Re: [PATCH] media: em28xx: fix silent audio capture on EM2860/TVP5150 reference design

[Mauro Carvalho Chehab <mchehab+huawei@kernel.org>]

On Sun, 13 Sep 2026 16:23:12 -0400
d01 <d01.devel@gmail.com> wrote:

> The EM2860/TVP5150 reference design (eb1a:5051, "USB 2861 Device",
> card=29) captures no analog audio.

Instead of touching the reference design, and except if you have the
real em2861 reference card (unlikely unless you're a manufacturer or
if some manufacturer donated one to you), you need to create a 
board-specific entry with the brand name used on it, and use
card=<number>.

Having a generic USB ID means that the vendor of your specific board 
was lazy enough to not store a different USB ID at the USB table inside
its eeprom.

It may also mean that the AC97 chip - either emp202 or a different
model - is wired on a non-standard way. Those chips have multiple 
inputs that are wired on different ways depending on the board.



> The ALSA device yields a flat noise
> floor of about -85 dBFS, identical with the audio cable plugged or
> unplugged and regardless of mixer settings, while video capture works
> normally.
> 
> Audio on this board is not on the em28xx vendor path. Chip config
> register 0x00 reads 0x50 -- bit 0x80 (vendor audio) is clear -- so the
> board exposes a USB Audio Class interface claimed by snd-usb-audio, fed
> by the EMP202 codec's ADC. Which input that ADC listens to is set by
> AC97_REC_SEL (0x1a).
> 
> em28xx_audio_analog_set() does program AC97_REC_SEL, but only under:
> 
> 	if (dev->ctl_aoutput & EM28XX_AOUT_PCM_IN)
> 
> ctl_aoutput comes from the board's per-input .aout field, falling back
> to EM28XX_AOUT_MASTER when it is unset (em28xx-video.c). This board's
> entry sets .amux but no .aout, so EM28XX_AOUT_PCM_IN is never set, the
> AC97_REC_SEL write is skipped, and the mux stays at its reset default
> of 0x0000 (Mic, which is not connected on these boards). Nothing is
> captured.
> 
> Set .aout on both inputs to select the PCM record path from Line In,
> matching the .amux already declared. ac97_return_record_select() maps
> EM28XX_AOUT_PCM_LINE to 4, which em28xx_audio_analog_set() mirrors into
> both channels as 0x0404.
> 
> EM28XX_AOUT_MASTER is included so the master volume continues to track
> the V4L2 volume control exactly as it did via the old fallback, keeping
> the change additive. It is not required for capture -- muting
> AC97_MASTER by hand while leaving everything else in place does not
> affect the recorded signal.
> 
> Diagnosed on eb1a:5051 by writing AC97_REC_SEL directly over usbfs with
> everything else held constant:
> 
>   AC97_REC_SEL = 0x0404 (Line In) -> -21.8 dBFS, line-level audio
>   AC97_REC_SEL = 0x0000 (Mic)     -> -85.4 dBFS, silence
> 
> Then verified with this patch applied and the rebuilt module loaded, with
> no userspace register writes at all: the driver programs AC97_REC_SEL to
> 0x0404 on its own at probe, and capture yields -25.0 dBFS of line-level
> audio once the mute control is cleared.
> 
> AC97_POWERDOWN (0x26) reads 0x420d throughout -- status bits ADC, ANL
> and REF all set -- so this is not an analog-power or GPIO problem. The
> codec is powered and simply listening to the wrong input.
> 
> Note that the V4L2 mute control defaults to 1, which gates the bridge
> audio path via EM28XX_XCLK_AUDIO_UNMUTE in R0F. That is a separate
> issue and is unchanged by this patch, so "v4l2-ctl -d /dev/videoN -c
> mute=0" is still required to hear anything.
> 
> Signed-off-by: d01 <d01.devel@gmail.com>

Please use your name instead of "d01".

> ---
>  drivers/media/usb/em28xx/em28xx-cards.c | 4 ++++
>  1 file changed, 4 insertions(+)
> 
> --- a/drivers/media/usb/em28xx/em28xx-cards.c
> +++ b/drivers/media/usb/em28xx/em28xx-cards.c
> @@ -1148,10 +1148,14 @@
>  			.type     = EM28XX_VMUX_COMPOSITE,
>  			.vmux     = TVP5150_COMPOSITE1,
>  			.amux     = EM28XX_AMUX_LINE_IN,
> +			.aout     = EM28XX_AOUT_MASTER | EM28XX_AOUT_PCM_IN |
> +				    EM28XX_AOUT_PCM_LINE,
>  		}, {
>  			.type     = EM28XX_VMUX_SVIDEO,
>  			.vmux     = TVP5150_SVIDEO,
>  			.amux     = EM28XX_AMUX_LINE_IN,
> +			.aout     = EM28XX_AOUT_MASTER | EM28XX_AOUT_PCM_IN |
> +				    EM28XX_AOUT_PCM_LINE,

Most boards don't require adjusting aout. On most cases, just amux is
needed. The above setup also sounds weird: why does it need 3 audio lines
there?

>  		} },
>  	},
>  	[EM2861_BOARD_PLEXTOR_PX_TV100U] = {



Thanks,
Mauro