On Wed, Aug 10, 2005 at 10:49:49AM -0600, Marc Aurele La France wrote:
>On Tue, 9 Aug 2005, David Dawes wrote:
>>On Tue, Aug 09, 2005 at 10:45:15AM -0600, Marc Aurele La France wrote:
>>>Our loader scheme has been, and is, reasonably good at dealing with
>>>incompatibilities between modules and the core binary. But it is not so
>>>good with ensuring modules are compatible with each other.
>
>>>As a case in point, I am now faced with strong motivation to rework the
>>>three Vbe.*InfoBlock structures currently exported by vbe.h.
>
>>>Now, it's not my purpose here to get into these motivations, but reworking
>>>these structures would have consequences, one of which I am unsure how to
>>>deal with. At minimum, what I need to do is ...
>
>>>a) prevent drivers compiled with the current vbe.h from using a vbe module
>>> created with the new vbe.h; and
>>>b) prevent drivers compiled with the new vbe.h from using a vbe module
>>> created with the current vbe.h.
>
>>>The hammer approach would suggest bumping the video driver ABI version
>>>number all the way up to 1.0. (It currently sits at 0.8). This is
>>>overkill as doing so would prevent the loading of _any_ version 0.*
>>>module,
>>>vendor-provided or not, making this "politically" unwise.
>
>>>Bumping this version number to 0.9 (or leaving it alone) is insufficient
>>>as
>>>it prevents neither "a)" nor "b)", so changes would be required.
>
>>>I've prototyped a way of dealing with "a)". Basically, I'd have the vbe
>>>module's Setup() function call the common layer to associate a
>>>(potentially
>>>extended) XF86ModReqInfo occurrence with the vbe module, which the loader
>>>would then use to check parent modules against.
>
>>>Dealing with "b)" is trickier. All I can think of right now would require
>>>changes to the way video drivers load the vbe module, something I'd like
>>>to
>>>stay away from, especially in the presence of vendor-provided source
>>>and/or
>>>binaries.
>
>>"b)" could be handled by changing the drivers to specify a vbe module
>>version requirement when loading the vbe module. I don't think that is
>>unreasonable given that child module version checking is already a part
>>of the loadModule interface.
>
>Yes. One issue with this is that it doesn't do anything for vendor-provided
>drivers.
Vendors would need to handle the "b)" case in their drivers.
>On the other hand, in this case, this would only need to be done to that
>subset of drivers that actually peek/poke inside the structures to be
>reworked, rather than passing structure occurrences around. This
>requirement
>would need to be clearly documented.
>
>Also, vbe.h could provide a macro to load the vbe module, the idea being to
>make the module control its own loading.
>
>>If we were to start adding module interface version checking for
>>other shared module interfaces, that would head off similar issues
>>in the future.
>
>Yes, and how a module is loaded could be controlled by its main header, or
>something like "xxxModule.h".
Yes.
>>Another option for handling both cases is to change the the name
>>of a required entry point in the vbe module (such as VBEInit and
>>VBEExtendedInit). There are cases where this will not result in a
>>clean failure (one of the flaws of the current loader's handling of
>>unresolved symbols), but it will fail before attempting to operate with
>>incompatible data structures.
>
>It would fail, yes, but not with an appropriate message.
The "a)" case can be made to fail cleanly by having the old names being
functions that print out an informative message and exit.
The "b)" case could be handled like this:
#define VBEExtendedInit(a,b,c) \
(xf86LoaderCheckSymbol("newVBEExtendedInit") : \
newVBEExtendedInit(a,b,c) ?
(FatalError(INFORMATIVE_MESSAGE), NULL))
However, I think it's better to control module loading from the module
public headers, thus incorporating interface version checking with the
interface definition and avoiding the need to resort to tricks like this.
David
_______________________________________________
Devel mailing list
Devel@XFree86.Org
http://XFree86.Org/mailman/listinfo/devel
