^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 1) ===============================
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 2) Realtek PC Beep Hidden Register
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 3) ===============================
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 4)
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 5) This file documents the "PC Beep Hidden Register", which is present in certain
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 6) Realtek HDA codecs and controls a muxer and pair of passthrough mixers that can
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 7) route audio between pins but aren't themselves exposed as HDA widgets. As far
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 8) as I can tell, these hidden routes are designed to allow flexible PC Beep output
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 9) for codecs that don't have mixer widgets in their output paths. Why it's easier
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 10) to hide a mixer behind an undocumented vendor register than to just expose it
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 11) as a widget, I have no idea.
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 12)
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 13) Register Description
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 14) ====================
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 15)
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 16) The register is accessed via processing coefficient 0x36 on NID 20h. Bits not
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 17) identified below have no discernible effect on my machine, a Dell XPS 13 9350::
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 18)
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 19) MSB LSB
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 20) +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 21) | |h|S|L| | B |R| | Known bits
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 22) +=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 23) |0|0|1|1| 0x7 |0|0x0|1| 0x7 | Reset value
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 24) +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 25)
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 26) 1Ah input select (B): 2 bits
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 27) When zero, expose the PC Beep line (from the internal beep generator, when
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 28) enabled with the Set Beep Generation verb on NID 01h, or else from the
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 29) external PCBEEP pin) on the 1Ah pin node. When nonzero, expose the headphone
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 30) jack (or possibly Line In on some machines) input instead. If PC Beep is
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 31) selected, the 1Ah boost control has no effect.
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 32)
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 33) Amplify 1Ah loopback, left (L): 1 bit
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 34) Amplify the left channel of 1Ah before mixing it into outputs as specified
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 35) by h and S bits. Does not affect the level of 1Ah exposed to other widgets.
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 36)
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 37) Amplify 1Ah loopback, right (R): 1 bit
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 38) Amplify the right channel of 1Ah before mixing it into outputs as specified
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 39) by h and S bits. Does not affect the level of 1Ah exposed to other widgets.
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 40)
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 41) Loopback 1Ah to 21h [active low] (h): 1 bit
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 42) When zero, mix 1Ah (possibly with amplification, depending on L and R bits)
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 43) into 21h (headphone jack on my machine). Mixed signal respects the mute
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 44) setting on 21h.
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 45)
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 46) Loopback 1Ah to 14h (S): 1 bit
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 47) When one, mix 1Ah (possibly with amplification, depending on L and R bits)
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 48) into 14h (internal speaker on my machine). Mixed signal **ignores** the mute
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 49) setting on 14h and is present whenever 14h is configured as an output.
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 50)
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 51) Path diagrams
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 52) =============
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 53)
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 54) 1Ah input selection (DIV is the PC Beep divider set on NID 01h)::
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 55)
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 56) <Beep generator> <PCBEEP pin> <Headphone jack>
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 57) | | |
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 58) +--DIV--+--!DIV--+ {1Ah boost control}
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 59) | |
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 60) +--(b == 0)--+--(b != 0)--+
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 61) |
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 62) >1Ah (Beep/Headphone Mic/Line In)<
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 63)
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 64) Loopback of 1Ah to 21h/14h::
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 65)
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 66) <1Ah (Beep/Headphone Mic/Line In)>
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 67) |
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 68) {amplify if L/R}
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 69) |
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 70) +-----!h-----+-----S-----+
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 71) | |
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 72) {21h mute control} |
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 73) | |
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 74) >21h (Headphone)< >14h (Internal Speaker)<
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 75)
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 76) Background
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 77) ==========
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 78)
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 79) All Realtek HDA codecs have a vendor-defined widget with node ID 20h which
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 80) provides access to a bank of registers that control various codec functions.
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 81) Registers are read and written via the standard HDA processing coefficient
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 82) verbs (Set/Get Coefficient Index, Set/Get Processing Coefficient). The node is
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 83) named "Realtek Vendor Registers" in public datasheets' verb listings and,
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 84) apart from that, is entirely undocumented.
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 85)
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 86) This particular register, exposed at coefficient 0x36 and named in commits from
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 87) Realtek, is of note: unlike most registers, which seem to control detailed
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 88) amplifier parameters not in scope of the HDA specification, it controls audio
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 89) routing which could just as easily have been defined using standard HDA mixer
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 90) and selector widgets.
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 91)
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 92) Specifically, it selects between two sources for the input pin widget with Node
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 93) ID (NID) 1Ah: the widget's signal can come either from an audio jack (on my
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 94) laptop, a Dell XPS 13 9350, it's the headphone jack, but comments in Realtek
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 95) commits indicate that it might be a Line In on some machines) or from the PC
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 96) Beep line (which is itself multiplexed between the codec's internal beep
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 97) generator and external PCBEEP pin, depending on if the beep generator is
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 98) enabled via verbs on NID 01h). Additionally, it can mix (with optional
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 99) amplification) that signal onto the 21h and/or 14h output pins.
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 100)
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 101) The register's reset value is 0x3717, corresponding to PC Beep on 1Ah that is
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 102) then amplified and mixed into both the headphones and the speakers. Not only
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 103) does this violate the HDA specification, which says that "[a vendor defined
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 104) beep input pin] connection may be maintained *only* while the Link reset
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 105) (**RST#**) is asserted", it means that we cannot ignore the register if we care
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 106) about the input that 1Ah would otherwise expose or if the PCBEEP trace is
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 107) poorly shielded and picks up chassis noise (both of which are the case on my
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 108) machine).
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 109)
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 110) Unfortunately, there are lots of ways to get this register configuration wrong.
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 111) Linux, it seems, has gone through most of them. For one, the register resets
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 112) after S3 suspend: judging by existing code, this isn't the case for all vendor
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 113) registers, and it's led to some fixes that improve behavior on cold boot but
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 114) don't last after suspend. Other fixes have successfully switched the 1Ah input
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 115) away from PC Beep but have failed to disable both loopback paths. On my
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 116) machine, this means that the headphone input is amplified and looped back to
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 117) the headphone output, which uses the exact same pins! As you might expect, this
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 118) causes terrible headphone noise, the character of which is controlled by the
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 119) 1Ah boost control. (If you've seen instructions online to fix XPS 13 headphone
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 120) noise by changing "Headphone Mic Boost" in ALSA, now you know why.)
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 121)
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 122) The information here has been obtained through black-box reverse engineering of
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 123) the ALC256 codec's behavior and is not guaranteed to be correct. It likely
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 124) also applies for the ALC255, ALC257, ALC235, and ALC236, since those codecs
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 125) seem to be close relatives of the ALC256. (They all share one initialization
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 126) function.) Additionally, other codecs like the ALC225 and ALC285 also have this
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 127) register, judging by existing fixups in ``patch_realtek.c``, but specific
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 128) data (e.g. node IDs, bit positions, pin mappings) for those codecs may differ
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 129) from what I've described here.
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