ABI Changes (was Re: Graphics Driver Frameworks and Security)

[Felix Kühling]

Am Dienstag, den 16.05.2006, 23:00 -0700 schrieb Andy Ritger:
> Thanks, guys, for bringing up the topic of ABI changes.
> 
> To set the record straight, there have been several ABI changes
> between Xorg 7.0 and the 7.1 RCs; not all of the ABI changes may
> be well known (or documented?).  The ABI changes that I am aware
> of are:
> 
>     - A devPrivates field was added to GlyphRec; this is needed,
>       for example, to store glyph data in video memory; impacts XAA,
>       EXA, and any acceleration architecture.
> 
>     - The XV PutImage driver proc was updated to take a DrawablePtr;
>       this is used so that drivers can integrate their XV
>       implementations with Damage and Composite.
> 
>     - The XF86 libc wrapper was removed.  Drivers/modules need to
>       link directly against the C library, rather than call the
>       XF86 libc wrappers.  Fortunately, dlloader drivers without
>       xf86 libc wrappers can also work on older X servers.
> 
>     - Field alignment within the DrawableRec data structure on 64-bit
>       platforms (bug 6924); this change was made as part of the
>       fix for bug 6438.  This change seems unfortunate, because A)
>       DrawableRec is so pervasive throughout the whole X server and
>       drivers, it is hard to abstract away the ABI differences,
>       B) bug 6438 really could have been fixed without changing
>       the ABI (DrawableRec could have been padded to retain the
>       same alignment after the fix), C) there seemed to be some
>       awareness that the change would alter the ABI, yet there was
>       no announcement or reving of the ABI version.
> 
>       It may be dangerously close to 7.1 release, but I would like
>       to suggest padding the DrawableRec data structure such that the
>       id field is still only 32-bits, as it is now (and therefore
>       still fixes bug 6438), but that the fields retain the same
>       alignment that they had previously.  Either Aaron Plattner
>       or I will provide a patch to bug 6924.

Isn't it too late for this? The ABI has changed and the ABI major
version was bumped already due to other changes. You will need separate
driver binaries for Xorg 7.0 and Xorg 7.1. So what's the benefit of
partially restoring the old ABI at this point?

Regards,
  Felix

> 
> Other ABI changes in the works for post-7.1 include the PCI
> management overhaul, and the removal of the MAXSCREENS constant.
> 
> >From NVIDIA's perspective, we understand that ABI breakages are
> needed for the advancement of X, and we certainly do not want to
> inhibit that.
> 
> At the same time, it seems good for ABI breakages to be thought-out
> and deliberate.  It is also desirable for there to be fewer ABI
> changes; if several ABI changes can be made at once, then we can
> have fewer revisions of the ABI, which means fewer ABI versions to
> support.  Fewer ABI versions is good not only for IHVs like NVIDIA
> and ATI, but it is also good for driver developers, packagers,
> and distributions too, because they have less complicated version
> dependencies to keep track of and a given version of a driver
> will work with a wider range of servers.  I would think this is
> particularly relevant with the modularization of the X source tree,
> and the decoupling of X server and X driver releases.
> 
> There is also a difference between ABI changes that can be managed
> within a single build of a driver versus ABI changes that really
> require multiple builds of a driver.  Something like the devPrivates
> addition to GlyphRec can be dealt with at runtime, but a change as
> pervasive as the alignment of DrawableRec's fields is difficult to 
> manage within a single build.
> 
> Thanks,
> - Andy Ritger
> 
> 
> 
[snip]

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| Felix Kühling <fxkuehl at gmx.de>                     http://fxk.de.vu |
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