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| <h1 id="newtron-flash-filesystem-nffs">Newtron Flash Filesystem (nffs)</h1> |
| <p>Mynewt includes the Newtron Flash File System (nffs). This file system is designed with two priorities that makes it suitable for embedded use: </p> |
| <ul> |
| <li>Minimal RAM usage</li> |
| <li>Reliability</li> |
| </ul> |
| <p>Mynewt also provides an abstraction layer API (fs) to allow you to swap out nffs with a different file system of your choice.</p> |
| <h3 id="description">Description</h3> |
| <h4 id="areas">Areas</h4> |
| <p>At the top level, an nffs disk is partitioned into <em>areas</em>. An area is a region of disk with the following properties:</p> |
| <ol> |
| <li>An area can be fully erased without affecting any other areas.</li> |
| <li>Writing to one area does not restrict writes to other areas.</li> |
| </ol> |
| <p><strong>Regarding property 1:</strong> Generally, flash hardware divides its memory space into "blocks." When erasing flash, entire blocks must be erased in a single operation; partial erases are not possible.</p> |
| <p><strong>Regarding property 2:</strong> Furthermore, some flash hardware imposes a restriction with regards to writes: writes within a block must be strictly sequential. For example, if you wish to write to the first 16 bytes of a block, you must write bytes 1 through 15 before writing byte 16. This restriction only applies at the block level; writes to one block have no effect on what parts of other blocks can be written.</p> |
| <p>Thus, each area must comprise a discrete number of blocks.</p> |
| <h4 id="initialization">Initialization</h4> |
| <p>Before nffs can be used, it must be initialized. There are two means of initializing an nffs file system:</p> |
| <ol> |
| <li>Restore an existing file system via detection.</li> |
| <li>Create a new file system via formatting.</li> |
| </ol> |
| <p>A typical initialization sequence is the following:</p> |
| <ol> |
| <li>Detect an nffs file system in a specific region of flash.</li> |
| <li>If no file system was detected, format a new file system in the same flash region.</li> |
| </ol> |
| <p>Both methods require the user to describe how the flash memory should be divided into nffs areas. This is accomplished with an array of <a href="../nffs_area_desc/">struct nffs_area_desc</a>.</p> |
| <p>Typically, a product's flash layout is exposed via its BSP-specific <code>bsp_flash_dev()</code> function. This function retrieves the layout of the specified flash device resident in the BSP. The result of this function can then be converted into the <code>struct nffs_area_desc[]</code> that nffs requires. The below example, taken from the slinky project, illustrates the nffs initialization procedure.</p> |
| <div class="codehilite" style="background: #ffffff"><pre style="line-height: 125%;"><span></span><code><span style="color: #177500">/*** hw/hal/include/hal/flash_map.h */</span> |
| |
| <span style="color: #177500">/*</span> |
| <span style="color: #177500"> * Flash area types</span> |
| <span style="color: #177500"> */</span> |
| <span style="color: #633820">#define FLASH_AREA_BOOTLOADER 0</span> |
| <span style="color: #633820">#define FLASH_AREA_IMAGE_0 1</span> |
| <span style="color: #633820">#define FLASH_AREA_IMAGE_1 2</span> |
| <span style="color: #633820">#define FLASH_AREA_IMAGE_SCRATCH 3</span> |
| <span style="color: #633820">#define FLASH_AREA_NFFS 4</span> |
| </code></pre></div> |
| |
| <div class="codehilite" style="background: #ffffff"><pre style="line-height: 125%;"><span></span><code><span style="color: #177500">/*** project/slinky/src/main.c */</span> |
| |
| <span style="color: #A90D91">int</span> |
| <span style="color: #000000">main</span>(<span style="color: #A90D91">int</span> <span style="color: #000000">argc</span>, <span style="color: #A90D91">char</span> <span style="color: #000000">**argv</span>) |
| { |
| <span style="color: #A90D91">int</span> <span style="color: #000000">rc</span>; |
| <span style="color: #A90D91">int</span> <span style="color: #000000">cnt</span>; |
| |
| <span style="color: #177500">/* NFFS_AREA_MAX is defined in the BSP-specified bsp.h header file. */</span> |
| <span style="color: #A90D91">struct</span> <span style="color: #3F6E75">nffs_area_desc</span> <span style="color: #000000">descs</span>[<span style="color: #000000">NFFS_AREA_MAX</span>]; |
| |
| <span style="color: #177500">/* Initialize nffs's internal state. */</span> |
| <span style="color: #000000">rc</span> <span style="color: #000000">=</span> <span style="color: #000000">nffs_init</span>(); |
| <span style="color: #000000">assert</span>(<span style="color: #000000">rc</span> <span style="color: #000000">==</span> <span style="color: #1C01CE">0</span>); |
| |
| <span style="color: #177500">/* Convert the set of flash blocks we intend to use for nffs into an array</span> |
| <span style="color: #177500"> * of nffs area descriptors.</span> |
| <span style="color: #177500"> */</span> |
| <span style="color: #000000">cnt</span> <span style="color: #000000">=</span> <span style="color: #000000">NFFS_AREA_MAX</span>; |
| <span style="color: #000000">rc</span> <span style="color: #000000">=</span> <span style="color: #000000">flash_area_to_nffs_desc</span>(<span style="color: #000000">FLASH_AREA_NFFS</span>, <span style="color: #000000">&cnt</span>, <span style="color: #000000">descs</span>); |
| <span style="color: #000000">assert</span>(<span style="color: #000000">rc</span> <span style="color: #000000">==</span> <span style="color: #1C01CE">0</span>); |
| |
| <span style="color: #177500">/* Attempt to restore an existing nffs file system from flash. */</span> |
| <span style="color: #A90D91">if</span> (<span style="color: #000000">nffs_detect</span>(<span style="color: #000000">descs</span>) <span style="color: #000000">==</span> <span style="color: #000000">FS_ECORRUPT</span>) { |
| <span style="color: #177500">/* No valid nffs instance detected; format a new one. */</span> |
| <span style="color: #000000">rc</span> <span style="color: #000000">=</span> <span style="color: #000000">nffs_format</span>(<span style="color: #000000">descs</span>); |
| <span style="color: #000000">assert</span>(<span style="color: #000000">rc</span> <span style="color: #000000">==</span> <span style="color: #1C01CE">0</span>); |
| } |
| <span style="color: #177500">/* [ ... ] */</span> |
| } |
| </code></pre></div> |
| |
| <p>After nffs has been initialized, the application can access the file system via the <a href="../../fs/fs/">file system abstraction layer</a>.</p> |
| <h3 id="configuration">Configuration</h3> |
| <p>The nffs file system is configured by populating fields in a global <a href="../nffs_config/">struct nffs_config</a> instance. Each field in the structure corresponds to a setting. All configuration must be done prior to calling nffs_init().</p> |
| <p>The global <code>struct nffs_config</code> instance is exposed in <code>nffs/nffs.h</code> as follows:</p> |
| <div class="codehilite" style="background: #ffffff"><pre style="line-height: 125%;"><span></span><code><span style="color: #A90D91">extern</span> <span style="color: #A90D91">struct</span> <span style="color: #3F6E75">nffs_config</span> <span style="color: #000000">nffs_config</span>; |
| </code></pre></div> |
| |
| <h3 id="data-structures">Data Structures</h3> |
| <p>The <code>fs/nffs</code> package exposes the following data structures:</p> |
| <table> |
| <thead> |
| <tr> |
| <th>Struct</th> |
| <th>Description</th> |
| </tr> |
| </thead> |
| <tbody> |
| <tr> |
| <td><a href="../nffs_area_desc/">struct nffs_area_desc</a></td> |
| <td>Descriptor for a single nffs area.</td> |
| </tr> |
| <tr> |
| <td><a href="../nffs_config/">struct nffs_config</a></td> |
| <td>Configuration struct for nffs.</td> |
| </tr> |
| </tbody> |
| </table> |
| <h3 id="api">API</h3> |
| <p>The functions available in this OS feature are:</p> |
| <table> |
| <thead> |
| <tr> |
| <th>Function</th> |
| <th>Description</th> |
| </tr> |
| </thead> |
| <tbody> |
| <tr> |
| <td><a href="../nffs_detect/">nffs_detect</a></td> |
| <td>Searches for a valid nffs file system among the specified areas.</td> |
| </tr> |
| <tr> |
| <td><a href="../nffs_format/">nffs_format</a></td> |
| <td>Erases all the specified areas and initializes them with a clean nffs file system.</td> |
| </tr> |
| <tr> |
| <td><a href="../nffs_init/">nffs_init</a></td> |
| <td>Initializes internal nffs memory and data structures.</td> |
| </tr> |
| </tbody> |
| </table> |
| <h3 id="miscellaneous-measures">Miscellaneous measures</h3> |
| <ul> |
| <li> |
| <p>RAM usage:</p> |
| <ul> |
| <li>24 bytes per inode</li> |
| <li>12 bytes per data block</li> |
| <li>36 bytes per inode cache entry</li> |
| <li>32 bytes per data block cache entry</li> |
| </ul> |
| </li> |
| <li> |
| <p>Maximum filename size: 256 characters (no null terminator required)</p> |
| </li> |
| <li>Disallowed filename characters: '/' and '\0'</li> |
| </ul> |
| <h3 id="internals">Internals</h3> |
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| <h3 id="future-enhancements">Future enhancements</h3> |
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| <li>Encryption.</li> |
| <li>Compression.</li> |
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