mirror of
https://github.com/outbackdingo/UltraGrid.git
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0a57fc4faf
* format is now entered as a parameter string (-g <mode>) * expect currently buffers as big as needed (exactly one frame) TODO: revide if isn't possible to read whole buffer at once (ideally the memmapped one, and extract frames by hand) * now it is needed to set /sys/class/sdivideo/sdivideorxX/bufsize * bump to new driver (2.7.2)
83 lines
2.9 KiB
Plaintext
83 lines
2.9 KiB
Plaintext
This file describes how memory for DMA buffers is allocated
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by the Master Linux driver. It is primarily of interest
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to programmers who wish to modify the driver.
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The Master Linux driver provides several DMA buffers
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for each interface. These buffers can range in size from
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a few bytes to several thousand bytes, may be different
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for each interface, and may number in the thousands.
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Allocating this memory becomes complex because of
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the limitations of the methods available to request
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DMA-capable memory in the Linux kernel.
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There are two basic methods of allocating DMA-capable memory
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in the Linux kernel:
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kmalloc() can allocate
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32 bytes, 64 bytes, 128 bytes,..., 131072 bytes
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in contiguous blocks.
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get_free_page() allocates one page.
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On the x86, one page is 4096 bytes.
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Allocating contiguous blocks becomes difficult as larger blocks
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are requested and memory becomes scarce.
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Allocating each buffer with a single kmalloc() is not acceptable
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because it is difficult or impossible to get larger blocks.
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Allocating each buffer with multiple get_free_page() calls
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is not acceptable because the memory wasted by smaller buffers
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is excessive.
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There are also issues in allocating memory for
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pointers to buffers and DMA descriptors.
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At present, memory allocation works like this:
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Memory for all of the buffers is allocated with get_free_page().
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For DVB ASI, the "tails" of buffers which are not integer multiples
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of the page size are "packed" into pages at the end of this memory;
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ie. several buffers may share a single page.
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For raw SDI and SDI audio/video, each tail has its own page.
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The pointers to these pages are stored in a kmalloc'd array.
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Another kmalloc'd array is used to store pointers to each page
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and tail in the order they will be accessed.
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Memory for the DMA descriptors is allocated with multiple
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pci_pool_alloc() calls. Pointers to these descriptors are stored
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in a kmalloc'd array.
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Example:
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Suppose we want one thousand 9588-byte buffers for
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one DVB ASI interface. Each buffer is made up of two 4096-byte pages
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plus one 1396-byte tail. Since up to two tails can fit
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in one page, the buffers will fit in 2500 pages.
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Tails are never split between multiple pages.
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Assuming 32-bit pointers, the 2500 pointers to these pages
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will fit in a 16384-byte array. Another 16384-byte array
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will hold the 3000 pointers to the pages and tails in
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the order they will be accessed.
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The total memory required for this example is
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1000 * 9588 + 2500 * 4 + 3000 * 4 = 9610000 bytes.
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The memory actually allocated is
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2500 * 4096 + 16384 + 16384 = 10272768 bytes.
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Example:
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Suppose we want ten thousand 1316-byte buffers
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for one DVB ASI interface. Since three buffers will fit in one page,
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all of the buffers will fit in 3334 pages. The 3334 pointers
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to these pages will fit in a 16384-byte array,
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and the 10000 pointers to the buffers will fit
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in a 65536-byte array.
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This example requires
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10000 * 1316 + 3334 * 4 + 10000 * 4 = 13213336 bytes
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and allocates
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3334 * 4096 + 16384 + 65536 = 13737984 bytes.
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