130 строки
6.3 KiB
ReStructuredText
130 строки
6.3 KiB
ReStructuredText
===============================
|
|
Realtek PC Beep Hidden Register
|
|
===============================
|
|
|
|
This file documents the "PC Beep Hidden Register", which is present in certain
|
|
Realtek HDA codecs and controls a muxer and pair of passthrough mixers that can
|
|
route audio between pins but aren't themselves exposed as HDA widgets. As far
|
|
as I can tell, these hidden routes are designed to allow flexible PC Beep output
|
|
for codecs that don't have mixer widgets in their output paths. Why it's easier
|
|
to hide a mixer behind an undocumented vendor register than to just expose it
|
|
as a widget, I have no idea.
|
|
|
|
Register Description
|
|
====================
|
|
|
|
The register is accessed via processing coefficient 0x36 on NID 20h. Bits not
|
|
identified below have no discernible effect on my machine, a Dell XPS 13 9350::
|
|
|
|
MSB LSB
|
|
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|
|
| |h|S|L| | B |R| | Known bits
|
|
+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+
|
|
|0|0|1|1| 0x7 |0|0x0|1| 0x7 | Reset value
|
|
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|
|
|
|
1Ah input select (B): 2 bits
|
|
When zero, expose the PC Beep line (from the internal beep generator, when
|
|
enabled with the Set Beep Generation verb on NID 01h, or else from the
|
|
external PCBEEP pin) on the 1Ah pin node. When nonzero, expose the headphone
|
|
jack (or possibly Line In on some machines) input instead. If PC Beep is
|
|
selected, the 1Ah boost control has no effect.
|
|
|
|
Amplify 1Ah loopback, left (L): 1 bit
|
|
Amplify the left channel of 1Ah before mixing it into outputs as specified
|
|
by h and S bits. Does not affect the level of 1Ah exposed to other widgets.
|
|
|
|
Amplify 1Ah loopback, right (R): 1 bit
|
|
Amplify the right channel of 1Ah before mixing it into outputs as specified
|
|
by h and S bits. Does not affect the level of 1Ah exposed to other widgets.
|
|
|
|
Loopback 1Ah to 21h [active low] (h): 1 bit
|
|
When zero, mix 1Ah (possibly with amplification, depending on L and R bits)
|
|
into 21h (headphone jack on my machine). Mixed signal respects the mute
|
|
setting on 21h.
|
|
|
|
Loopback 1Ah to 14h (S): 1 bit
|
|
When one, mix 1Ah (possibly with amplification, depending on L and R bits)
|
|
into 14h (internal speaker on my machine). Mixed signal **ignores** the mute
|
|
setting on 14h and is present whenever 14h is configured as an output.
|
|
|
|
Path diagrams
|
|
=============
|
|
|
|
1Ah input selection (DIV is the PC Beep divider set on NID 01h)::
|
|
|
|
<Beep generator> <PCBEEP pin> <Headphone jack>
|
|
| | |
|
|
+--DIV--+--!DIV--+ {1Ah boost control}
|
|
| |
|
|
+--(b == 0)--+--(b != 0)--+
|
|
|
|
|
>1Ah (Beep/Headphone Mic/Line In)<
|
|
|
|
Loopback of 1Ah to 21h/14h::
|
|
|
|
<1Ah (Beep/Headphone Mic/Line In)>
|
|
|
|
|
{amplify if L/R}
|
|
|
|
|
+-----!h-----+-----S-----+
|
|
| |
|
|
{21h mute control} |
|
|
| |
|
|
>21h (Headphone)< >14h (Internal Speaker)<
|
|
|
|
Background
|
|
==========
|
|
|
|
All Realtek HDA codecs have a vendor-defined widget with node ID 20h which
|
|
provides access to a bank of registers that control various codec functions.
|
|
Registers are read and written via the standard HDA processing coefficient
|
|
verbs (Set/Get Coefficient Index, Set/Get Processing Coefficient). The node is
|
|
named "Realtek Vendor Registers" in public datasheets' verb listings and,
|
|
apart from that, is entirely undocumented.
|
|
|
|
This particular register, exposed at coefficient 0x36 and named in commits from
|
|
Realtek, is of note: unlike most registers, which seem to control detailed
|
|
amplifier parameters not in scope of the HDA specification, it controls audio
|
|
routing which could just as easily have been defined using standard HDA mixer
|
|
and selector widgets.
|
|
|
|
Specifically, it selects between two sources for the input pin widget with Node
|
|
ID (NID) 1Ah: the widget's signal can come either from an audio jack (on my
|
|
laptop, a Dell XPS 13 9350, it's the headphone jack, but comments in Realtek
|
|
commits indicate that it might be a Line In on some machines) or from the PC
|
|
Beep line (which is itself multiplexed between the codec's internal beep
|
|
generator and external PCBEEP pin, depending on if the beep generator is
|
|
enabled via verbs on NID 01h). Additionally, it can mix (with optional
|
|
amplification) that signal onto the 21h and/or 14h output pins.
|
|
|
|
The register's reset value is 0x3717, corresponding to PC Beep on 1Ah that is
|
|
then amplified and mixed into both the headphones and the speakers. Not only
|
|
does this violate the HDA specification, which says that "[a vendor defined
|
|
beep input pin] connection may be maintained *only* while the Link reset
|
|
(**RST#**) is asserted", it means that we cannot ignore the register if we care
|
|
about the input that 1Ah would otherwise expose or if the PCBEEP trace is
|
|
poorly shielded and picks up chassis noise (both of which are the case on my
|
|
machine).
|
|
|
|
Unfortunately, there are lots of ways to get this register configuration wrong.
|
|
Linux, it seems, has gone through most of them. For one, the register resets
|
|
after S3 suspend: judging by existing code, this isn't the case for all vendor
|
|
registers, and it's led to some fixes that improve behavior on cold boot but
|
|
don't last after suspend. Other fixes have successfully switched the 1Ah input
|
|
away from PC Beep but have failed to disable both loopback paths. On my
|
|
machine, this means that the headphone input is amplified and looped back to
|
|
the headphone output, which uses the exact same pins! As you might expect, this
|
|
causes terrible headphone noise, the character of which is controlled by the
|
|
1Ah boost control. (If you've seen instructions online to fix XPS 13 headphone
|
|
noise by changing "Headphone Mic Boost" in ALSA, now you know why.)
|
|
|
|
The information here has been obtained through black-box reverse engineering of
|
|
the ALC256 codec's behavior and is not guaranteed to be correct. It likely
|
|
also applies for the ALC255, ALC257, ALC235, and ALC236, since those codecs
|
|
seem to be close relatives of the ALC256. (They all share one initialization
|
|
function.) Additionally, other codecs like the ALC225 and ALC285 also have this
|
|
register, judging by existing fixups in ``patch_realtek.c``, but specific
|
|
data (e.g. node IDs, bit positions, pin mappings) for those codecs may differ
|
|
from what I've described here.
|