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

[Jiada Wang <jiada_wang@mentor.com>]

From: Andreas Pape <apape@de.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.

Signed-off-by: Andreas Pape <apape@de.adit-jv.com>
Signed-off-by: Jiada Wang <jiada_wang@mentor.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