EXA framework question

Michel Dänzer michel at tungstengraphics.com
Sun Jun 15 05:21:42 PDT 2008

On Sun, 2008-06-15 at 04:51 +0100, Steven J Newbury wrote:
> On Sat, 2008-06-14 at 19:50 -0700, Mohan Parthasarathy wrote:
> > 
> > 
> > On 6/14/08, Steven J Newbury <steve at snewbury.org.uk> wrote: 
> >         On Sat, 2008-06-14 at 06:47 -0700, Mohan Parthasarathy wrote:
> >         > Hi,
> >         > i am not sure i understand. X itself does not need it but
> >         still needed
> >         >  for e.g., text mode console access. Is that what you mean ?
> >         > Mohan
> >         EXA is completely unrelated to fbdev drivers.  DRM will
> >         eventually
> >         largely replace fbdev drivers once kernel modesetting support
> >         goes
> >         mainstream, at that point X will be using the kernel
> >         modesetting
> >         support.  At present, fbdev provides modesetting for console
> >         (non-X -
> >         DirectFB, text etc) only while all current X video drivers
> >         have built-in
> >         modesetting either through VBIOS calls or by programming the
> >         hw
> >         directly. EXA is an acceleration infrastructure for X
> >         replacing XAA and
> >         makes use of DRM in some cases to provide higher performance.
> >  
> > Great. That clarifies a lot. In EXA, the X driver in userspace
> > controls
> > the hardware through MMIO and there is no kernel component at all
> > except for the modesetting unlike the older XAA model where there
> > was an X driver and the framebuffer driver. Right ?
> Not exactly.  XAA is/was mostly MMIO, EXA is much more flexible and as
> noted some drivers make use of the DRM to set up DMA command buffers
> etc.  

Drivers can and do do that with XAA as well.

> Modesetting is what the framebuffer (Linux fbdev) drivers do, this
> doesn't have anything to do with the XAA or EXA acceleration
> architectures.

That is the main point: XAA and EXA are just two different acceleration
architectures with different emphasis on accelerated operations (XAA:
legacy X11 rendering, EXA: RENDER extension / compositing). Neither has
any direct influence on how drivers choose to implement their basic
acceleration hooks, let alone on modesetting.

> Earlier versions of the earlier accelerated X drivers provided UseFBDev
> as an option, examples would have been the mga and ATI drivers.  As far
> as I , this is no longer the case and the option has been removed.

It's been removed from drivers that support RandR 1.2 because the kernel
fbdev interface does not provide everything needed for that. It's still
available in other drivers though.

Earthling Michel Dänzer           |          http://tungstengraphics.com
Libre software enthusiast         |          Debian, X and DRI developer

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