Re: [Xenomai-core] RTDM fd support.

[Gilles Chanteperdrix <gilles.chanteperdrix@xenomai.org>]

Jan Kiszka wrote:
> Gilles Chanteperdrix wrote:
>> Hi Jan,
>>
>> from discussions on the mailing list, it seems that we are going to need
>> that unified file descriptors thing. However, since everybody wants
>> 2.5.0 to be released ASAP, we should try to think about any changes for
>> this support which would break the ABI, do them now, and keep the rest
>> for later.
>>
>> One such problem is the translation which currently exists between rtdm
>> file descriptors and descriptors passed to the posix skin, by adding
>> 1024 - 128. So, I propose to fix this issue.
>>
>> The idea Philippe had, and which I tend to agree to, was, in case of
>> open/socket/accept, to open("/dev/null"), and use an association table
>> somewhere to associate with the kernel-space descriptor number. Since
>> we are at it, this association table could in fact be the file
>> descriptor table, which we would put in the core skin ppd. The actual
>> data structure should be sparse, a linked list does not scale, so,
>> probably a hash would do. (I could also propose a solution based on avl
>> trees, but their implementation is not nearly as simple).
>>
>> Additionnally, this would allow our open/socket to conform to posix
>> which states that open should return the lowest free file descriptor.
>>
>> Should I propose a patch in that direction? Do you see any other
>> possible cause of ABI breakage when we migrate to an unified file
>> descriptors structure?
> 
> Right now this sounds like a plan - but I don't feel 100% comfortable to
> predict that we will get along with it. Converting some skin-specific
> service to a generic one involves quite a lot of refactoring. It is not
> really unlikely that we define some ABI now that will later turn out to
> be insufficient for what we want to achieve.

For the posix skin, I can live with a two hops solution right now, and
implement the one-hop solution later.

user-space fd
      |
      | core skin fd table
      v
kernel-space fd
      |
      | posix skin registry
      v
actual object

If we do this, there is not much re-factoring involved.

The question really boils down to whether you accept this solution for
the rtdm skin too.

-- 
					    Gilles.