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iommu: Documentation: rearrange, update kernel-parameters

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Add text for some undescribed iommu= parameters (merge, nomerge,
biomerge, panic, nopanic, pt, nopt). Add "usedac" and its description.
Add that iommu=pt is equivalent to iommu.passthrough=1
and that iommu=nopt is equivalent to iommu.passthrough=0.

Move the PPC/POWERNV heading & its option "nobypass" to a separate
area since the current "iommu=" applies only to X86 (according to
its heading).

Unindent the AMD GART IOMMU options heading to make it stand out.
Also add its kconfig symbol name to be explicit about what these
options apply to.

Make sure that the IOMMU options that are listed under AMD Gart
HW IOMMU-specific options are only for that product; i.e., add "force"
there and move "merge", "nomerge", and "panic" to the general IOMMU
options area.

Signed-off-by: Randy Dunlap <rdunlap@infradead.org>
Signed-off-by: Jonathan Corbet <corbet@lwn.net>
Message-ID: <20260528054611.1524937-1-rdunlap@infradead.org>
This commit is contained in:
Randy Dunlap 2026-05-27 22:46:11 -07:00 committed by Jonathan Corbet
parent af794e79d8
commit a41ac48a16
1 changed files with 33 additions and 17 deletions
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50
Documentation/admin-guide/kernel-parameters.txt
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@ -2580,23 +2580,41 @@ Kernel parameters
Don't force hardware IOMMU usage when it is not
needed. (default).
biomerge
panic
nopanic
merge
Do scatter-gather (SG) merging. Implies "force"
(experimental).
nomerge
Don't do scatter-gather (SG) merging.
biomerge
Do scatter-gather (SG) merging. Implies "force"
(experimental). [same as "merge"]
panic
Always panic when IOMMU overflows.
nopanic
Don't panic on IOMMU overflows.
pt
Use passththrough mode by default
(Equivalent to iommu.passthrough=1)
nopt
Use translated mode for DMA by default
(Equivalent to iommu.passthrough=0)
soft
Use software bounce buffering (SWIOTLB) (default for
Intel machines). This can be used to prevent the usage
of an available hardware IOMMU.
pt
nopt
nobypass [PPC/POWERNV]
Disable IOMMU bypass, using IOMMU for PCI devices.
usedac
Use the DAC on VIA PCI bridge
(default: disable the VIA PCI bridge DAC)
AMD Gart HW IOMMU-specific options:
AMD Gart HW IOMMU-specific options: (CONFIG_GART_IOMMU)
<size>
Set the size of the remapping area in bytes.
@ -2604,6 +2622,9 @@ Kernel parameters
allowed
Overwrite iommu off workarounds for specific chipsets
force
Overwrite iommu off workarounds for specific chipsets
fullflush
Flush IOMMU on each allocation (default).
@ -2614,21 +2635,16 @@ Kernel parameters
Allocate an own aperture over RAM with size
32MB<<order. (default: order=1, i.e. 64MB)
merge
Do scatter-gather (SG) merging. Implies "force"
(experimental).
nomerge
Don't do scatter-gather (SG) merging.
noaperture
Ask the IOMMU not to touch the aperture for AGP.
noagp
Don't initialize the AGP driver and use full aperture.
panic
Always panic when IOMMU overflows.
iommu= [PPC/POWERNV]
nobypass
Disable IOMMU bypass, using IOMMU for PCI devices.
iommu.forcedac= [ARM64,X86,EARLY] Control IOVA allocation for PCI devices.
Format: { "0" | "1" }