anymore. + +6.4. Finding why MSI cannot be enabled on a device + +Assuming that MSI are not enabled on a device, you should look at +dmesg to find messages that quirks may output when disabling MSI +on some devices, some bridges or even globally. +Then, lspci -t gives the list of bridges above a device. Reading +/sys/bus/pci/devices/0000:00:0e/msi_bus will tell you whether MSI +are enabled (1) or disabled (0). In 0 is found in a single bridge +msi_bus file above the device, MSI cannot be enabled. + +7. FAQ Q1. Are there any limitations on using the MSI? |