preliminary docs for pva links

documentation/mainpage.h

@@ -10,7 +10,8 @@ runs inside an EPICS IOC process and allows clients
 to make requests to access the Process Variables (PVs)
 within.
 
-Documentation of @ref qsrv_config including @ref qsrv_group_def.
+Documentation of @ref qsrv_config including @ref qsrv_group_def
+and @ref qsrv_link configuration.
 
 @subsection qsrv_build Building
 

documentation/qsrvpage.h

@@ -141,4 +141,127 @@ record(ai, "...") {
 }
 @endcode
 
+@subsection qsrv_link PVAccess Links
+
+When built against Base >= 3.16.1, support is enabled for PVAccess links,
+which are analogous to Channel Access (CA) links.  However, the syntax
+for PVA links is quite different.
+
+@warning The PVA Link syntax shown below is provisional and subject to change.
+
+A simple configuration using defaults is
+
+@code
+record(longin, "tgt") {}
+record(longin, "src") {
+    field(INP, {pva:"tgt"})
+}
+@endcode
+
+This is a shorthand for
+
+@code
+record(longin, "tgt") {}
+record(longin, "src") {
+    field(INP, {pva:{pv:"tgt"}})
+}
+@endcode
+
+Some additional keys (beyond "pv") may be used.
+Defaults are shown below:
+
+@code
+record(longin, "tgt") {}
+record(longin, "src") {
+    field(INP, {pva:{
+        pv:"tgt",
+        field:"",   # may be a sub-field
+        Q:4,        # monitor queue depth
+        proc:none,  # Request record processing (side-effects).
+        sevr:false, # Maximize severity.
+        monorder:0, # Order of record processing as a result of CP and CPP
+        defer:false # Defer put
+    }})
+}
+@endcode
+
+@subsubsection qsrv_link_pv pv: Target PV name
+
+The PV name to search for.
+This is the same name which could be used with 'pvget' or other client tools.
+
+@subsubsection qsrv_link_field field: Structure field name
+
+The name of a sub-field of the remotely provided Structure.
+By default, an empty string "" uses the top-level Structure.
+
+If the top level structure, or a sub-structure is selected, then
+it is expeccted to conform to NTScalar, NTScalarArray, or NTEnum
+to extract value and meta-data.
+
+If the sub-field is an PVScalar or PVScalarArray, then a value
+will be taken from it, but not meta-data will be available.
+
+@todo Ability to traverse through unions and into structure arrays (as with group mappings).
+
+@subsubsection qsrv_link_Q Q: Monitor queue depth
+
+Requests a certain monitor queue depth.
+The server may, or may not, take this into consideration when selecting
+a queue depth.
+
+@subsubsection qsrv_link_pipeline pipeline: Monitor flow control
+
+Expect that the server supports PVA monitor flow control.
+If not, then the subscription will stall (ick.)
+
+@subsubsection qsrv_link_proc proc: Request record processing (side-effects)
+
+The meaning of this option depends on the direction of the link.
+
+For output links, this option allows a request for remote processing (side-effects).
+
+@li none (default) - Make no special request.  Uses a server specific default.
+@li false, "NPP" - Request to skip processing.
+@li true, "PP" - Request to force processing.
+@li "CP", "CPP" - For output links, an alias for "PP".
+
+For input links, this option controls whether the record containing
+the PVA link will be processed when subscription events are received.
+
+@li none (default), false, "NPP" - Do not process on subscription updates.
+@li true, "CP" - Always process on subscription updates.
+@li "PP", "CPP" - Process on subscription updates if SCAN=Passive
+
+@subsubsection qsrv_link_sevr sevr: Alarm propagation
+
+This option controls whether reading a value from an input PVA link
+has the addition effect of propagating any alarm via the Maximize
+Severity process.
+
+@warning Not yet implemented
+
+@li false - Do not maximize severity.
+@li true - Maximize alarm severity
+@li "MSI" - Maximize only if the remote severity is INVALID.
+
+@subsubsection qsrv_link_monorder monorder: Monitor processing order
+
+When multiple record target the same target PV, and request processing
+on subscription updates.  This option allows the order of processing
+to be specified.
+
+Record are processed in increasing order.
+monorder=-1 is processed before monorder=0.
+Both are processed before monorder=1.
+
+@subsubsection qsrv_link_defer defer: Defer put
+
+By default (defer=false) an output link will immediately
+start a PVA Put operation.  defer=true will store the
+new value in an internal cache, but not start a PVA Put.
+
+This option, in combination with field: allows a single
+Put to contain updates to multiple sub-fields.
+
 */