1 ===============================
2 Realtek PC Beep Hidden Register
3 ===============================
4
5 This file documents the "PC Beep Hidden Register", which is present in certain
6 Realtek HDA codecs and controls a muxer and pair of passthrough mixers that can
7 route audio between pins but aren't themselves exposed as HDA widgets. As far
8 as I can tell, these hidden routes are designed to allow flexible PC Beep output
9 for codecs that don't have mixer widgets in their output paths. Why it's easier
10 to hide a mixer behind an undocumented vendor register than to just expose it
11 as a widget, I have no idea.
12
13 Register Description
14 ====================
15
16 The register is accessed via processing coefficient 0x36 on NID 20h. Bits not
17 identified below have no discernible effect on my machine, a Dell XPS 13 9350::
18
19 MSB LSB
20 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
21 | |h|S|L| | B |R| | Known bits
22 +=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+
23 |0|0|1|1| 0x7 |0|0x0|1| 0x7 | Reset value
24 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
25
26 1Ah input select (B): 2 bits
27 When zero, expose the PC Beep line (from the internal beep generator, when
28 enabled with the Set Beep Generation verb on NID 01h, or else from the
29 external PCBEEP pin) on the 1Ah pin node. When nonzero, expose the headphone
30 jack (or possibly Line In on some machines) input instead. If PC Beep is
31 selected, the 1Ah boost control has no effect.
32
33 Amplify 1Ah loopback, left (L): 1 bit
34 Amplify the left channel of 1Ah before mixing it into outputs as specified
35 by h and S bits. Does not affect the level of 1Ah exposed to other widgets.
36
37 Amplify 1Ah loopback, right (R): 1 bit
38 Amplify the right channel of 1Ah before mixing it into outputs as specified
39 by h and S bits. Does not affect the level of 1Ah exposed to other widgets.
40
41 Loopback 1Ah to 21h [active low] (h): 1 bit
42 When zero, mix 1Ah (possibly with amplification, depending on L and R bits)
43 into 21h (headphone jack on my machine). Mixed signal respects the mute
44 setting on 21h.
45
46 Loopback 1Ah to 14h (S): 1 bit
47 When one, mix 1Ah (possibly with amplification, depending on L and R bits)
48 into 14h (internal speaker on my machine). Mixed signal **ignores** the mute
49 setting on 14h and is present whenever 14h is configured as an output.
50
51 Path diagrams
52 =============
53
54 1Ah input selection (DIV is the PC Beep divider set on NID 01h)::
55
56 <Beep generator> <PCBEEP pin> <Headphone jack>
57 | | |
58 +--DIV--+--!DIV--+ {1Ah boost control}
59 | |
60 +--(b == 0)--+--(b != 0)--+
61 |
62 >1Ah (Beep/Headphone Mic/Line In)<
63
64 Loopback of 1Ah to 21h/14h::
65
66 <1Ah (Beep/Headphone Mic/Line In)>
67 |
68 {amplify if L/R}
69 |
70 +-----!h-----+-----S-----+
71 | |
72 {21h mute control} |
73 | |
74 >21h (Headphone)< >14h (Internal Speaker)<
75
76 Background
77 ==========
78
79 All Realtek HDA codecs have a vendor-defined widget with node ID 20h which
80 provides access to a bank of registers that control various codec functions.
81 Registers are read and written via the standard HDA processing coefficient
82 verbs (Set/Get Coefficient Index, Set/Get Processing Coefficient). The node is
83 named "Realtek Vendor Registers" in public datasheets' verb listings and,
84 apart from that, is entirely undocumented.
85
86 This particular register, exposed at coefficient 0x36 and named in commits from
87 Realtek, is of note: unlike most registers, which seem to control detailed
88 amplifier parameters not in scope of the HDA specification, it controls audio
89 routing which could just as easily have been defined using standard HDA mixer
90 and selector widgets.
91
92 Specifically, it selects between two sources for the input pin widget with Node
93 ID (NID) 1Ah: the widget's signal can come either from an audio jack (on my
94 laptop, a Dell XPS 13 9350, it's the headphone jack, but comments in Realtek
95 commits indicate that it might be a Line In on some machines) or from the PC
96 Beep line (which is itself multiplexed between the codec's internal beep
97 generator and external PCBEEP pin, depending on if the beep generator is
98 enabled via verbs on NID 01h). Additionally, it can mix (with optional
99 amplification) that signal onto the 21h and/or 14h output pins.
100
101 The register's reset value is 0x3717, corresponding to PC Beep on 1Ah that is
102 then amplified and mixed into both the headphones and the speakers. Not only
103 does this violate the HDA specification, which says that "[a vendor defined
104 beep input pin] connection may be maintained *only* while the Link reset
105 (**RST#**) is asserted", it means that we cannot ignore the register if we care
106 about the input that 1Ah would otherwise expose or if the PCBEEP trace is
107 poorly shielded and picks up chassis noise (both of which are the case on my
108 machine).
109
110 Unfortunately, there are lots of ways to get this register configuration wrong.
111 Linux, it seems, has gone through most of them. For one, the register resets
112 after S3 suspend: judging by existing code, this isn't the case for all vendor
113 registers, and it's led to some fixes that improve behavior on cold boot but
114 don't last after suspend. Other fixes have successfully switched the 1Ah input
115 away from PC Beep but have failed to disable both loopback paths. On my
116 machine, this means that the headphone input is amplified and looped back to
117 the headphone output, which uses the exact same pins! As you might expect, this
118 causes terrible headphone noise, the character of which is controlled by the
119 1Ah boost control. (If you've seen instructions online to fix XPS 13 headphone
120 noise by changing "Headphone Mic Boost" in ALSA, now you know why.)
121
122 The information here has been obtained through black-box reverse engineering of
123 the ALC256 codec's behavior and is not guaranteed to be correct. It likely
124 also applies for the ALC255, ALC257, ALC235, and ALC236, since those codecs
125 seem to be close relatives of the ALC256. (They all share one initialization
126 function.) Additionally, other codecs like the ALC225 and ALC285 also have this
127 register, judging by existing fixups in ``patch_realtek.c``, but specific
128 data (e.g. node IDs, bit positions, pin mappings) for those codecs may differ
129 from what I've described here.
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