Re: radeon, apertures & memory mapping

["Ville Syrjälä" <syrjala@sci.fi>]

On Sun, Mar 13, 2005 at 08:20:45PM +1100, Benjamin Herrenschmidt wrote:
> On Sun, 2005-03-13 at 10:22 +0200, Ville Syrjälä wrote:
> 
> > > 
> > >  - We only really need to bother about CPU access for the framebuffer
> > > itself (and possibly the cursor). That is normal non-accelerated fbdev
> > > operations an mmap'ing of the framebuffer in user space. This is not
> > > really a problem if that is limited to some part of vram. It puts a
> > > small constraint on the allocation of video memory: the framebuffer has
> > > to be near the beginning.
> > 
> > It will limit DirectFB to access only CONFIG_APER_SIZE. DirectFB needs CPU 
> > access to offscreen memory for software fallbacks and explicit user 
> > access. Any other compositing window system would need to do the same. If 
> > the video memory manager ever gets done then it shouldn't be a major 
> > problem because the kernel could blit the data to/from the inaccesible 
> > part without the application even realizing it. Although direct access 
> > might be useful in that case also since it could reduce pressure on the 
> > GART address space.
> 
> Yes, that means direct access will be limited to half of the vram on
> some setups and not on others, depending on how the BIOS sets up the
> card. Is this a real issue ? I don't think so personally. Especially
> since directfb could make use of DRM to do DMA blits either from main
> memory or from AGP space...

AGP as it's currently used is pretty much pointless for software fallbacks 
since reading from AGP memory is nearly as slow as reading from video 
memory.

> Or things can be put in accessible space,
> and blitted elsewhere using the accelerator.

This could work (and it would avoid DRM which in my book is a plus) but 
it's not very nice to have to copy the data twice.

> That "half" of vram is plenty enough for a framebuffer (and more). it's
> only an issue when you start doing very large offscreen surfaces. Do you
> have much usage of those without DMA ?

I have about 26MB of video memory used when running XDirectFB with GNOME, 
epiphany and 4 gnome-terminals, and I also have some videos playing on the 
TV at the same time. That's on a 32MB G400 BTW.


I must be missing something something obvious because I don't quite 
understand what major drawbacks there are with the non-overlapping mode. 
As I see it you get at least the same amount of CPU accessible memory as 
you get in the overlapping mode.

-- 
Ville Syrjälä
syrjala@sci.fi
http://www.sci.fi/~syrjala/


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