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

From: Takashi Iwai
Date: Thu Feb 21 2013 - 04:03:44 EST


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@xxxxxxxxxxxxx>
> ---
> 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
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