<!DOCTYPE html PUBLIC "-//W3C//DTD HTML 4.01 Transitional//EN"> <html> <head> <meta name="generator" content= "HTML Tidy for Linux/x86 (vers 1 September 2005), see www.w3.org"> <meta http-equiv="Content-Type" content= "text/html; charset=us-ascii"> <title>Chapter 10. Allocating DMA Buffers on 64-bit Platforms</title> <meta name="generator" content="DocBook XSL Stylesheets V1.68.1"> <link rel="start" href="index.html" title= "NVIDIA Accelerated Linux Graphics Driver README and Installation Guide"> <link rel="up" href="installationandconfiguration.html" title= "Part I. Installation and Configuration Instructions"> <link rel="prev" href="knownissues.html" title= "Chapter 9. Known Issues"> <link rel="next" href="openglenvvariables.html" title= "Chapter 11. Specifying OpenGL Environment Variable Settings"> </head> <body> <div class="navheader"> <table width="100%" summary="Navigation header"> <tr> <th colspan="3" align="center">Chapter 10. Allocating DMA Buffers on 64-bit Platforms</th> </tr> <tr> <td width="20%" align="left"><a accesskey="p" href= "knownissues.html">Prev</a> </td> <th width="60%" align="center">Part I. Installation and Configuration Instructions</th> <td width="20%" align="right"> <a accesskey="n" href= "openglenvvariables.html">Next</a></td> </tr> </table> <hr></div> <div class="chapter" lang="en"> <div class="titlepage"> <div> <div> <h2 class="title"><a name="dma_issues" id= "dma_issues"></a>Chapter 10. Allocating DMA Buffers on 64-bit Platforms</h2> </div> </div> </div> <p>NVIDIA GPUs have limits on how much physical memory they can address. This directly impacts DMA buffers, as a DMA buffer allocated in physical memory that is unaddressable by the NVIDIA GPU cannot be used (or may be truncated, resulting in bad memory accesses).</p> <p>All pre-PCI Express GPUs and non-Native PCI Express GPUs (often known as bridged GPUs) are limited to 32 bits of physical address space, which corresponds to 4 GB of memory. On a system with greater than 4 GB of memory, allocating usable DMA buffers can be a problem. Native PCI Express GPUs are capable of addressing greater than 32 bits of physical address space and do not experience the same problems.</p> <p>Newer kernels provide a simple way to allocate memory that is guaranteed to reside within the 32 bit physical address space. Linux 2.6.15 provides this functionality with the __GFP_DMA32 interface. Kernels earlier than this version provide a software I/O TLB on Intel's EM64T and IOMMU support on AMD's AMD64 platform.</p> <p>Unfortunately, some problems exist with both interfaces. Early implementations of the Linux SWIOTLB set aside a very small amount of memory for its memory pool (only 4 MB). Also, when this memory pool is exhausted, some SWIOTLB implementations forcibly panic the kernel. This is also true for some implementations of the IOMMU interface.</p> <p>The NVIDIA Linux driver does not support the SWIOTLB. NVIDIA recommends that users of Intel's EM64T platform upgrade to Linux 2.6.11 or a more recent Linux kernel.</p> <p>On AMD's AMD64 platform, the size of the IOMMU can be configured in the system BIOS or, if no IOMMU BIOS option is available, using the 'iommu=memaper' kernel parameter. This kernel parameter expects an order and instructs the Linux kernel to create an IOMMU of size 32 MB^order overlapping physical memory. If the system's default IOMMU is smaller than 64 MB, the Linux kernel automatically replaces it with a 64 MB IOMMU.</p> <p>To reduce the risk of stability problems as a result of IOMMU space exhaustion on the X86-64 platform, the NVIDIA Linux driver internally limits its use of these interfaces. By default, the driver will not use more than 60 MB of IOMMU space, leaving at least 4 MB for the rest of the system (assuming a 64 MB IOMMU).</p> <p>This limit can be adjusted with the 'NVreg_RemapLimit' NVIDIA kernel module option. Specifically, if the IOMMU is larger than 64 MB, the limit can be adjusted to take advantage of the additional space. The 'NVreg_RemapLimit' option expects the size argument in bytes.</p> <p>NVIDIA recommends leaving 4 MB available for the rest of the system when changing the limit. For example, if the internal limit is to be relaxed to account for a 128 MB IOMMU, the recommended remap limit is 124 MB. This remap limit can be specified by passing 'NVreg_RemapLimit=0x7c00000' to the NVIDIA kernel module.</p> <p>Also see the 'The X86-64 platform (AMD64/EM64T) and early Linux 2.6 kernels' section in <a href= "knownissues.html#x86_64_platform">The X86-64 platform (AMD64/EM64T) and early Linux 2.6 kernels</a>.</p> </div> <div class="navfooter"> <hr> <table width="100%" summary="Navigation footer"> <tr> <td width="40%" align="left"><a accesskey="p" href= "knownissues.html">Prev</a> </td> <td width="20%" align="center"><a accesskey="u" href= "installationandconfiguration.html">Up</a></td> <td width="40%" align="right"> <a accesskey="n" href= "openglenvvariables.html">Next</a></td> </tr> <tr> <td width="40%" align="left" valign="top"> Chapter 9. Known Issues </td> <td width="20%" align="center"><a accesskey="h" href= "index.html">Home</a></td> <td width="40%" align="right" valign="top"> Chapter 11. Specifying OpenGL Environment Variable Settings</td> </tr> </table> </div> </body> </html>
distrib
>
Mageia
>
7
>
i586
>
media
>
nonfree-updates
>
by-pkgid
>
a720cb256c173980abbcc5128212ffd6
>
files
>
14
