Messages in this thread |  | | | Subject | Re: Dropped frames in video capture process | | Date | Fri, 6 Nov 1998 00:31:48 +0100 (MET) | | From | (Rogier Wolff) |
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Alan Cox wrote: > > frames are coming at a rate of 60Hz, and there are 4 buffers, if > > interrupt is not serviced within 64 msec frame would be lost. First of > > all - is this normal for Linux to have a latency over 50msec, or > > something else is wrong on my system (K6,233MHZ). > > Dangerous assumption here - who says the IRQ latency is the problem. Suppose > you are in fact losing frames because the Xserver is creating enough PCI > activity to make the capture device fifo overrun ?
I have to agree with Alan here. I've made a driver that also handled a video capture device, which could do 60 frames per second.
If the CPU would do a memcpy, (with Linux' efficient memcpy implementation), then the memory would be busy enough that the chips' FIFO would overrun.
This was most noticable with two cards running at the same time, but could also be provoked with just one card. It depends a bit on the motherboard too (the bus arbiter should give more priority to the PCI device, and let the CPU just take the rest. I didn't have enough money to buy a PCI bus analyser to look into it fully. I expect more modern chipsets to be better able to handle this...)
Yes, a back-of-the-envelope calculation will show that it is Completely impossible for the PCI bus not to be able to handle
60 * 512 * 240 * 4 = 29 Mb/s 1/s pix/line lines bytes/pixel = bytes/sec
becuase everything says 133Mbyte per seconde for PCI bus, but in practise it is quite hard to get the 30Mb per second to run faultlessly.
What chip are you using as a video/pci interface? ZR36120?
Roger.
-- | Most people would die sooner than think.... | R.E.Wolff@BitWizard.nl | in fact, most do. -- Bertrand Russsell | phone: +31-15-2137555 We write Linux device drivers for any device you may have! fax: ..-2138217
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