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Subject[PATCH] Re: data corruption with nvidia chipsets and IDE/SATA drives // memory hole mapping related bug?!
On Thu, 14 Dec 2006, Muli Ben-Yehuda wrote:

> On Wed, Dec 13, 2006 at 09:34:16PM +0100, Karsten Weiss wrote:
>
> > BTW: It would be really great if this area of the kernel would get some
> > more and better documentation. The information at
> > linux-2.6/Documentation/x86_64/boot_options.txt is very terse. I had to
> > read the code to get a *rough* idea what all the "iommu=" options
> > actually do and how they interact.
>
> Patches happily accepted :-)

Well, you asked for it. :-) So here's my little contribution. Please
*double* *check*!

(BTW: I would like to know what "DAC" and "SAC" means in this context)

===

From: Karsten Weiss <K.Weiss@science-computing.de>

Patch summary:

- Better explanation of some of the iommu kernel parameter options.
- "32MB<<order" instead of "32MB^order".
- Mention the default "order".
- SWIOTLB config help text
- removed the duplication of the iommu kernel parameter documentation.
- mention Documentation/x86_64/boot-options.txt in
Documentation/kernel-parameters.txt
- list the four existing PCI DMA mapping implementations of arch x86_64

Signed-off-by: Karsten Weiss <knweiss@science-computing.de>

---

--- linux-2.6.19/arch/x86_64/kernel/pci-dma.c.original 2006-12-14 11:15:38.348598021 +0100
+++ linux-2.6.19/arch/x86_64/kernel/pci-dma.c 2006-12-14 12:14:48.176967312 +0100
@@ -223,30 +223,10 @@
}
EXPORT_SYMBOL(dma_set_mask);

-/* iommu=[size][,noagp][,off][,force][,noforce][,leak][,memaper[=order]][,merge]
- [,forcesac][,fullflush][,nomerge][,biomerge]
- size set size of iommu (in bytes)
- noagp don't initialize the AGP driver and use full aperture.
- off don't use the IOMMU
- leak turn on simple iommu leak tracing (only when CONFIG_IOMMU_LEAK is on)
- memaper[=order] allocate an own aperture over RAM with size 32MB^order.
- noforce don't force IOMMU usage. Default.
- force Force IOMMU.
- merge Do lazy merging. This may improve performance on some block devices.
- Implies force (experimental)
- biomerge Do merging at the BIO layer. This is more efficient than merge,
- but should be only done with very big IOMMUs. Implies merge,force.
- nomerge Don't do SG merging.
- forcesac For SAC mode for masks <40bits (experimental)
- fullflush Flush IOMMU on each allocation (default)
- nofullflush Don't use IOMMU fullflush
- allowed overwrite iommu off workarounds for specific chipsets.
- soft Use software bounce buffering (default for Intel machines)
- noaperture Don't touch the aperture for AGP.
- allowdac Allow DMA >4GB
- nodac Forbid DMA >4GB
- panic Force panic when IOMMU overflows
-*/
+/*
+ * See <Documentation/x86_64/boot-options.txt> for the iommu kernel parameter
+ * documentation.
+ */
__init int iommu_setup(char *p)
{
iommu_merge = 1;
--- linux-2.6.19/arch/x86_64/Kconfig.original 2006-12-14 11:37:35.832142506 +0100
+++ linux-2.6.19/arch/x86_64/Kconfig 2006-12-14 11:47:24.346056710 +0100
@@ -431,8 +431,8 @@
on systems with more than 3GB. This is usually needed for USB,
sound, many IDE/SATA chipsets and some other devices.
Provides a driver for the AMD Athlon64/Opteron/Turion/Sempron GART
- based IOMMU and a software bounce buffer based IOMMU used on Intel
- systems and as fallback.
+ based hardware IOMMU and a software bounce buffer based IOMMU used
+ on Intel systems and as fallback.
The code is only active when needed (enough memory and limited
device) unless CONFIG_IOMMU_DEBUG or iommu=force is specified
too.
@@ -458,6 +458,11 @@
# need this always selected by IOMMU for the VIA workaround
config SWIOTLB
bool
+ help
+ Support for a software bounce buffer based IOMMU used on Intel
+ systems which don't have a hardware IOMMU. Using this code
+ PCI devices with 32bit memory access only are able to be
+ used on systems with more than 3 GB.

config X86_MCE
bool "Machine check support" if EMBEDDED
--- linux-2.6.19/Documentation/x86_64/boot-options.txt.original 2006-12-14 11:11:32.099300994 +0100
+++ linux-2.6.19/Documentation/x86_64/boot-options.txt 2006-12-14 12:10:24.028009890 +0100
@@ -180,35 +180,66 @@
pci=lastbus=NUMBER Scan upto NUMBER busses, no matter what the mptable says.
pci=noacpi Don't use ACPI to set up PCI interrupt routing.

-IOMMU
+IOMMU (input/output memory management unit)
+
+ Currently four x86_64 PCI DMA mapping implementations exist:
+
+ 1. <arch/x86_64/kernel/pci-nommu.c>: use no hardware/software IOMMU at all
+ (e.g. because you have < 3 GB memory).
+ Kernel boot message: "PCI-DMA: Disabling IOMMU"
+
+ 2. <arch/x86_64/kernel/pci-gart.c>: AMD GART based hardware IOMMU.
+ Kernel boot message: "PCI-DMA: using GART IOMMU"
+
+ 3. <arch/x86_64/kernel/pci-swiotlb.c> : Software IOMMU implementation. Used
+ e.g. if there is no hardware IOMMU in the system and it is need because
+ you have >3GB memory or told the kernel to us it (iommu=soft))
+ Kernel boot message: "PCI-DMA: Using software bounce buffering
+ for IO (SWIOTLB)"
+
+ 4. <arch/x86_64/pci-calgary.c> : IBM Calgary hardware IOMMU. Used in IBM
+ pSeries and xSeries servers. This hardware IOMMU supports DMA address
+ mapping with memory protection, etc.
+ Kernel boot message: "PCI-DMA: Using Calgary IOMMU"

iommu=[size][,noagp][,off][,force][,noforce][,leak][,memaper[=order]][,merge]
[,forcesac][,fullflush][,nomerge][,noaperture]
- size set size of iommu (in bytes)
- noagp don't initialize the AGP driver and use full aperture.
- off don't use the IOMMU
- leak turn on simple iommu leak tracing (only when CONFIG_IOMMU_LEAK is on)
- memaper[=order] allocate an own aperture over RAM with size 32MB^order.
- noforce don't force IOMMU usage. Default.
- force Force IOMMU.
- merge Do SG merging. Implies force (experimental)
- nomerge Don't do SG merging.
- forcesac For SAC mode for masks <40bits (experimental)
- fullflush Flush IOMMU on each allocation (default)
- nofullflush Don't use IOMMU fullflush
- allowed overwrite iommu off workarounds for specific chipsets.
- soft Use software bounce buffering (default for Intel machines)
- noaperture Don't touch the aperture for AGP.
- allowdac Allow DMA >4GB
- When off all DMA over >4GB is forced through an IOMMU or bounce
- buffering.
- nodac Forbid DMA >4GB
- panic Always panic when IOMMU overflows
+ size set size of IOMMU (in bytes)
+ noagp don't initialize the AGP driver and use full aperture.
+ off don't initialize and use any kind of IOMMU.
+ leak turn on simple iommu leak tracing (only when
+ CONFIG_IOMMU_LEAK is on)
+ memaper[=order] allocate an own aperture over RAM with size 32MB<<order.
+ (default: order=1, i.e. 64MB)
+ noforce don't force hardware IOMMU usage when it is not needed.
+ (default).
+ force Force the use of the hardware IOMMU even when it is
+ not actually needed (e.g. because < 3 GB memory).
+ merge Do scather-gather (SG) merging. Implies force (experimental)
+ nomerge Don't do scather-gather (SG) merging.
+ forcesac For SAC mode for masks <40bits (experimental)
+ fullflush Flush AMD GART based hardware IOMMU on each allocation
+ (default)
+ nofullflush Don't use IOMMU fullflush
+ allowed overwrite iommu off workarounds for specific chipsets.
+ soft Use software bounce buffering (SWIOTLB) (default for Intel
+ machines). This can be used to prevent the usage
+ of a available hardware IOMMU.
+ noaperture Ask the AMD GART based hardware IOMMU driver not to
+ touch the aperture for AGP.
+ allowdac Allow DMA >4GB
+ When off all DMA over >4GB is forced through an IOMMU or
+ bounce buffering.
+ nodac Forbid DMA >4GB
+ panic Always panic when IOMMU overflows

swiotlb=pages[,force]
+ pages Prereserve that many 128K pages for the software IO bounce
+ buffering.
+ force Force all IO through the software TLB.

- pages Prereserve that many 128K pages for the software IO bounce buffering.
- force Force all IO through the software TLB.
+ Settings for the IBM Calgary hardware IOMMU currently found in IBM
+ pSeries and xSeries machines:

calgary=[64k,128k,256k,512k,1M,2M,4M,8M]
calgary=[translate_empty_slots]
--- linux-2.6.19/Documentation/kernel-parameters.txt.original 2006-12-14 11:03:46.584429749 +0100
+++ linux-2.6.19/Documentation/kernel-parameters.txt 2006-12-14 11:11:22.172025378 +0100
@@ -104,6 +104,9 @@
Do not modify the syntax of boot loader parameters without extreme
need or coordination with <Documentation/i386/boot.txt>.

+There are also arch-specific kernel-parameters not documented here.
+See for example <Documentation/x86_64/boot-options.txt>.
+
Note that ALL kernel parameters listed below are CASE SENSITIVE, and that
a trailing = on the name of any parameter states that that parameter will
be entered as an environment variable, whereas its absence indicates that
--
__________________________________________creating IT solutions
Dipl.-Inf. Karsten Weiss science + computing ag
phone: +49 7071 9457 452 Hagellocher Weg 73
teamline: +49 7071 9457 681 72070 Tuebingen, Germany
email: knweiss@science-computing.de www.science-computing.de
-
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