EXA framework question

Steven J Newbury steve at snewbury.org.uk
Sat Jun 14 20:51:15 PDT 2008

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.  Modesetting is what the framebuffer (Linux fbdev) drivers do, this
doesn't have anything to do with the XAA or EXA acceleration
architectures.  Currently, all of the accelerated X drivers either set
up the video modes themselves or through video BIOS calls.

>         There is also a DDX driver for fbdev which uses the kernel
>         fbdev driver
>         to setup the graphics but provides no hw
>         acceleration.  Various eariler
>         versions of some video drivers featured a "UseFBDev" option
>         which used
>         fbdev for modesetting.
> I am a little confused by this. Before EXA, XAA framework used the
> framebuffer driver
> for the acceleration.

As I clarified above, the framebuffer drivers aren't really accelerated,
there is some text console acceleration, but that is it.

>  You are talking about two drivers above : DDX driver to setup
> graphics and some drivers that used "UseFBDev" for modesetting. Could
Yes, there is an fbdev X driver that uses fbdev to setup the video mode
and provide access to the framebuffer.  It isn't accelerated, but it is
very useful on hardware which has a kernel fbdev driver but for which
there is no X driver (embedded hardware often falls into this category).

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.

>  you
> clarify ? Sorry, i am pretty new to all these..
> thanks
> mohan

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