Gentoo x84_64 system with AMD GFX card (R9 285).

gst-play-1.0 test.mp4 gives:

Press 'k' to see a list of keyboard shortcuts.
Now playing /home/user/Downloads/test.mp4
libva info: VA-API version 0.38.1
libva info: va_getDriverName() returns 0
libva info: Trying to open /usr/lib64/va/drivers/fglrx_drv_video.so
libva info: Found init function __vaDriverInit_0_33
libva info: va_openDriver() returns 0
Redistribute latency...
ERROR Cannot access memory for read and write operation. for file:///home/user/Downloads/test.mp4
ERROR debug information: /var/tmp/portage/media-plugins/gst-plugins-libav-1.6.3/work/gst-libav-1.6.3/ext/libav/gstavviddec.c(1242): get_output_buffer (): /GstPlayBin:playbin/GstURIDecodeBin:uridecodebin0/GstDecodeBin:decodebin0/avdec_h264:avdec_h264-0:
The video memory allocated from downstream pool could not mapped forread and write.
Reached end of play list.

vainfo:
libva info: VA-API version 0.38.1
libva info: va_getDriverName() returns 0
libva info: Trying to open /usr/lib64/va/drivers/fglrx_drv_video.so
libva info: Found init function __vaDriverInit_0_33
libva info: va_openDriver() returns 0
vainfo: VA-API version: 0.38 (libva 1.6.2)
vainfo: Driver version: AMD MMD 1.0
vainfo: Supported profile and entrypoints
      VAProfileMPEG2Simple            :	VAEntrypointVLD
      VAProfileMPEG2Main              :	VAEntrypointVLD
      VAProfileMPEG4Simple            :	VAEntrypointVLD
      VAProfileMPEG4AdvancedSimple    :	VAEntrypointVLD
      VAProfileMPEG4Main              :	VAEntrypointVLD
      VAProfileH264Baseline           :	VAEntrypointVLD
      VAProfileH264Main               :	VAEntrypointVLD
      VAProfileH264High               :	VAEntrypointVLD
      VAProfileVC1Simple              :	VAEntrypointVLD
      VAProfileVC1Main                :	VAEntrypointVLD
      VAProfileVC1Advanced            :	VAEntrypointVLD

mpv --hwdec=vaapi runs fine, so it seems to be something gst specific.

By the way, is there a simple way to blacklist an installed plugin? I'd like to disable vaapi while it's not working without just deleting it and fighting my package manager.