Re: bttv driver questions

[Daniel Glöckner <daniel-gl@xxxxxxx>]

On Sat, Aug 30, 2008 at 12:01:47PM +0200, Jean Delvare wrote:
> My assumption that there was a
> VBI interrupt was wrong, probably because my video source is a VCR
> and it doesn't send any information during VBI?

When there is an application requesting VBI capture, there will be a
VBI interrupt, regardless of the content in those lines.

> > There is only one program with jumps that are patched at runtime to
> > point to the program fragments for capture.
> 
> And this all happens magically inside the BT878 without the bttv
> driver having to care? Wow! Tricky.

No, this is how bttv does it.
Other implementations may use a single loop without jumps.

> In my case there's a PCI Express-to-PCI bridge on the path, so I
> presume that this acts as the target. I suppose that the board was
> designed that way precisely to make sure that no extra latency would
> happen on the PCI bus due to the host being slow/busy. If the bridge
> has large enough buffers, it should be easy for it to send the data
> down to the host bridge, given that PCI Express x1 is much faster
> than PCI.

It's not that much faster. Of the 250MB/s a lot is lost to overhead,
especially when there are mostly short packets. And there may be other
bottlenecks before the data reaches the RAM.

> > I think for competing Bt878s the smallest trigger point in combination
> > with a high latency counter should perform best.
> 
> I don't quite follow you here. Care to explain how you reached this
> conclusion?

A smaller trigger point will make the PCI bus less idle but the average
FIFO fill will be lower.

Yesterday I wrote a small program that simulates a number of PCI masters
with constant data rate filling their FIFOs. There is a simple round
robin arbiter and neither the target nor the master needs wait cycles.

For 5 masters with 24.2MB/s each (peak data rate of YUY2 640x480 NTSC),
a latency counter of 254, and a FIFO trigger of 4, the bus is never idle.
The maximum FIFO fill is 16 DWords. With a latency counter below 20,
the FIFO fill rises infinitely.

With a FIFO trigger of 32 and a latency counter of 254, the maximum fill
is 33 DWords and the bus is 4.5% idle.

Those 17 less FIFO entries in the 4-entry-trigger case can buffer for 93
PCI cycles. The 4.5% idle cycles in the 32-entry-trigger case are wasted
if there is no other master on the bus, as is the case when the Bt878s are
behind a bridge.

In reality there are always idle phases during syncs and additional
traffic will be generated to fetch RISC instructions and to access
registers.

  Daniel

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