[linux-dvb] Re: [PATCH] fix for stream corruption on budget /Nova-T cards

[John Dalgliesh <johnd@syd.microforte.com.au>]

On Sun, 31 Aug 2003, Jon Burgess wrote:

> Stefan Betermieux wrote:
> 
> > Hi,
> > 
> > unfortunatly, the patch doesn't do the trick for me, I still have to use 
> > another PCI burst mode. I have taken a look at your patch and I have noticed 
> > that your workaround is executed every second call of vpeirq() on my system. 
> > Is this a normal behaviour?
> 
> I don't think it is normal behaviour. I see about 64 IRQ's / second.
> The buffer is 188kB, if we assume the DVB-T stream is about 2MByte/s, we 
> should half fill the buffer every 2048 / (188/2) = so I reckon that it 
> should hit the 0%/50% mark approximately 21 times/s. Perhaps it is every 
> second call if your stream rate is higher.
>
...
> 
> I wonder why we bother using both the ODD & EVEN buffer at all. The 
> distinction doesn't appear relevant when the chip is dealing with raw 
> DVB data and not interlaced video fields. Perhaps we should just use one 
> field buffer and double it's size. This should simplify things a bit, it 
> seems to be this transition between the two fields which causes the 
> ambiguous DMA pointer in the first place.

Just FYI, this was the approach I took with the Mac driver. Originally I'd
tried doing the same as the linux driver (two fields), but I was having
problems and couldn't see a good reason why you'd want to use that
ODD/EVEN stuff, so I went to one field and everything worked fine. Plus 
it's a lot saner (IMHO).

> > On the other hand, I can see a connection between your patch and mine. If I 
> > reduce the burst threshold, then the buffer doesn't get filled up so much and 
> > the difference between olddma and newdma should be smaller. 
> 
> As far as I can tell, the burst threshold just controls how often the 
> data goes from the FIFO to the main memory via PCI DMA.
> 
> It is the LIMIT parameter which defines how often the IRQ should fire 
> and hence how much data should be available each time the IRQ is called.
> 
...

{P^/



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