[alsa-devel] [PATCH - usb-audio 1/1] ALSA: usb-audio: Allow changing from a bad sample rate

[Takashi Iwai]

On Tue, 10 Jul 2018 22:27:43 +0200,
Adam Goode wrote:
> 
> If the audio device is externally clocked and set to a rate that does
> not match the external clock, the clock will never be valid and we cannot
> set the rate successfully. To fix this, allow a rate change even if
> the clock is initially invalid, and validate again after the rate is
> changed.
> 
> This fixes problems with MOTU UltraLite AVB hardware over USB.
> 
> Signed-off-by: Adam Goode <agoode at google.com>

The logic looks good, but just a few minor nitpicking below.


> diff --git a/sound/usb/clock.c b/sound/usb/clock.c
> index c79749613fa6..9e9019464f91 100644
> --- a/sound/usb/clock.c
> +++ b/sound/usb/clock.c
> @@ -513,14 +513,26 @@ static int set_sample_rate_v2v3(struct snd_usb_audio *chip, int iface,
>  	bool writeable;
>  	u32 bmControls;
>  
> +	/* First, try to find a valid clock. This may trigger
> +	 * automatic clock selection if the current clock is not
> +	 * valid. */

The "*/" should put in another line.

>  	clock = snd_usb_clock_find_source(chip, fmt->protocol,
>  					  fmt->clock, true);
> -	if (clock < 0)
> -		return clock;
> +	if (clock < 0) {
> +		/* We did not find a valid clock, but that might be
> +		 * because the current sample rate does not match an
> +		 * external clock source. Try again without validation
> +		 * and we will do another validation after setting the
> +		 * rate. */

Ditto.

> +		clock = snd_usb_clock_find_source(chip, fmt->protocol,
> +						  fmt->clock, false);
> +		if (clock < 0)
> +			return clock;
> +	}
>  
>  	prev_rate = get_sample_rate_v2v3(chip, iface, fmt->altsetting, clock);
>  	if (prev_rate == rate)
> -		return 0;
> +		return uac_clock_source_is_valid(chip, fmt->protocol, clock) ? 0 : -ENXIO;

It's better to have a single return path, so let's use here
"goto validation", and...

>  	if (fmt->protocol == UAC_VERSION_3) {
>  		struct uac3_clock_source_descriptor *cs_desc;
> @@ -577,7 +589,8 @@ static int set_sample_rate_v2v3(struct snd_usb_audio *chip, int iface,
>  		snd_usb_set_interface_quirk(dev);
>  	}
>  
> -	return 0;
> +	/* validate clock after rate change */
> +	return uac_clock_source_is_valid(chip, fmt->protocol, clock) ? 0 : -ENXIO;

... put here like

  validation:
	if (!uac_clock_source_is_valid(chip, fmt->protocol, clock))
		return  -ENXIO;
	return 0;


thanks,

Takashi