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xf86drm: warn about GEM handle reference counting 

Users need to be careful when using drmPrimeHandleToFD or
drmPrimeFDToHandle directly. Mention GBM as a solution.

See [1] for an example mistake.

[1]: https://gitlab.freedesktop.org/drm/nouveau/-/issues/43#note_772661

Signed-off-by: Simon Ser <contact@emersion.fr>
Reviewed-by: Daniel Vetter <daniel.vetter@ffwll.ch>
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Simon Ser 2021-01-21 11:11:38 +01:00 committed by Emil Velikov
parent 523b3658aa
commit 632f59fcbf
1 changed files with 18 additions and 0 deletions
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18
xf86drm.h
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@ -813,6 +813,24 @@ extern char *drmGetDeviceNameFromFd(int fd);
extern char *drmGetDeviceNameFromFd2(int fd);
extern int drmGetNodeTypeFromFd(int fd);
/* Convert between GEM handles and DMA-BUF file descriptors.
*
* Warning: since GEM handles are not reference-counted and are unique per
* DRM file description, the caller is expected to perform its own reference
* counting. drmPrimeFDToHandle is guaranteed to return the same handle for
* different FDs if they reference the same underlying buffer object. This
* could even be a buffer object originally created on the same DRM FD.
*
* When sharing a DRM FD with an API such as EGL or GBM, the caller must not
* use drmPrimeHandleToFD nor drmPrimeFDToHandle. A single user-space
* reference-counting implementation is necessary to avoid double-closing GEM
* handles.
*
* Two processes can't share the same DRM FD and both use it to create or
* import GEM handles, even when using a single user-space reference-counting
* implementation like GBM, because GBM doesn't share its state between
* processes.
*/
extern int drmPrimeHandleToFD(int fd, uint32_t handle, uint32_t flags, int *prime_fd);
extern int drmPrimeFDToHandle(int fd, int prime_fd, uint32_t *handle);