[linux-dvb] buffer underruns with budget-av/-ci (vpeirq)

Matthias Dahl mldvb at mortal-soul.de
Sat Oct 14 16:27:12 CEST 2006

Hi everyone.

I own a KNC ONE DVB Plus with a Cineview module (Alphacrypt Light). From the 
beginning I ran into data corruption problems with the mpeg stream as soon as 
I made the usual X <-> VT switch or put some load on the Athlon64 3500+ 
machine with its RAID 5 (3 SATA disks).

After some looking around, I soon discovered it was related to a buffer 
underrun because the following showed up in my logs every time a corruption 

saa7146 (0) vpeirq: used 1 times >80% of buffer (65612 bytes now)

I searched the mailing list archives and learned this problem had been 
reported earlier and that I should increase the DMA buffer size- which I did. 
Running now at the maximum of 1410kb, things have pretty much been fixed. I 
can put load on the machine, do random X <-> VT switches and everything works 
most of the time. But there still is the occasional underrun even though the 
machine is not under load.

For example, I am running mythtv on my box for scheduling recordings. From 
time to time, I get 3-4 underrun msgs in a short time (a matter of minutes), 
even though nothing is recording and the machine is far from being loaded. I 
don't know what mythtvbackend is doing then (maybe checking for EPG updates 
from time to time).

Here the relevant parts of my log when the dvb modules are being loaded:

Oct 14 09:19:22 saa7146: found saa7146 @ mem ffffc20000014000 (revision 1, irq 
66) (0x1894,0x0021).
Oct 14 09:19:22 saa7146 (0): dma buffer size 1443840
Oct 14 09:19:22 DVB: registering new adapter (KNC1 DVB-C Plus).
Oct 14 09:19:22 adapter failed MAC signature check
Oct 14 09:19:22 encoded MAC from EEPROM was 
Oct 14 09:19:22 saa7146_vv: saa7146 (0): registered device video0 [v4l2]
Oct 14 09:19:22 KNC1-0: MAC addr = 00:09:d6:6d:61:b5
Oct 14 09:19:22 DVB: registering frontend 0 (Philips TDA10021 DVB-C)...
Oct 14 09:19:22 budget-av: ci interface initialised.
Oct 14 09:19:22 budget-av: cam inserted A
Oct 14 09:19:22 dvb_ca adapter 0: DVB CAM detected and initialised 
Oct 14 09:19:22 saa7146: register extension 'budget_ci dvb'.
Oct 14 09:19:35 budget-av: cam inserted A
Oct 14 09:19:36 dvb_ca adapter 0: DVB CAM detected and initialised 

If there is anything I can do to help fix or investigate this, please let me 
know. I am more than willing to help...

Have a nice weekend and thanks for any help in advance,

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