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PCI: Update MSI-HOWTO.txt according to pci_msi_supported()

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Update MSI-HOWTO.txt according to pci_msi_supported().

Signed-off-by: Brice Goglin <brice@myri.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>
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Brice Goglin 2006-10-05 10:24:42 +02:00 committed by Greg Kroah-Hartman
parent 0306ebfa3b
commit 0cc2b3763e
1 changed files with 62 additions and 1 deletions
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Documentation/MSI-HOWTO.txt
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@ -470,7 +470,68 @@ LOC: 324553 325068
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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?