Hi All,
After receiving a fair bit of feedback (thanks!), I've updated the EGL_EXT_image_dma_buf_import spec
and expanded it to resolve a number of the issues. Please find the latest draft below and let me
know any additional feedback you might have, either on the lists or by private e-mail - I don't mind
which.
I think the only remaining issue now is if we need a mechanism whereby an application can query
which drm_fourcc.h formats EGL supports or if just failing with EGL_BAD_MATCH when the application
has use one EGL doesn't support is sufficient. Any thoughts?
Cheers,
Tom
--------------------8<--------------------
Name
EXT_image_dma_buf_import
Name Strings
EGL_EXT_image_dma_buf_import
Contributors
Jesse Barker
Rob Clark
Tom Cooksey
Contacts
Jesse Barker (jesse 'dot' barker 'at' linaro 'dot' org)
Tom Cooksey (tom 'dot' cooksey 'at' arm 'dot' com)
Status
DRAFT
Version
Version 4, October 04, 2012
Number
EGL Extension ???
Dependencies
EGL 1.2 is required.
EGL_KHR_image_base is required.
The EGL implementation must be running on a Linux kernel supporting the
dma_buf buffer sharing mechanism.
This extension is written against the wording of the EGL 1.2 Specification.
Overview
This extension allows creating an EGLImage from a Linux dma_buf file
descriptor or multiple file descriptors in the case of multi-plane YUV
images.
New Types
None
New Procedures and Functions
None
New Tokens
Accepted by the <target> parameter of eglCreateImageKHR:
EGL_LINUX_DMA_BUF_EXT
Accepted as an attribute in the <attrib_list> parameter of
eglCreateImageKHR:
EGL_LINUX_DRM_FOURCC_EXT
EGL_DMA_BUF_PLANE0_FD_EXT
EGL_DMA_BUF_PLANE0_OFFSET_EXT
EGL_DMA_BUF_PLANE0_PITCH_EXT
EGL_DMA_BUF_PLANE1_FD_EXT
EGL_DMA_BUF_PLANE1_OFFSET_EXT
EGL_DMA_BUF_PLANE1_PITCH_EXT
EGL_DMA_BUF_PLANE2_FD_EXT
EGL_DMA_BUF_PLANE2_OFFSET_EXT
EGL_DMA_BUF_PLANE2_PITCH_EXT
EGL_YUV_COLOR_SPACE_HINT_EXT
EGL_SAMPLE_RANGE_HINT_EXT
EGL_YUV_CHROMA_HORIZONTAL_SITING_HINT_EXT
EGL_YUV_CHROMA_VERTICAL_SITING_HINT_EXT
Accepted as the value for the EGL_YUV_COLOR_SPACE_HINT_EXT attribute:
EGL_ITU_REC601_EXT
EGL_ITU_REC709_EXT
EGL_ITU_REC2020_EXT
Accepted as the value for the EGL_SAMPLE_RANGE_HINT_EXT attribute:
EGL_YUV_FULL_RANGE_EXT
EGL_YUV_NARROW_RANGE_EXT
Accepted as the value for the EGL_YUV_CHROMA_HORIZONTAL_SITING_HINT_EXT &
EGL_YUV_CHROMA_VERTICAL_SITING_HINT_EXT attributes:
EGL_YUV_CHROMA_SITING_0_EXT
EGL_YUV_CHROMA_SITING_0_5_EXT
Additions to Chapter 2 of the EGL 1.2 Specification (EGL Operation)
Add to section 2.5.1 "EGLImage Specification" (as defined by the
EGL_KHR_image_base specification), in the description of
eglCreateImageKHR:
"Values accepted for <target> are listed in Table aaa, below.
+-------------------------+--------------------------------------------+
| <target> | Notes |
+-------------------------+--------------------------------------------+
| EGL_LINUX_DMA_BUF_EXT | Used for EGLImages imported from Linux |
| | dma_buf file descriptors |
+-------------------------+--------------------------------------------+
Table aaa. Legal values for eglCreateImageKHR <target> parameter
...
If <target> is EGL_LINUX_DMA_BUF_EXT, <dpy> must be a valid display, <ctx>
must be EGL_NO_CONTEXT, and <buffer> must be NULL, cast into the type
EGLClientBuffer. The details of the image is specified by the attributes
passed into eglCreateImageKHR. Required attributes and their values are as
follows:
* EGL_WIDTH & EGL_HEIGHT: The logical dimensions of the buffer in pixels
* EGL_LINUX_DRM_FOURCC_EXT: The pixel format of the buffer, as specified
by drm_fourcc.h and used as the pixel_format parameter of the
drm_mode_fb_cmd2 ioctl.
* EGL_DMA_BUF_PLANE0_FD_EXT: The dma_buf file descriptor of plane 0 of
the image.
* EGL_DMA_BUF_PLANE0_OFFSET_EXT: The offset from the start of the
dma_buf of the first sample in plane 0, in bytes.
* EGL_DMA_BUF_PLANE0_PITCH_EXT: The number of bytes between the start of
subsequent rows of samples in plane 0. May have special meaning for
non-linear formats.
For images in an RGB color-space or those using a single-plane YUV format,
only the first plane's file descriptor, offset & pitch should be specified.
For semi-planar YUV formats, the chroma samples are stored in plane 1 and
for fully planar formats, U-samples are stored in plane 1 and V-samples are
stored in plane 2. Planes 1 & 2 are specified by the following attributes,
which have the same meanings as defined above for plane 0:
* EGL_DMA_BUF_PLANE1_FD_EXT
* EGL_DMA_BUF_PLANE1_OFFSET_EXT
* EGL_DMA_BUF_PLANE1_PITCH_EXT
* EGL_DMA_BUF_PLANE2_FD_EXT
* EGL_DMA_BUF_PLANE2_OFFSET_EXT
* EGL_DMA_BUF_PLANE2_PITCH_EXT
In addition to the above required attributes, the application may also
provide hints as to how the data should be interpreted by the GL. If any of
these hints are not specified, the GL will guess based on the pixel format
passed as the EGL_LINUX_DRM_FOURCC_EXT attribute or may fall-back to some
default value. Not all GLs will be able to support all combinations of
these hints and are free to use whatever settings they choose to achieve
the closest possible match.
* EGL_YUV_COLOR_SPACE_HINT_EXT: The color-space the data is in. Only
relevant for images in a YUV format, ignored when specified for an
image in an RGB format. Accepted values are:
EGL_ITU_REC601_EXT, EGL_ITU_REC709_EXT & EGL_ITU_REC2020_EXT.
* EGL_YUV_CHROMA_HORIZONTAL_SITING_HINT_EXT &
EGL_YUV_CHROMA_VERTICAL_SITING_HINT_EXT: Where chroma samples are
sited relative to luma samples when the image is in a sub-sampled
format. When the image is not using chroma sub-sampling, the luma and
chroma samples are assumed to be co-sited. Siting is split into the
vertical and horizontal and is in a fixed range. A siting of zero
means the first luma sample is taken from the same position in that
dimension as the chroma sample. This is best illustrated in the
diagram below:
(0.5, 0.5) (0.0, 0.5) (0.0, 0.0)
+ + + + + + + + * + * +
x x x x
+ + + + + + + + + + + +
+ + + + + + + + * + * +
x x x x
+ + + + + + + + + + + +
Luma samples (+), Chroma samples (x) Chrome & Luma samples (*)
Note this attribute is ignored for RGB images and non sub-sampled
YUV images. Accepted values are: EGL_YUV_CHROMA_SITING_0_EXT (0.0)
& EGL_YUV_CHROMA_SITING_0_5_EXT (0.5)
* EGL_SAMPLE_RANGE_HINT_EXT: The numerical range of samples. Only
relevant for images in a YUV format, ignored when specified for
images in an RGB format. Accepted values are: EGL_YUV_FULL_RANGE_EXT
(0-256) & EGL_YUV_NARROW_RANGE_EXT (16-235).
If eglCreateImageKHR is successful for a EGL_LINUX_DMA_BUF_EXT target, the
EGL takes ownership of the file descriptor and is responsible for closing
it, which it may do at any time while the EGLDisplay is initialized."
Add to the list of error conditions for eglCreateImageKHR:
"* If <target> is EGL_LINUX_DMA_BUF_EXT and <buffer> is not NULL, the
error EGL_BAD_PARAMETER is generated.
* If <target> is EGL_LINUX_DMA_BUF_EXT, and the list of attributes is
incomplete, EGL_BAD_PARAMETER is generated.
* If <target> is EGL_LINUX_DMA_BUF_EXT, and the EGL_LINUX_DRM_FOURCC_EXT
attribute is set to a format not supported by the EGL, EGL_BAD_MATCH
is generated.
* If <target> is EGL_LINUX_DMA_BUF_EXT, and the EGL_LINUX_DRM_FOURCC_EXT
attribute indicates a single-plane format, EGL_BAD_ATTRIBUTE is
generated if any of the EGL_DMA_BUF_PLANE1_* or EGL_DMA_BUF_PLANE2_*
attributes are specified.
* If <target> is EGL_LINUX_DMA_BUF_EXT and the value specified for
EGL_YUV_COLOR_SPACE_HINT_EXT is not EGL_ITU_REC601_EXT,
EGL_ITU_REC709_EXT or EGL_ITU_REC2020_EXT, EGL_BAD_ATTRIBUTE is
generated.
* If <target> is EGL_LINUX_DMA_BUF_EXT and the value specified for
EGL_SAMPLE_RANGE_HINT_EXT is not EGL_YUV_FULL_RANGE_EXT or
EGL_YUV_NARROW_RANGE_EXT, EGL_BAD_ATTRIBUTE is generated.
* If <target> is EGL_LINUX_DMA_BUF_EXT and the value specified for
EGL_YUV_CHROMA_HORIZONTAL_SITING_HINT_EXT or
EGL_YUV_CHROMA_VERTICAL_SITING_HINT_EXT is not
EGL_YUV_CHROMA_SITING_0_EXT or EGL_YUV_CHROMA_SITING_0_5_EXT,
EGL_BAD_ATTRIBUTE is generated.
* If <target> is EGL_LINUX_DMA_BUF_EXT and one or more of the values
specified for a plane's pitch or offset isn't supported by EGL,
EGL_BAD_ACCESS is generated.
* If <target> is EGL_LINUX_DMA_BUF_EXT and eglCreateImageKHR fails,
EGL does not retain ownership of the file descriptor and it is the
responsibility of the application to close it."
Issues
1. Should this be a KHR or EXT extension?
ANSWER: EXT. Khronos EGL working group not keen on this extension as it is
seen as contradicting the EGLStream direction the specification is going in.
The working group recommends creating additional specs to allow an EGLStream
producer/consumer connected to v4l2/DRM or any other Linux interface.
2. Should this be a generic any platform extension, or a Linux-only
extension which explicitly states the handles are dma_buf fds?
ANSWER: There's currently no intention to port this extension to any OS not
based on the Linux kernel. Consequently, this spec can be explicitly written
against Linux and the dma_buf API.
3. Does ownership of the file descriptor pass to the EGL library?
ANSWER: If eglCreateImageKHR is successful, EGL assumes ownership of the
file descriptors and is responsible for closing them.
4. How are the different YUV color spaces handled (BT.709/BT.601)?
ANSWER: The pixel formats defined in drm_fourcc.h only specify how the data
is laid out in memory. It does not define how that data should be
interpreted. Added a new EGL_YUV_COLOR_SPACE_HINT_EXT attribute to allow the
application to specify which color space the data is in to allow the GL to
choose an appropriate set of co-efficients if it needs to convert that data
to RGB for example.
5. What chroma-siting is used for sub-sampled YUV formats?
ANSWER: The chroma siting is not specified by either the v4l2 or DRM APIs.
This is similar to the color-space issue (4) in that the chroma siting
doesn't affect how the data is stored in memory. However, the GL will need
to know the siting in order to filter the image correctly. While the visual
impact of getting the siting wrong is minor, provision should be made to
allow an application to specify the siting if desired. Added additional
EGL_YUV_CHROMA_HORIZONTAL_SITING_HINT_EXT &
EGL_YUV_CHROMA_VERTICAL_SITING_HINT_EXT attributes to allow the siting to
be specified using a set of pre-defined values (0 or 0.5).
6. How can an application query which formats the EGL implementation
supports?
PROPOSAL: Don't provide a query mechanism but instead add an error condition
that EGL_BAD_MATCH is raised if the EGL implementation doesn't support that
particular format.
7. Which image formats should be supported and how is format specified?
Seem to be two options 1) specify a new enum in this specification and
enumerate all possible formats. 2) Use an existing enum already in Linux,
either v4l2_mbus_pixelcode and/or those formats listed in drm_fourcc.h?
ANSWER: Go for option 2) and just use values defined in drm_fourcc.h.
8. How can AYUV images be handled?
ANSWER: At least on fourcc.org and in drm_fourcc.h, there only seems to be
a single AYUV format and that is a packed format, so everything, including
the alpha component would be in the first plane.
9. How can you import interlaced images?
ANSWER: Interlaced frames are usually stored with the top & bottom fields
interleaved in a single buffer. As the fields would need to be displayed as
at different times, the application would create two EGLImages from the same
buffer, one for the top field and another for the bottom. Both EGLImages
would set the pitch to 2x the buffer width and the second EGLImage would use
a suitable offset to indicate it started on the second line of the buffer.
This should work regardless of whether the data is packed in a single plane,
semi-planar or multi-planar.
If each interlaced field is stored in a separate buffer then it should be
trivial to create two EGLImages, one for each field's buffer.
10. How are semi-planar/planar formats handled that have a different
width/height for Y' and CbCr such as YUV420?
ANSWER: The spec says EGL_WIDTH & EGL_HEIGHT specify the *logical* width and
height of the buffer in pixels. For pixel formats with sub-sampled Chroma
values, it should be trivial for the EGL implementation to calculate the
width/height of the Chroma sample buffers using the logical width & height
and by inspecting the pixel format passed as the EGL_LINUX_DRM_FOURCC_EXT
attribute. I.e. If the pixel format says it's YUV420, the Chroma buffer's
width = EGL_WIDTH/2 & height =EGL_HEIGHT/2.
11. How are Bayer formats handled?
ANSWER: As of Linux 2.6.34, drm_fourcc.h does not include any Bayer formats.
However, future kernel versions may add such formats in which case they
would be handled in the same way as any other format.
12. Should the spec support buffers which have samples in a "narrow range"?
Content sampled from older analogue sources typically don't use the full
(0-256) range of the data type storing the sample and instead use a narrow
(16-235) range to allow some headroom & toeroom in the signals to avoid
clipping signals which overshoot slightly during processing. This is
sometimes known as signals using "studio swing".
ANSWER: Add a new attribute to define if the samples use a narrow 16-235
range or the full 0-256 range.
13. Specifying the color space and range seems cumbersome, why not just
allow the application to specify the full YUV->RGB color conversion matrix?
ANSWER: Some hardware may not be able to use an arbitrary conversion matrix
and needs to select an appropriate pre-defined matrix based on the color
space and the sample range.
14. How do you handle EGL implementations which have restrictions on pitch
and/or offset?
ANSWER: Buffers being imported using dma_buf pretty much have to be
allocated by a kernel-space driver. As such, it is expected that a system
integrator would make sure all devices which allocate buffers suitable for
exporting make sure they use a pitch supported by all possible importers.
However, it is still possible eglCreateImageKHR can fail due to an
unsupported pitch. Added a new error to the list indicating this.
15. Should this specification also describe how to export an existing
EGLImage as a dma_buf file descriptor?
ANSWER: No. Importing and exporting buffers are two separate operations and
importing an existing dma_buf fd into an EGLImage is useful functionality in
itself. Agree that exporting an EGLImage as a dma_buf fd is useful, E.g. it
could be used by an OpenMAX IL implementation's OMX_UseEGLImage function to
give access to the buffer backing an EGLImage to video hardware. However,
exporting can be split into a separate extension specification.
Revision History
#4 (Tom Cooksey, October 04, 2012)
- Fixed issue numbering!
- Added issues 8 - 15.
- Promoted proposal for Issue 3 to be the answer.
- Added an additional attribute to allow an application to specify the color
space as a hint which should address issue 4.
- Added an additional attribute to allow an application to specify the chroma
siting as a hint which should address issue 5.
- Added an additional attribute to allow an application to specify the sample
range as a hint which should address the new issue 12.
- Added language to end of error section clarifying who owns the fd passed
to eglCreateImageKHR if an error is generated.
#3 (Tom Cooksey, August 16, 2012)
- Changed name from EGL_EXT_image_external and re-written language to
explicitly state this for use with Linux & dma_buf.
- Added a list of issues, including some still open ones.
#2 (Jesse Barker, May 30, 2012)
- Revision to split eglCreateImageKHR functionality from export
Functionality.
- Update definition of EGLNativeBufferType to be a struct containing a list
of handles to support multi-buffer/multi-planar formats.
#1 (Jesse Barker, March 20, 2012)
- Initial draft.
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