Re: [PATCH] ALSA: usb: Fix Processing Unit Descriptor parsers

[Takashi Iwai <tiwai@suse.de>]

At Thu, 21 Feb 2013 01:55:50 +0000,
Pawel Moll wrote:
> 
> Commit 99fc86450c439039d2ef88d06b222fd51a779176 "ALSA: usb-mixer:
> parse descriptors with structs" introduced a set of useful parsers
> for descriptors. Unfortunately the parses for the Processing Unit
> Descriptor came with a very subtle bug...
> 
> Functions uac_processing_unit_iProcessing() and
> uac_processing_unit_specific() were indexing the baSourceID array
> forgetting the fields before the iProcessing and process-specific
> descriptors.
> 
> The problem was observed with Sound Blaster Extigy mixer,
> where nNrModes in Up/Down-mix Processing Unit Descriptor
> was accessed at offset 10 of the descriptor (value 0)
> instead of offset 15 (value 7). In result the resulting
> control had interesting limit values:
> 
> Simple mixer control 'Channel Routing Mode Select',0
>   Capabilities: volume volume-joined penum
>   Playback channels: Mono
>   Capture channels: Mono
>   Limits: 0 - -1
>   Mono: -1 [100%]
> 
> Fixed by starting from the bmControls, which was calculated
> correctly, instead of baSourceID.
> 
> Now the mentioned control is fine:
> 
> Simple mixer control 'Channel Routing Mode Select',0
>   Capabilities: volume volume-joined penum
>   Playback channels: Mono
>   Capture channels: Mono
>   Limits: 0 - 6
>   Mono: 0 [0%]
> 
> Signed-off-by: Pawel Moll <mail@pawelmoll.com>
> ---
>  include/uapi/linux/usb/audio.h |    6 ++++--
>  1 file changed, 4 insertions(+), 2 deletions(-)
> 
> diff --git a/include/uapi/linux/usb/audio.h b/include/uapi/linux/usb/audio.h
> index ac90037..d2314be 100644
> --- a/include/uapi/linux/usb/audio.h
> +++ b/include/uapi/linux/usb/audio.h
> @@ -384,14 +384,16 @@ static inline __u8 uac_processing_unit_iProcessing(struct uac_processing_unit_de
>  						   int protocol)
>  {
>  	__u8 control_size = uac_processing_unit_bControlSize(desc, protocol);
> -	return desc->baSourceID[desc->bNrInPins + control_size];
> +	return *(uac_processing_unit_bmControls(desc, protocol)
> +			+ control_size);
>  }
>  
>  static inline __u8 *uac_processing_unit_specific(struct uac_processing_unit_descriptor *desc,
>  						 int protocol)
>  {
>  	__u8 control_size = uac_processing_unit_bControlSize(desc, protocol);
> -	return &desc->baSourceID[desc->bNrInPins + control_size + 1];
> +	return uac_processing_unit_bmControls(desc, protocol)
> +			+ control_size + 1;
>  }

Looks good to me.

An alternative would be to change uac_processing_unit_bControlSize()
to correct the header size, but I don't think it's good, as
bControlSize fields in other units show the value including the header
size.

Daniel, what do you think?


thanks,

Takashi