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Name
NV_stream_frame_limits
Name Strings
EGL_NV_stream_frame_limits
Contributors
Daniel Kartch
Contacts
Daniel Kartch, NVIDIA (dkartch 'at' nvidia.com)
Status
Draft
Version
Version 4 - October 27, 2016
Number
EGL Extension #113
Dependencies
Requires EGL_KHR_stream
Interacts with EGL_EXT_stream_consumer_egloutput
Overview
Some stream consumers may allow more than one frame to be acquired
at a time, so that applications can operate on sequences of images
rather than individual images. This in turn may lead to producers
allocating additional buffers to keep the fifo full while fulfilling
the consumer's needs. Applications may wish to limit the resources
allocated for a given stream, and some stream implementations may be
able to operate more efficiently if they know in advance how many
buffers will be used.
This extension defines two new stream attributes which provide hints
as to how many frames the application will require, allowing the
implementation to plan accordingly.
New functions
None
New tokens
Accepted as an attribute name in the <attrib_list> parameter of
eglCreateStreamKHR and as the <attribute> parameter of
eglQueryStreamKHR.
EGL_PRODUCER_MAX_FRAME_HINT_NV 0x3337
EGL_CONSUMER_MAX_FRAME_HINT_NV 0x3338
Add to "Table 3.10.4.4 EGLStream Attributes"
Attribute Read/Write Type Section
------------------------------ ---------- ------ ----------
EGL_PRODUCER_MAX_FRAME_HINT_NV io EGLint 3.10.4.x
EGL_CONSUMER_MAX_FRAME_HINT_NV io EGLint 3.10.4.x+1
Add new subsections to section "3.10.4 EGLStream Attributes"
3.10.4.x EGL_PRODUCER_MAX_FRAME_HINT_NV Attribute
The EGL_PRODUCER_MAX_FRAME_HINT_NV attribute indicates a limit on how
many outstanding frames the producer application intends to have at
any given time. This includes all frames currently being generated,
waiting in in the stream's mailbox or FIFO, and held by the consumer.
Its default value is EGL_DONT_CARE.
The implementation may make use of this hint to determine how many
buffers or other resources to allocate for the stream. It is not
necessarily an error for an application to attempt to insert more
than this many frames into the stream at once. However, exceeding
available resources may cause a producer to block or return an error,
as per its specification.
3.10.4.x+1 EGL_CONSUMER_MAX_FRAME_HINT_NV Attribute
The EGL_CONSUMER_MAX_FRAME_HINT_NV attribute indicates a limit on how
many frames the consumer application intends to acquire at the same
time. Its default value EGL_DONT_CARE.
The implementation may make use of this hint to determine how many
buffers or other resources to allocate for the stream. It is not
necessarily an error for an application to attempt to acquire more
than this many frames at once. However, exceeding available resources
may cause the consumer or producer to block or return an error, as per
their specifications.
Add to the description of eglStreamConsumerOutputEXT in the
EGL_KHR_stream_consumer_egloutput extension
When the producer generates frames faster than the output device can
display them, <stream>'s EGL_CONSUMER_MAX_FRAME_HINT_NV attribute can
be used to throttle the output. No more than the specified number of
frames will be scheduled for display at a time. If specified, the value
should be set to at least 2, to allow one frame to be displayed while
another is acquired and scheduled for display.
Issues
1. Is a generic stream extension really necessary, or can such
limits instead be imposed in the producer and consumer
interfaces?
RESOLVED: Yes, it is necessary. There are several use cases
where an application may need to impose limits and cannot do so
through the producer and consumer interfaces:
a) The producer and client interfaces are already published and
do not allow room for extension to impose limits.
b) The stream is cross-process, and one process needs to impose
limits on the endpoint provided by the other process.
In addition, a common method for imposing such limits simplifies
programming of large application suites which make use of
multiple types of producers and consumers, and allows the limits
on producer and consumer endpoints to be set to compatible
values.
2. Should the attributes be hints or hard limits?
RESOLVED: Hints. The variety of possible producers and consumers
makes it difficult to specify what the behavior should be if a
hard limit is exceeded. The goal here is to allow the application
to coordinate its resource requirements with the implementation.
If it fails to limit itself to the hinted values, we allow
producers or consumers to block or fail as appropriate for their
interfaces, but do not require it.
Revision History
#4 (October 27, 2016) Daniel Kartch
- Clean up for publication
#3 (September 14, 2106) Daniel Kartch
- Switched from hard limits to hints
#2 (January 8, 2016) Daniel Kartch
- Assigned enum values
#1 (October 30, 2015) Daniel Kartch
- Initial draft
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