Re: TLV question - ranges not in volume order

[David Henningsson <david.henningsson@canonical.com>]

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On 05/08/2013 02:14 PM, Takashi Iwai wrote:
> At Wed, 08 May 2013 14:09:31 +0200,
> Clemens Ladisch wrote:
>>
>> David Henningsson wrote:
>>> I recently came across a strange ASoC volume control. In this case,
>>> the values are not in volume order, i e, the control is declared
>>> something like below.
>>>
>>> However alsamixer seems not to handle this "reordering" correctly. My
>>> question is if this is something that should be fixed in alsamixer (or
>>> possibly alsa-lib?), or does one need to write some kind of mapping
>>> table in the kernel, just to make something come in volume order to
>>> userspace?
>>
>> Items in a DB_RANGE container must be ordered by their values *and* by
>> their dB values.  This implies that larger values must correspond with
>> larger dB values, whis is also required for all other mixer controls.
>>
>> If the register values are unordered, the driver must reorder them.
>
> Yep.  Although there is no explicit definition, it's an implicit rule,
> as we have never implemented the parser code to allow the disorder.
>
> We should add a text mentioning that somewhere.  A patch is welcome ;)

Like this? (Feel free to apply if you think it's good)



-- 
David Henningsson, Canonical Ltd.
https://launchpad.net/~diwic

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>From 19778a06ef5bd995ba1e800dc14bbe7ff27dcb97 Mon Sep 17 00:00:00 2001
From: David Henningsson <david.henningsson@canonical.com>
Date: Wed, 8 May 2013 15:28:49 +0200
Subject: [PATCH] ALSA: Add comment for control TLV API

Userspace is not meant to have to handle all strange dB ranges,
so add a specification comment.

Signed-off-by: David Henningsson <david.henningsson@canonical.com>
---
 include/sound/tlv.h |    5 ++++-
 1 file changed, 4 insertions(+), 1 deletion(-)

diff --git a/include/sound/tlv.h b/include/sound/tlv.h
index 28c65e1..84fc31a 100644
--- a/include/sound/tlv.h
+++ b/include/sound/tlv.h
@@ -74,7 +74,10 @@
 #define DECLARE_TLV_DB_LINEAR(name, min_dB, max_dB)	\
 	unsigned int name[] = { TLV_DB_LINEAR_ITEM(min_dB, max_dB) }
 
-/* dB range container */
+/* dB range container. Items in dB range container must be ordered
+by their values and by their dB values. This implies that larger values
+must correspond with larger dB values (which is also required for all
+other mixer controls). */
 /* Each item is: <min> <max> <TLV> */
 #define TLV_DB_RANGE_ITEM(...) \
 	TLV_ITEM(SNDRV_CTL_TLVT_DB_RANGE, __VA_ARGS__)
-- 
1.7.9.5


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