1<html><head><meta http-equiv="Content-Type" content="text/html; charset=ANSI_X3.4-1968"><title>Non-Contiguous Buffers</title><meta name="generator" content="DocBook XSL Stylesheets V1.78.1"><link rel="home" href="index.html" title="Writing an ALSA Driver"><link rel="up" href="buffer-and-memory.html" title="Chapter 11. Buffer and Memory Management"><link rel="prev" href="buffer-and-memory-external-hardware.html" title="External Hardware Buffers"><link rel="next" href="buffer-and-memory-vmalloced.html" title="Vmalloc'ed Buffers"></head><body bgcolor="white" text="black" link="#0000FF" vlink="#840084" alink="#0000FF"><div class="navheader"><table width="100%" summary="Navigation header"><tr><th colspan="3" align="center">Non-Contiguous Buffers</th></tr><tr><td width="20%" align="left"><a accesskey="p" href="buffer-and-memory-external-hardware.html">Prev</a> </td><th width="60%" align="center">Chapter 11. Buffer and Memory Management</th><td width="20%" align="right"> <a accesskey="n" href="buffer-and-memory-vmalloced.html">Next</a></td></tr></table><hr></div><div class="section"><div class="titlepage"><div><div><h2 class="title" style="clear: both"><a name="buffer-and-memory-non-contiguous"></a>Non-Contiguous Buffers</h2></div></div></div><p> 2 If your hardware supports the page table as in emu10k1 or the 3 buffer descriptors as in via82xx, you can use the scatter-gather 4 (SG) DMA. ALSA provides an interface for handling SG-buffers. 5 The API is provided in <code class="filename"><sound/pcm.h></code>. 6 </p><p> 7 For creating the SG-buffer handler, call 8 <code class="function">snd_pcm_lib_preallocate_pages()</code> or 9 <code class="function">snd_pcm_lib_preallocate_pages_for_all()</code> 10 with <code class="constant">SNDRV_DMA_TYPE_DEV_SG</code> 11 in the PCM constructor like other PCI pre-allocator. 12 You need to pass <code class="function">snd_dma_pci_data(pci)</code>, 13 where pci is the struct <span class="structname">pci_dev</span> pointer 14 of the chip as well. 15 The <span class="type">struct snd_sg_buf</span> instance is created as 16 substream->dma_private. You can cast 17 the pointer like: 18 19 </p><div class="informalexample"><pre class="programlisting"> 20 21 struct snd_sg_buf *sgbuf = (struct snd_sg_buf *)substream->dma_private; 22 23 </pre></div><p> 24 </p><p> 25 Then call <code class="function">snd_pcm_lib_malloc_pages()</code> 26 in the <em class="structfield"><code>hw_params</code></em> callback 27 as well as in the case of normal PCI buffer. 28 The SG-buffer handler will allocate the non-contiguous kernel 29 pages of the given size and map them onto the virtually contiguous 30 memory. The virtual pointer is addressed in runtime->dma_area. 31 The physical address (runtime->dma_addr) is set to zero, 32 because the buffer is physically non-contiguous. 33 The physical address table is set up in sgbuf->table. 34 You can get the physical address at a certain offset via 35 <code class="function">snd_pcm_sgbuf_get_addr()</code>. 36 </p><p> 37 When a SG-handler is used, you need to set 38 <code class="function">snd_pcm_sgbuf_ops_page</code> as 39 the <em class="structfield"><code>page</code></em> callback. 40 (See <a class="link" href="pcm-interface-operators.html#pcm-interface-operators-page-callback" title="page callback"> 41 <em class="citetitle">page callback section</em></a>.) 42 </p><p> 43 To release the data, call 44 <code class="function">snd_pcm_lib_free_pages()</code> in the 45 <em class="structfield"><code>hw_free</code></em> callback as usual. 46 </p></div><div class="navfooter"><hr><table width="100%" summary="Navigation footer"><tr><td width="40%" align="left"><a accesskey="p" href="buffer-and-memory-external-hardware.html">Prev</a> </td><td width="20%" align="center"><a accesskey="u" href="buffer-and-memory.html">Up</a></td><td width="40%" align="right"> <a accesskey="n" href="buffer-and-memory-vmalloced.html">Next</a></td></tr><tr><td width="40%" align="left" valign="top">External Hardware Buffers </td><td width="20%" align="center"><a accesskey="h" href="index.html">Home</a></td><td width="40%" align="right" valign="top"> Vmalloc'ed Buffers</td></tr></table></div></body></html> 47