Re: Is sendfile all that sexy?

[Rogier Wolff]

Linus Torvalds wrote:
> > I wrote a driver for a zoran-chipset frame-grabber card. The "natural"
> > way to save a video stream was exactly the way it came out of the
> > card. And the card was structured that you could put on an "mpeg
> > decoder" (or encoder) chip, and you could DMA the stream directly into
> > that chip.
> 
> Ehh..
> 
> And how many of these chips are out on the market?
> 
> Would you agree that it is less than 0.01% of all PC hardware? Like MUCH
> less?

Someone asked me to write a driver for one of these cards. I was
assuming that most of them work like this. And I'm never wrong, you
know... 

> > The way soundcards are commonly programmed, they don't play from their
> > own memory, but from main memory. However, they all can play from
> > their own memory. 
> 
> And how do you synchronize the streams etc? It's a nasty piece of
> business, and direct PCI-PCI streaming is not the answer.
> 
> > > And you wouldn't need a new memory zone - the kernel wouldn't ever touch
> > > the memory anyway, you'd just ioremap() it if you needed to access it
> > > programmatically in addition to the streaming of data off disk.
> > 
> > That's the way things currently work. If you start thinking about it
> > as a NUMA, it may improve the situation for "common users" too. 
> > 
> > A PC is a NUMA machine! We have disk (swap) and main memory. We also
> > have a frame buffer, which doesn't currently fit into our memory
> > architecture.
> 
> Don't be silly. It fints _fine_ in our memory architecture. We map it to
> xfree86, and we're done with it.
> 
> Using the frame buffer for "backing store" for normal  memory is not worth
> it. That's what disks are for. Frame buffers are _way_ too small to be
> interesting as a memory resource.

It's a silly small resource that suddenly becomes usable should the
right infrastructure be in place. It isn't. You're not planning on doing
it soonish. Neither am I. 

			Roger. 

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