[win-pv-devel] more changes to PDO revision numbering

[Paul Durrant <Paul.Durrant@xxxxxxxxxx>]

Hi,

  I've recently realised that the way in which PDO revisions are assigned is 
flawed. The nested-for-loop implementation doesn't guarantee that a particular 
revision corresponds to a particular combination of interface versions as was 
my intention, and also the combinatorial explosion that occurs whenever one 
interface version is incremented means we burn through PDO revisions pretty 
rapidly.  So, I propose a change...
  We don't really need to have a revision for *every* possible combination of 
interfaces. A bus driver implementing the newest  version of one interface and 
the oldest version of another possibly would never exist since often multiple 
interfaces may be updated by a single (series of) commit(s). I therefore think 
that we should move to an implementation that uses a table mapping a set of 
interface versions to a PDO revision. To do this, and maintain working 
combinations of drivers, we will need to follow some rules though:

1) The last line of the table should always map the latest interface versions 
to the highest PDO revision
2) A patch should never *modify* a line in the table, only add or remove 
(although I think we can allow that for patches within the same series)
3) A line should not be removed from the table until no child driver relies on 
binding to that PDO revision [1]. I.e. child drivers get updated to bind to the 
latest PDO revision and *then* old revisions are retired.

  Under this scheme, I think we can also drop the need to register individual 
interface versions between drivers (i.e. the current provider/subscriber 
scheme) since we can infer what interface versions a driver is using by looking 
at the MatchingDeviceId value in the device software key.

  To get from where we are to this new scheme I also propose that we just start 
with a single line table in each bus driver (XENBUS and XENVIF) mapping the 
latest interface versions to the current highest PDO revision. The latest 
versions of all child drivers are currently binding to the highest PDO 
revisions so rule 3 is adhered to.

  If anyone has any thoughts on this please let me know, otherwise I will make 
changes this Friday (July 15th).

    Paul

[1] It may be necessary to deliberately make incompatible changes and thus, in 
such exceptional circumstances, a single series may add a new line to the table 
and remove all existing lines but we should clearly try to avoid this where 
possible. In this case though, patch series to update child drivers should be 
simultaneously available and committed to the relevant repositories within the 
smallest possible time window.

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