pub enum BufferCollectionTokenGroupRequest {
Sync {
responder: BufferCollectionTokenGroupSyncResponder,
},
Close {
control_handle: BufferCollectionTokenGroupControlHandle,
},
SetName {
priority: u32,
name: String,
control_handle: BufferCollectionTokenGroupControlHandle,
},
SetDebugClientInfo {
name: String,
id: u64,
control_handle: BufferCollectionTokenGroupControlHandle,
},
SetDebugTimeoutLogDeadline {
deadline: i64,
control_handle: BufferCollectionTokenGroupControlHandle,
},
SetVerboseLogging {
control_handle: BufferCollectionTokenGroupControlHandle,
},
GetNodeRef {
responder: BufferCollectionTokenGroupGetNodeRefResponder,
},
IsAlternateFor {
node_ref: Event,
responder: BufferCollectionTokenGroupIsAlternateForResponder,
},
CreateChild {
payload: BufferCollectionTokenGroupCreateChildRequest,
control_handle: BufferCollectionTokenGroupControlHandle,
},
CreateChildrenSync {
rights_attenuation_masks: Vec<Rights>,
responder: BufferCollectionTokenGroupCreateChildrenSyncResponder,
},
AllChildrenPresent {
control_handle: BufferCollectionTokenGroupControlHandle,
},
}Expand description
The sysmem implementation is guaranteed to be consistent with a logical / conceptual model as follows:
As usual, a logical allocation considers either the root and all nodes with connectivity to the root that don’t transit an AttachToken(), or a sub-tree rooted at an AttachToken() token and all nodes with connectivity to that subtree that don’t transit another AttachToken(). This is called the logical allocation pruned sub-tree, or pruned sub-tree for short.
During constraints aggregation, each BufferCollectionTokenGroup will select a single child token among its children. The rest of the children will appear to fail the logical allocation, while the selected child may succeed.
When more than one BufferCollectionTokenGroup exists in the overall logical allocation pruned sub-tree, the relative priority between two groups is equivalent to their ordering in a DFS pre-order iteration of the tree, with parents higher priority than children, and left children higher priority than right children.
When a particular child of a group is selected (whether provisionally during a constraints aggregation attempt, or as a final selection), the non-selection of other children of the group can potentially “hide” other groups under those non-selected children.
Within a logical allocation, aggregation is attempted first by provisionally selecting the child 0 of the highest-priority group, and child 0 of the next highest-priority group that isn’t hidden by the provisional selections so far, etc.
If that aggregation attempt fails, aggregation will be attempted with the ordinal 0 child of all the same groups except the lowest priority non-hidden group which will provisionally select its ordinal 1 child (and then child 2 and so on). If a new lowest-priority group is un-hidden as provisional selections are updated, that newly un-hidden lowest-priority group has all its children considered in order, before changing the provisional selection in the former lowest-priority group. In terms of result, this is equivalent to systematic enumeration of all possible combinations of choices in a counting-like order updating the lowest-priority group the most often and the highest-priority group the least often. Rather than actually attempting aggregation with all the combinations, we can skip over combinations which are redundant/equivalent due to hiding without any change to the result.
Attempted aggregations of enumerated non-equivalent combinations of choices continue in this manner until either (a) all aggregation attempts fail in which case the overall logical allocation fails, or (b) until an attempted aggregation succeeds, in which case buffer allocation (if needed) is attempted once. If buffer allocation based on the first successful aggregation fails, the overall logical allocation fails (there is no buffer allocation retry / re-attempt). If buffer allocation succeeds (or is not needed), the logical allocation succeeds.
If this prioritization scheme cannot reasonably work for your usage of sysmem, please contact sysmem folks to discuss potentially adding a way to achieve what you need.
Please avoid creating a large number of BufferCollectionTokenGroup(s) per logical allocation, especially with large number of children overall, and especially in cases where aggregation may reasonably be expected to often fail using ordinal 0 children and possibly with later children as well. We anticipate mitigating potentially high time complexity of evaluating too many child combinations/selections across too many groups by simply failing logical allocation beyond a certain (fairly high, but not huge) max number of considered group child combinations/selections. More advanced (and more complicated) mitigation is not anticipated to be practically necessary or worth the added complexity. Please contact sysmem folks if the max limit is getting hit or if you anticipate it getting hit, to discuss potential options.
Prefer to use multiple ImageFormatConstraints in a single BufferCollectionConstraints when feasible (when a participant just needs to express the ability to work with more than a single PixelFormat, with sysmem choosing which PixelFormat to use among those supported by all participants).
Similar to BufferCollectionToken and BufferCollection, closure of the BufferCollectionTokenGroup channel without sending Close() first will cause logical buffer collection failure (or sub-tree failure if using SetDispensable() or AttachToken() and the BufferCollectionTokenGroup is part of a sub-tree under such a node that doesn’t propagate failure to its parent).
Variants§
Sync
Ensure that previous messages, including Duplicate() messages on a token, collection, or group, have been received server side.
Calling BufferCollectionToken.Sync() on a token that isn’t/wasn’t a valid sysmem token risks the Sync() hanging forever. See ValidateBufferCollectionToken() for one way to mitigate the possibility of a hostile/fake BufferCollectionToken at the cost of one round trip. Another way is to pass the token to BindSharedCollection(), which also validates the token as part of exchanging it for a BufferCollection channel, and BufferCollection Sync() can then be used.
After a Sync(), it’s then safe to send the client end of token_request to another participant knowing the server will recognize the token when it’s sent into BindSharedCollection() by the other participant.
Other options include waiting for each token.Duplicate() to complete individually (using separate call to token.Sync() after each), or calling Sync() on BufferCollection after the token has been turned in via BindSharedCollection().
Another way to mitigate is to avoid calling Sync() on the token, and instead later deal with potential failure of BufferCollection.Sync() if the original token was invalid. This option can be preferable from a performance point of view, but requires client code to delay sending tokens duplicated from this token until after client code has converted the duplicating token to a BufferCollection and received successful response from BufferCollection.Sync().
Prefer using BufferCollection.Sync() instead, when feasible (see above). When BufferCollection.Sync() isn’t feasible, the caller must already know that this token is/was valid, or BufferCollectionToken.Sync() may hang forever. See ValidateBufferCollectionToken() to check token validity first if the token isn’t already known to be (is/was) valid.
Fields
responder: BufferCollectionTokenGroupSyncResponderClose
On a BufferCollectionToken channel:
Normally a participant will convert a BufferCollectionToken into a BufferCollection view, but a participant is also free to Close() the token (and then close the channel immediately or shortly later in response to server closing its end), which avoids causing logical buffer collection failure. Normally an unexpected token channel close will cause logical buffer collection failure (the only exceptions being certain cases involving AttachToken() or SetDispensable()).
On a BufferCollection channel:
By default the server handles unexpected failure of a BufferCollection by failing the whole logical buffer collection. Partly this is to expedite closing VMO handles to reclaim memory when any participant fails. If a participant would like to cleanly close a BufferCollection view without causing logical buffer collection failure, the participant can send Close() before closing the client end of the BufferCollection channel. If this is the last BufferCollection view, the logical buffer collection will still go away. The Close() can occur before or after SetConstraints(). If before SetConstraints(), the buffer collection won’t require constraints from this node in order to allocate. If after SetConstraints(), the constraints are retained and aggregated along with any subsequent logical allocation(s), despite the lack of channel connection.
On a BufferCollectionTokenGroup channel:
By default, unexpected failure of a BufferCollectionTokenGroup will trigger failure of the logical BufferCollectionTokenGroup and will propagate failure to its parent. To close a BufferCollectionTokenGroup channel without failing the logical group or propagating failure, send Close() before closing the channel client endpoint.
If Close() occurs before AllChildrenPresent(), the logical buffer collection will still fail despite the Close() (because sysmem can’t be sure whether all relevant children were created, so it’s ambiguous whether all relevant constraints will be provided to sysmem). If Close() occurs after AllChildrenPresent(), the children and all their constraints remain intact (just as they would if the BufferCollectionTokenGroup channel had remained open), and the close doesn’t trigger or propagate failure.
Fields
control_handle: BufferCollectionTokenGroupControlHandleSetName
Set a name for VMOs in this buffer collection. The name may be truncated shorter. The name only affects VMOs allocated after it’s set - this call does not rename existing VMOs. If multiple clients set different names then the larger priority value will win.
SetDebugClientInfo
Set information about the current client that can be used by sysmem to help debug leaking memory and hangs waiting for constraints. |name| can be an arbitrary string, but the current process name (see fsl::GetCurrentProcessName()) is a good default. |id| can be an arbitrary id, but the current process ID (see fsl::GetCurrentProcessKoid()) is a good default.
Also used when verbose logging is enabled (see SetVerboseLogging()) to indicate which client is closing their channel first, leading to sub-tree failure (which can be normal if the purpose of the sub-tree is over, but if happening earlier than expected, the client-channel-specific name can help diagnose where the failure is first coming from, from sysmem’s point of view).
By default (unless overriden by this message or using Allocator.SetDebugClientInfo()), a Node will copy info from its parent Node at the time the child Node is created. While this can be better than nothing, it’s often better for each participant to use Node.SetDebugClientInfo() or Allocator.SetDebugClientInfo() to keep the info directly relevant to the current client. Also, SetVerboseLogging() can be used to help disambiguate if a Node is suspected of having info that was copied from its parent.
SetDebugTimeoutLogDeadline
Sysmem logs a warning if not all clients have set constraints 5 seconds after creating a collection. Clients can call this method to change when the log is printed. If multiple client set the deadline, it’s unspecified which deadline will take effect.
SetVerboseLogging
Verbose logging includes constraints set via SetConstraints() from each client along with info set via SetDebugClientInfo() and the structure of the tree of Node(s).
Normally sysmem prints only a single line complaint when aggregation fails, with just the specific detailed reason that aggregation failed, with minimal context. While this is often enough to diagnose a problem if only a small change was made and the system had been working before the small change, it’s often not particularly helpful for getting a new buffer collection to work for the first time. Especially with more complex trees of nodes, involving things like AttachToken(), SetDispensable(), BufferCollectionTokenGroup nodes, and associated sub-trees of nodes, verbose logging may help in diagnosing what the tree looks like and why it’s failing a logical allocation, or why a tree or sub-tree is failing sooner than expected.
The intent of the extra logging is to be acceptable from a performance point of view, if only enabled on a low number of buffer collections. If we’re not tracking down a bug, we shouldn’t send this message.
If too many participants leave verbose logging enabled, we may end up needing to require that system-wide sysmem verbose logging be permitted via some other setting, to avoid sysmem spamming the log too much due to this message.
This may be a NOP for some nodes due to intentional policy associated with the node, if we don’t trust a node enough to let it turn on verbose logging.
Fields
control_handle: BufferCollectionTokenGroupControlHandleGetNodeRef
This gets an event handle that can be used as a parameter to IsAlternateFor() called on any Node. The client will not be granted the right to signal this event, as this handle should only be used as proof that the client obtained this handle from this Node.
Because this is a get not a set, no Sync() is needed between the GetNodeRef() and the call to IsAlternateFor(), despite the two calls potentially being on different channels.
See also IsAlternateFor().
Fields
IsAlternateFor
This checks whether the calling node is in a subtree rooted at a different child token of a common parent BufferCollectionTokenGroup, in relation to the passed-in node_ref.
This call is for assisting with admission control de-duplication, and with debugging.
The node_ref must be obtained using GetNodeRef() of a BufferCollectionToken, BufferCollection, or BufferCollectionTokenGroup.
The node_ref can be a duplicated handle; it’s not necessary to call GetNodeRef() for every call to IsAlternateFor().
If a calling token may not actually be a valid token at all due to a potentially hostile/untrusted provider of the token, call ValidateBufferCollectionToken() first instead of potentially getting stuck indefinitely if IsAlternateFor() never responds due to a calling token not being a real token (not really talking to sysmem). Another option is to call BindSharedCollection with this token first which also validates the token along with converting it to a BufferCollection, then call BufferCollection IsAlternateFor().
error values:
ZX_ERR_NOT_FOUND means the node_ref wasn’t found within the same logical buffer collection as the calling Node. Before logical allocation and within the same logical allocation sub-tree, this essentially means that the node_ref was never part of this logical buffer collection, since before logical allocation all node_refs that come into existence remain in existence at least until logical allocation (including Node(s) that have done a Close() and closed their channel), and for ZX_ERR_NOT_FOUND to be returned, this Node’s channel needs to still be connected server side, which won’t be the case if the whole logical allocation has failed. After logical allocation or in a different logical allocation sub-tree there are additional potential reasons for this error. For example a different logical allocation (separated from this Node(s) logical allocation by an AttachToken() or SetDispensable()) can fail its sub-tree deleting those Node(s), or a BufferCollectionTokenGroup may exist and may select a different child sub-tree than the sub-tree the node_ref is in causing deletion of the node_ref Node. The only time sysmem keeps a Node around after that Node has no corresponding channel is when Close() is used and the Node’s sub-tree has not yet failed. Another reason for this error is if the node_ref is an eventpair handle with sufficient rights, but isn’t actually a real node_ref obtained from GetNodeRef().
ZX_ERR_INVALID_ARGS means the caller passed a node_ref that isn’t an eventpair handle, or doesn’t have the needed rights expected on a real node_ref.
No other failing status codes are returned by this call. However, sysmem may add additional codes in future, so the client should have sensible default handling for any failing status code.
On success, is_alternate has the following meaning:
- true - The first parent node in common between the calling node and the node_ref Node is a BufferCollectionTokenGroup. This means that the calling Node and the node_ref Node will not have both their constraints apply - rather sysmem will choose one or the other of the constraints - never both. This is because only one child of a BufferCollectionTokenGroup is selected during logical allocation, with only that one child’s sub-tree contributing to constraints aggregation.
- false - The first parent node in common between the calling Node and the node_ref Node is not a BufferCollectionTokenGroup. Currently, this means the first parent node in common is a BufferCollectionToken or BufferCollection (regardless of not Close()ed or Close()ed). This means that the calling Node and the node_ref Node may have both their constraints apply during constraints aggregation of the logical allocation, if both Node(s) are selected by any parent BufferCollectionTokenGroup(s) involved. In this case, there is no BufferCollectionTokenGroup that will directly prevent the two Node(s) from both being selected and their constraints both aggregated, but even when false, one or both Node(s) may still be eliminated from consideration if one or both Node(s) has a direct or indirect parent BufferCollectionTokenGroup which selects a child sub-tree other than the sub-tree containing the calling Node or node_ref Node.
CreateChild
Create a child token. Before passing the client end of this token to BindSharedCollection(), completion of Sync() after CreateChild() is required. Or the client can use CreateChildrenSync() which essentially includes the Sync().
token_request - the server end of the new token channel.
rights_attenuation_mask - If ZX_RIGHT_SAME_RIGHTS, the created token allows the holder to get the same rights to buffers as the parent token (of the group) had.
Fields
control_handle: BufferCollectionTokenGroupControlHandleCreateChildrenSync
Create 1 or more child tokens at once, synchronously. In contrast to CreateChild(), no Sync() completion is required before passing the client end of a returned token to BindSharedCollection().
The size of the rights_attentuation_mask determines the number of created child tokens.
The lower-index child tokens are higher priority (attempted sooner) than higher-index child tokens.
As per all child tokens, successful aggregation will choose exactly one child among all created children (across all children created across potentially multiple calls to CreateChild() and CreateChildrenSync()).
The maximum permissible total number of children per group, and total number of nodes in an overall tree (from the root) are capped to limits which are not configurable via these protocols.
Fields
AllChildrenPresent
AllChildrenPresent()
After creating all children, the client must call AllChildrenPresent() to inform sysmem that no more children will be created, so that sysmem can know when it’s ok to start aggregating constraints.
If Close() is to be sent, it should be sent after AllChildrenPresent(), else failure of the group and propagation of the failure to the group’s parent will still be triggered.
Fields
control_handle: BufferCollectionTokenGroupControlHandleImplementations§
source§impl BufferCollectionTokenGroupRequest
impl BufferCollectionTokenGroupRequest
pub fn into_sync(self) -> Option<BufferCollectionTokenGroupSyncResponder>
pub fn into_close(self) -> Option<BufferCollectionTokenGroupControlHandle>
pub fn into_set_name( self ) -> Option<(u32, String, BufferCollectionTokenGroupControlHandle)>
pub fn into_set_debug_client_info( self ) -> Option<(String, u64, BufferCollectionTokenGroupControlHandle)>
pub fn into_set_debug_timeout_log_deadline( self ) -> Option<(i64, BufferCollectionTokenGroupControlHandle)>
pub fn into_set_verbose_logging( self ) -> Option<BufferCollectionTokenGroupControlHandle>
pub fn into_get_node_ref( self ) -> Option<BufferCollectionTokenGroupGetNodeRefResponder>
pub fn into_is_alternate_for( self ) -> Option<(Event, BufferCollectionTokenGroupIsAlternateForResponder)>
pub fn into_create_child( self ) -> Option<(BufferCollectionTokenGroupCreateChildRequest, BufferCollectionTokenGroupControlHandle)>
pub fn into_create_children_sync( self ) -> Option<(Vec<Rights>, BufferCollectionTokenGroupCreateChildrenSyncResponder)>
pub fn into_all_children_present( self ) -> Option<BufferCollectionTokenGroupControlHandle>
sourcepub fn method_name(&self) -> &'static str
pub fn method_name(&self) -> &'static str
Name of the method defined in FIDL