file_systems.html

ADI 公司blackfin系列的用户使用文挡。

</p>

<p>
 The key code components are: 
</p>
<ul>
<li class="level1"><div class="li"> register a file system</div>
</li>
<li class="level1"><div class="li"> define how to read a superblock</div>
</li>
</ul>

<p>
 Once you read a superblock you then define the file operations allowed by that file systemo The basic building block of a file system is the inode. An inode defines an entity in the file system. Typically such an entity can be  
</p>
<ul>
<li class="level1"><div class="li"> a regular file</div>
</li>
<li class="level1"><div class="li"> a directory</div>
</li>
<li class="level1"><div class="li"> a link to another entry</div>
</li>
</ul>

<p>
 For each different type of inode the file system registers different methods for handling reads and writes to that particular inode. There are inode operations and file operations.
</p>

<p>
Directory inode operations include 
</p>
<ul>
<li class="level1"><div class="li"> create  - create a file in the directory</div>
</li>
<li class="level1"><div class="li"> mkdir - make a sub directory </div>
</li>
<li class="level1"><div class="li"> mknod - make a special device file</div>
</li>
<li class="level1"><div class="li"> symlink - make a link to another file</div>
</li>
<li class="level1"><div class="li"> rename - change the name of an entry</div>
</li>
<li class="level1"><div class="li"> unlink - delete a file ( when no longer used )</div>
</li>
</ul>

<p>
Refer to the code for more details
</p>

</div>
<!-- SECTION [3417-4573] -->
<h2><a name="file_system_exampledebugfs" id="file_system_exampledebugfs">File System Example: Debugfs</a></h2>
<div class="level2">

<p>
Created by Greg Kroah-Hartman debugfs is an in-kernel filesystem which gas been designed to help kernel developers easily access debug data from userspace.
</p>

<p>
It has been designed to be easy to use and also it is optional. If you dont mount the debugfs the filesystem , and its kernel memory overhead, is not there.
</p>

<p>
 Debugfs is available as a patch which also contains a few use examples.
</p>

<p>
NOTE that all the code below is in <strong>kernel space</strong> 
</p>

<p>
 You can create a debug directory as follows:
</p>
<pre class="code c">&nbsp;
<span class="kw4">struct</span> dentry *debugfs_create_dir<span class="br0">&#40;</span><span class="kw4">const</span> <span class="kw4">char</span> *name, <span class="kw4">struct</span> dentry *parent<span class="br0">&#41;</span>;
&nbsp;
<span class="kw4">struct</span> dentry *mydebdir;
mydebdir = debugfs_create_dir<span class="br0">&#40;</span><span class="st0">"mydebug"</span>, <span class="kw2">NULL</span><span class="br0">&#41;</span>;
&nbsp;</pre>
<p>
Then, for example, to export a single value <strong>mydebugch</strong> to userspace:
</p>
<pre class="code c"><span class="kw4">struct</span> dentry *debugfs_create_u8<span class="br0">&#40;</span><span class="kw4">const</span> <span class="kw4">char</span> *name,
                                 mode_t mode,
                                 <span class="kw4">struct</span> dentry *parent, u8*value<span class="br0">&#41;</span>;
&nbsp;
&nbsp;
&nbsp;
<span class="co1">//Example</span>
&nbsp;
u8 mydebugch; <span class="br0">&#40;</span>also u16 , u32 <span class="br0">&#41;</span>
debugfs_create_u8<span class="br0">&#40;</span><span class="st0">"mydebugch"</span>, S_IWUGO, mydebdir, &amp;mydebugchar<span class="br0">&#41;</span>;</pre>
<p>
 Two lines of code provide the following entry in the file system
</p>
<pre class="code">


    /debug/mydebug/mydebugchar


</pre>

<p>
 For your own parser you will need to define a file operations table with suitable  open , seek, read and write functions.
</p>
<pre class="code c">&nbsp;
<span class="kw4">static</span> <span class="kw4">int</span> my_open<span class="br0">&#40;</span><span class="kw4">struct</span> inode *inode, <span class="kw4">struct</span> file *file<span class="br0">&#41;</span>
<span class="br0">&#123;</span>
        <span class="kw1">return</span> <span class="nu0">0</span>;
<span class="br0">&#125;</span>
&nbsp;
<span class="kw4">static</span> loff_t my_lseek<span class="br0">&#40;</span><span class="kw4">struct</span> file *file, loff_t offset, <span class="kw4">int</span> orig<span class="br0">&#41;</span>
<span class="br0">&#123;</span>
        loff_t retval = -EINVAL;
&nbsp;
        down<span class="br0">&#40;</span>&amp;file-&gt;f_dentry-&gt;d_inode-&gt;i_sem<span class="br0">&#41;</span>;
        <span class="kw1">switch</span><span class="br0">&#40;</span>orig<span class="br0">&#41;</span> <span class="br0">&#123;</span>
        <span class="kw1">case</span> <span class="nu0">0</span>:
                <span class="kw1">if</span> <span class="br0">&#40;</span>offset &gt; <span class="nu0">0</span><span class="br0">&#41;</span> <span class="br0">&#123;</span>
                        file-&gt;f_pos = offset;
                        retval = file-&gt;f_pos;
                <span class="br0">&#125;</span>
                <span class="kw2">break</span>;
        <span class="kw1">case</span> <span class="nu0">1</span>:
                <span class="kw1">if</span> <span class="br0">&#40;</span><span class="br0">&#40;</span>offset + file-&gt;f_pos<span class="br0">&#41;</span> &gt; <span class="nu0">0</span><span class="br0">&#41;</span> <span class="br0">&#123;</span>
                        file-&gt;f_pos += offset;
                        retval = file-&gt;f_pos;
                <span class="br0">&#125;</span>
                <span class="kw2">break</span>;
        <span class="kw1">default</span>:
                <span class="kw2">break</span>;
        <span class="br0">&#125;</span>
        up<span class="br0">&#40;</span>&amp;file-&gt;f_dentry-&gt;d_inode-&gt;i_sem<span class="br0">&#41;</span>;
        <span class="kw1">return</span> retval;
<span class="br0">&#125;</span>
&nbsp;
<span class="kw4">static</span> ssize_t my_read<span class="br0">&#40;</span><span class="kw4">struct</span> file *file, <span class="kw4">char</span> __user *user_buf,
                                  size_t count, loff_t *ppos<span class="br0">&#41;</span>
<span class="br0">&#123;</span>
        <span class="kw4">char</span> buf<span class="br0">&#91;</span><span class="nu0">32</span><span class="br0">&#93;</span>;
        u32 *val = file-&gt;private_data;
        <span class="kw4">int</span> len;
        len = sprintf<span class="br0">&#40;</span>buf,<span class="st0">"my val is %d <span class="es0">\n</span>"</span>,val<span class="br0">&#41;</span>;
&nbsp;
        <span class="kw1">return</span> simple_read_from_buffer<span class="br0">&#40;</span>user_buf, count, ppos, buf, len<span class="br0">&#41;</span>;
<span class="br0">&#125;</span>
&nbsp;
<span class="kw4">static</span> ssize_t my_write<span class="br0">&#40;</span><span class="kw4">struct</span> file *file, <span class="kw4">const</span> <span class="kw4">char</span> __user *user_buf,
                              size_t count, loff_t *ppos<span class="br0">&#41;</span>
<span class="br0">&#123;</span>
        <span class="kw4">char</span> buf<span class="br0">&#91;</span><span class="nu0">32</span><span class="br0">&#93;</span>;
        <span class="kw4">int</span> buf_size;
        <span class="kw4">int</span> scanc;
        u32 *val = file-&gt;private_data;
        buf_size = min<span class="br0">&#40;</span>count, <span class="br0">&#40;</span><span class="kw4">sizeof</span><span class="br0">&#40;</span>buf<span class="br0">&#41;</span><span class="nu0">-1</span><span class="br0">&#41;</span><span class="br0">&#41;</span>;
        <span class="kw1">if</span> <span class="br0">&#40;</span>copy_from_user<span class="br0">&#40;</span>buf, user_buf, buf_size<span class="br0">&#41;</span><span class="br0">&#41;</span>
                <span class="kw1">return</span> -EFAULT;
&nbsp;
        sscanf<span class="br0">&#40;</span>buf,<span class="st0">"Set val to %d<span class="es0">\n</span>"</span>,val<span class="br0">&#41;</span>;
&nbsp;
        <span class="kw1">return</span> count;
<span class="br0">&#125;</span></pre>
<p>
Create a file using debugfs the call is
</p>
<pre class="code c"><span class="kw4">static</span> <span class="kw4">struct</span> file_operations my_fops = <span class="br0">&#123;</span>
        .<span class="me1">read</span> =         my_read,
        .<span class="me1">write</span> =        my_write,
        .<span class="me1">open</span> =         my_open,
        .<span class="me1">llseek</span> =       my_lseek,
<span class="br0">&#125;</span>;
&nbsp;</pre>
<p>
Then use the <strong>create_file</strong> system function to make the entry in debugfs:
</p>
<pre class="code c"><span class="kw4">struct</span> dentry *debugfs_create_file<span class="br0">&#40;</span><span class="kw4">const</span> <span class="kw4">char</span> *name,
                                   mode_t mode,
                                   <span class="kw4">struct</span> dentry *parent, <span class="kw4">void</span> *data,
                                   <span class="kw4">struct</span> file_operations *fops<span class="br0">&#41;</span>;
&nbsp;
<span class="kw4">static</span> <span class="kw4">int</span> mysval;
&nbsp;
debugfs_create_file<span class="br0">&#40;</span><span class="st0">"mystruct"</span>,
                        <span class="nu0">0444</span>,
                        mydebdir,
                        &amp;mysval,
                        my_fops<span class="br0">&#41;</span>;
&nbsp;
&nbsp;</pre>
</div>
<!-- SECTION [4574-] --></body>
</html>