[alsa-devel] [PATCH v4 07/15] ALSA: usb-audio: correct ep use_count semantics (add set_param flag)

[Takashi Iwai]

At Sun,  6 Oct 2013 22:31:12 +0200,
Eldad Zack wrote:
> 
> Currently, use_count is used in snd_usb_endpoint_set_params to
> ensure the parameters don't get changed for an in-use endpoint.
> 
> However, there is a subtle condition where this check fails -
> if hw_params is called on both substreams before calling prepare (for
> playback) or start trigger (for capture): the endpoint is not yet
> started, i.e., snd_usb_endpoint_start() does not yet increment use_count.
> 
> This adds a flag to check if the parameters are set, but does not
> omit checking the use_count.
> 
> Signed-off-by: Eldad Zack <eldad at fogrefinery.com>
> 
> ---
> v2: Cleaned up the patch, now it does only one thing (add the flag
> and the check).

I guess this logic doesn't work if a capture stream is re-prepared
before started.  I know that you'll change the capture stream starting
in the later patch, but then this patch must be applied after that,
because it implicitly assumes the scheme.


Takashi

> ---
>  sound/usb/card.h     | 1 +
>  sound/usb/endpoint.c | 6 +++++-
>  2 files changed, 6 insertions(+), 1 deletion(-)
> 
> diff --git a/sound/usb/card.h b/sound/usb/card.h
> index ca98a9b..1014136 100644
> --- a/sound/usb/card.h
> +++ b/sound/usb/card.h
> @@ -51,6 +51,7 @@ struct snd_usb_endpoint {
>  	struct snd_usb_audio *chip;
>  
>  	int use_count;
> +	bool param_set;
>  	int ep_num;		/* the referenced endpoint number */
>  	int type;		/* SND_USB_ENDPOINT_TYPE_* */
>  	unsigned long flags;
> diff --git a/sound/usb/endpoint.c b/sound/usb/endpoint.c
> index 2685660..9acc864 100644
> --- a/sound/usb/endpoint.c
> +++ b/sound/usb/endpoint.c
> @@ -472,6 +472,8 @@ struct snd_usb_endpoint *snd_usb_add_endpoint(struct snd_usb_audio *chip,
>  		ep->syncmaxsize = le16_to_cpu(get_endpoint(alts, 1)->wMaxPacketSize);
>  	}
>  
> +	ep->param_set = false;
> +
>  	list_add_tail(&ep->list, &chip->ep_list);
>  
>  __exit_unlock:
> @@ -780,11 +782,12 @@ int snd_usb_endpoint_set_params(struct snd_usb_endpoint *ep,
>  {
>  	int err;
>  
> -	if (ep->use_count != 0) {
> +	if (ep->param_set || ep->use_count != 0) {
>  		snd_printk(KERN_WARNING "Unable to change format on ep #%x: already in use\n",
>  			   ep->ep_num);
>  		return -EBUSY;
>  	}
> +	ep->param_set = true;
>  
>  	/* release old buffers, if any */
>  	release_urbs(ep, 0);
> @@ -937,6 +940,7 @@ void snd_usb_endpoint_stop(struct snd_usb_endpoint *ep)
>  		ep->sync_slave = NULL;
>  		ep->retire_data_urb = NULL;
>  		ep->prepare_data_urb = NULL;
> +		ep->param_set = false;
>  		set_bit(EP_FLAG_STOPPING, &ep->flags);
>  	}
>  }
> -- 
> 1.8.1.5
>