linux-kernel - Re: [PATCH v2 1/2] ALSA: usb-audio: more tolerant packetsize

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Message-ID: <s5h37i15y43.wl-tiwai@suse.de>
Date:   Tue, 06 Dec 2016 08:13:16 +0100
From:   Takashi Iwai <tiwai@...e.de>
To:     "Jiada Wang" <jiada_wang@...tor.com>
Cc:     <clemens@...isch.de>, <perex@...ex.cz>, <o-takashi@...amocchi.jp>,
        <alsa-devel@...a-project.org>, <apape@...adit-jv.com>,
        <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH v2 1/2] ALSA: usb-audio: more tolerant packetsize

On Tue, 06 Dec 2016 06:46:14 +0100,
Jiada Wang wrote:
> 
> From: Andreas Pape <apape@...adit-jv.com>
> 
> since commit 57e6dae1087b ("ALSA: usb-audio: do not trust too-big
> wMaxPacketSize values"), the expected packetsize is always limited
> to nominal + 25%. It was discovered, that some devices have a much
> higher jitter in used packetsizes than 25% which would result in BABBLE
> condition and dropping of packets.
> A better solution is so assume the jitter to be the nominal packetsize:
> -one nearly empty packet followed by a almost 150% sized one.
> 
> V2: changed to assume max frequency is +50 of nominal packetsize.

Clemens, are you OK with this change?


Takashi

> 
> Signed-off-by: Andreas Pape <apape@...adit-jv.com>
> Signed-off-by: Jiada Wang <jiada_wang@...tor.com>
> ---
>  sound/usb/endpoint.c | 4 ++--
>  1 file changed, 2 insertions(+), 2 deletions(-)
> 
> diff --git a/sound/usb/endpoint.c b/sound/usb/endpoint.c
> index c470251..a2931f4 100644
> --- a/sound/usb/endpoint.c
> +++ b/sound/usb/endpoint.c
> @@ -632,8 +632,8 @@ static int data_ep_set_params(struct snd_usb_endpoint *ep,
>  	ep->stride = frame_bits >> 3;
>  	ep->silence_value = pcm_format == SNDRV_PCM_FORMAT_U8 ? 0x80 : 0;
>  
> -	/* assume max. frequency is 25% higher than nominal */
> -	ep->freqmax = ep->freqn + (ep->freqn >> 2);
> +	/* assume max. frequency is 50% higher than nominal */
> +	ep->freqmax = ep->freqn + (ep->freqn >> 1);
>  	/* Round up freqmax to nearest integer in order to calculate maximum
>  	 * packet size, which must represent a whole number of frames.
>  	 * This is accomplished by adding 0x0.ffff before converting the
> -- 
> 2.9.3
>