Re: [Dri-devel] my X-Kernel question

Re: [Dri-devel] my X-Kernel question

[Peter Surda]

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On Mon, Oct 22, 2001 at 02:27:23AM -0400, volodya@mindspring.com wrote:
> The biggest reason against this is that X (as it is now) support not only
> Linux but many other OSes: in particular BSD(s) and Solaris. Moving
> stuff into Linux kernel creates a fork of the drivers which is
> undesirable..
That's a lame excuse. I'm using Linux so I won't suffer from Windows, why
should I suffer because of BSD or Solaris?

<Rant>
About the precise vsync thingy we're talking about in xpert: we need kernel
support anyway. So why instead of calling a video driver in kernel "lame" and
"uncool" and adding a strange inflexible function god-knows-where, shouldn't
we move the whole driver structure to kernel? Drivers for every other device
type are in kernel. What would the anti-video-in-kernel-guys think if I
claimed that network cards should have userspace "drivers" in sort of "uber
daemon" and if an app wants to make a TCP connection it should contact this
"uber daemon"? I don't want to have staroffice in kernel, but the DRIVER
STRUCTURE. For a great UI, we need DMA, vsync and devices communicating with
each other directly or with little overhead. Why insist on doing this in
userspace? The reasons to put it into kernel aren't speed, but because it's
much more easier to add/maintain drivers, add functionality, share code and do
fancy stuff. DRI is a very good example of what I mean.
</Rant>

Short explaination of "the precise vsync thingy": For fluent video playback it
is necessary to precisely coordinate number of frames the monitor displays.
It is very visible on a TV. When I have a 25fps video, it should be EXACTLY
"one frame of data == one frame on TV". Currently, I can tell the card (ATI)
to blit on vsync (so it won't tear), but I can't tell it "don't miss a frame",
or "block until vsync". This results in visible "jumps" when suddenly the same
picture is staying on screen for the double duration than the others and it
sucks and I can't do anything about it without SOME kernel support. Telling
Xserver to poll for vsync and eat CPU is lame.

>                    Vladimir Dergachev
Bye,

Peter Surda (Shurdeek) <shurdeek@panorama.sth.ac.at>, ICQ 10236103, +436505122023

--
                   Disc space - The final frontier.

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Re: [Dri-devel] my X-Kernel question

[Alan Cox]

> we move the whole driver structure to kernel? Drivers for every other device

Not really. 

> STRUCTURE. For a great UI, we need DMA, vsync and devices communicating with
> each other directly or with little overhead. Why insist on doing this in

A video driver has to have extremely good latency, syscalls are overhead that
you generally do not want. There are specific things you want kernel help
with - agp management (and thus AGP DMA), context switching on DRI and maybe
some day interrupt handling for video vsync events and wiring them into
the XSync extension.

The rest is a bit questionable as a kernel space candidate, but if you 
want it in kernel go ahead - XFree86 supports both models.