2 files changed, 67 insertions, 1 deletions

diff --git a/Documentation/MSI-HOWTO.txt b/Documentation/MSI-HOWTO.txt
index c70306abb7b2..5c34910665d1 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?
 
diff --git a/Documentation/kernel-parameters.txt b/Documentation/kernel-parameters.txt
index ff571f9298e0..dd00fd556a60 100644
--- a/Documentation/kernel-parameters.txt
+++ b/Documentation/kernel-parameters.txt
@@ -1231,6 +1231,11 @@ and is between 256 and 4096 characters. It is defined in the file
                                 machine check when some devices' config space
                                 is read. But various workarounds are disabled
                                 and some IOMMU drivers will not work.
+                bfsort          Sort PCI devices into breadth-first order.
+                                This sorting is done to get a device
+                                order compatible with older (<= 2.4) kernels.
+                nobfsort        Don't sort PCI devices into breadth-first order.
+
         pcmv=           [HW,PCMCIA] BadgePAD 4
 
         pd.             [PARIDE]