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ALSA: Update the documentation for changes of proc files

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Signed-off-by: Takashi Iwai <tiwai@suse.de>
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Takashi Iwai 2010-04-13 12:55:09 +02:00
parent 02f4865fa4
commit c56a3b1847
1 changed files with 17 additions and 10 deletions
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27
Documentation/DocBook/writing-an-alsa-driver.tmpl
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@ -5518,34 +5518,41 @@ struct _snd_pcm_runtime {
]]>
</programlisting>
</informalexample>
For the raw data, <structfield>size</structfield> field must be
set properly. This specifies the maximum size of the proc file access.
</para>
<para>
The callback is much more complicated than the text-file
version. You need to use a low-level I/O functions such as
The read/write callbacks of raw mode are more direct than the text mode.
You need to use a low-level I/O functions such as
<function>copy_from/to_user()</function> to transfer the
data.
<informalexample>
<programlisting>
<![CDATA[
static long my_file_io_read(struct snd_info_entry *entry,
static ssize_t my_file_io_read(struct snd_info_entry *entry,
void *file_private_data,
struct file *file,
char *buf,
unsigned long count,
unsigned long pos)
size_t count,
loff_t pos)
{
long size = count;
if (pos + size > local_max_size)
size = local_max_size - pos;
if (copy_to_user(buf, local_data + pos, size))
if (copy_to_user(buf, local_data + pos, count))
return -EFAULT;
return size;
return count;
}
]]>
</programlisting>
</informalexample>
If the size of the info entry has been set up properly,
<structfield>count</structfield> and <structfield>pos</structfield> are
guaranteed to fit within 0 and the given size.
You don't have to check the range in the callbacks unless any
other condition is required.
</para>
</chapter>