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[ALSA] document basic TLV stuff 

Add documentation about how to define dB scale information for mixer
controls.

Signed-off-by: Clemens Ladisch <clemens@ladisch.de>
Signed-off-by: Jaroslav Kysela <perex@suse.cz>
This commit is contained in:
Clemens Ladisch 2007-09-10 08:05:19 +02:00 committed by Jaroslav Kysela
parent 950fb626dc
commit d1761d1b14
1 changed files with 59 additions and 2 deletions
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61
Documentation/sound/alsa/DocBook/writing-an-alsa-driver.tmpl
View File

@ -18,8 +18,8 @@
</affiliation>
</author>
<date>July 26, 2007</date>
<edition>0.3.6.1</edition>
<date>September 10, 2007</date>
<edition>0.3.7</edition>
<abstract>
<para>
@ -3472,6 +3472,13 @@ struct _snd_pcm_runtime {
(casted to unsigned long) of some record to this field, too.
</para>
<para>
The <structfield>tlv</structfield> field can be used to provide
metadata about the control; see the
<link linkend="control-interface-tlv">
<citetitle>Metadata</citetitle></link> subsection.
</para>
<para>
The other three are
<link linkend="control-interface-callbacks"><citetitle>
@ -3871,6 +3878,56 @@ struct _snd_pcm_runtime {
</para>
</section>
<section id="control-interface-tlv">
<title>Metadata</title>
<para>
To provide information about the dB values of a mixer control, use
on of the <constant>DECLARE_TLV_xxx</constant> macros from
<filename>&lt;sound/tlv.h&gt;</filename> to define a variable
containing this information, set the<structfield>tlv.p
</structfield> field to point to this variable, and include the
<constant>SNDRV_CTL_ELEM_ACCESS_TLV_READ</constant> flag in the
<structfield>access</structfield> field; like this:
<informalexample>
<programlisting>
<![CDATA[
static DECLARE_TLV_DB_SCALE(db_scale_my_control, -4050, 150, 0);
static struct snd_kcontrol_new my_control __devinitdata = {
...
.access = SNDRV_CTL_ELEM_ACCESS_READWRITE |
SNDRV_CTL_ELEM_ACCESS_TLV_READ,
...
.tlv.p = db_scale_my_control,
};
]]>
</programlisting>
</informalexample>
</para>
<para>
The <function>DECLARE_TLV_DB_SCALE</function> macro defines
information about a mixer control where each step in the control's
value changes the dB value by a constant dB amount.
The first parameter is the name of the variable to be defined.
The second parameter is the minimum value, in units of 0.01 dB.
The third parameter is the step size, in units of 0.01 dB.
Set the fourth parameter to 1 if the minimum value actually mutes
the control.
</para>
<para>
The <function>DECLARE_TLV_DB_LINEAR</function> macro defines
information about a mixer control where the control's value affects
the output linearly.
The first parameter is the name of the variable to be defined.
The second parameter is the minimum value, in units of 0.01 dB.
The third parameter is the maximum value, in units of 0.01 dB.
If the minimum value mutes the control, set the second parameter to
<constant>TLV_DB_GAIN_MUTE</constant>.
</para>
</section>
</chapter>