From 158350aae16a4e666e261da0bb4d91c0601a3edd Mon Sep 17 00:00:00 2001 From: Simon Ser Date: Thu, 16 Feb 2023 13:09:45 +0000 Subject: drm: document DRM_IOCTL_GEM_CLOSE This is a bit tricky, because of the ref'counting considerations. See also [1] for more discussion about this topic. Since this is kernel docs, I've decided to elaborate a bit less on the user-space details. [1]: https://gitlab.freedesktop.org/mesa/drm/-/merge_requests/110 Signed-off-by: Simon Ser Cc: Daniel Vetter Cc: Pekka Paalanen Cc: Daniel Stone Reviewed-by: Daniel Vetter Acked-by: Pekka Paalanen Link: https://patchwork.freedesktop.org/patch/msgid/20230216130934.156541-2-contact@emersion.fr --- include/uapi/drm/drm.h | 13 +++++++++++++ 1 file changed, 13 insertions(+) (limited to 'include/uapi/drm') diff --git a/include/uapi/drm/drm.h b/include/uapi/drm/drm.h index 642808520d92..c39fefb54613 100644 --- a/include/uapi/drm/drm.h +++ b/include/uapi/drm/drm.h @@ -972,6 +972,19 @@ extern "C" { #define DRM_IOCTL_GET_STATS DRM_IOR( 0x06, struct drm_stats) #define DRM_IOCTL_SET_VERSION DRM_IOWR(0x07, struct drm_set_version) #define DRM_IOCTL_MODESET_CTL DRM_IOW(0x08, struct drm_modeset_ctl) +/** + * DRM_IOCTL_GEM_CLOSE - Close a GEM handle. + * + * GEM handles are not reference-counted by the kernel. User-space is + * responsible for managing their lifetime. For example, if user-space imports + * the same memory object twice on the same DRM file description, the same GEM + * handle is returned by both imports, and user-space needs to ensure + * &DRM_IOCTL_GEM_CLOSE is performed once only. The same situation can happen + * when a memory object is allocated, then exported and imported again on the + * same DRM file description. The &DRM_IOCTL_MODE_GETFB2 IOCTL is an exception + * and always returns fresh new GEM handles even if an existing GEM handle + * already refers to the same memory object before the IOCTL is performed. + */ #define DRM_IOCTL_GEM_CLOSE DRM_IOW (0x09, struct drm_gem_close) #define DRM_IOCTL_GEM_FLINK DRM_IOWR(0x0a, struct drm_gem_flink) #define DRM_IOCTL_GEM_OPEN DRM_IOWR(0x0b, struct drm_gem_open) -- cgit v1.2.3 From 61a55f8b1ebcde68bc7bfd57435b1b3bb1121b8f Mon Sep 17 00:00:00 2001 From: Simon Ser Date: Fri, 17 Feb 2023 16:22:01 +0000 Subject: drm: document expectations for GETFB2 handles There are two important details missing from the docs: - If the memory object backing the FB already has a GEM handle, it's not re-used, a new one is generated. - Aliased planes will return the same GEM handle. v2: document how user-space can obtain DMA-BUF FDs without leaking handles (Pekka) Signed-off-by: Simon Ser Cc: Daniel Vetter Cc: Pekka Paalanen Cc: Daniel Stone Acked-by: Pekka Paalanen Reviewed-by: Daniel Vetter Link: https://patchwork.freedesktop.org/patch/msgid/20230217162151.59996-1-contact@emersion.fr --- include/uapi/drm/drm.h | 14 ++++++++++++-- 1 file changed, 12 insertions(+), 2 deletions(-) (limited to 'include/uapi/drm') diff --git a/include/uapi/drm/drm.h b/include/uapi/drm/drm.h index c39fefb54613..292e4778a2f4 100644 --- a/include/uapi/drm/drm.h +++ b/include/uapi/drm/drm.h @@ -1117,8 +1117,13 @@ extern "C" { * struct as the output. * * If the client is DRM master or has &CAP_SYS_ADMIN, &drm_mode_fb_cmd2.handles - * will be filled with GEM buffer handles. Planes are valid until one has a - * zero handle -- this can be used to compute the number of planes. + * will be filled with GEM buffer handles. Fresh new GEM handles are always + * returned, even if another GEM handle referring to the same memory object + * already exists on the DRM file description. The caller is responsible for + * removing the new handles, e.g. via the &DRM_IOCTL_GEM_CLOSE IOCTL. The same + * new handle will be returned for multiple planes in case they use the same + * memory object. Planes are valid until one has a zero handle -- this can be + * used to compute the number of planes. * * Otherwise, &drm_mode_fb_cmd2.handles will be zeroed and planes are valid * until one has a zero &drm_mode_fb_cmd2.pitches. @@ -1126,6 +1131,11 @@ extern "C" { * If the framebuffer has a format modifier, &DRM_MODE_FB_MODIFIERS will be set * in &drm_mode_fb_cmd2.flags and &drm_mode_fb_cmd2.modifier will contain the * modifier. Otherwise, user-space must ignore &drm_mode_fb_cmd2.modifier. + * + * To obtain DMA-BUF FDs for each plane without leaking GEM handles, user-space + * can export each handle via &DRM_IOCTL_PRIME_HANDLE_TO_FD, then immediately + * close each unique handle via &DRM_IOCTL_GEM_CLOSE, making sure to not + * double-close handles which are specified multiple times in the array. */ #define DRM_IOCTL_MODE_GETFB2 DRM_IOWR(0xCE, struct drm_mode_fb_cmd2) -- cgit v1.2.3 From 6068771673a38efc31ed5b99645176b4d3a33129 Mon Sep 17 00:00:00 2001 From: Simon Ser Date: Fri, 17 Feb 2023 16:22:04 +0000 Subject: drm: document DRM_IOCTL_PRIME_HANDLE_TO_FD and PRIME_FD_TO_HANDLE v2: mention caps, note that the IOCTLs might fail, document that user-space needs a data structure to keep track of the handles (Daniel V.) Signed-off-by: Simon Ser Cc: Daniel Vetter Cc: Pekka Paalanen Cc: Daniel Stone Reviewed-by: Daniel Vetter Acked-by: Pekka Paalanen Link: https://patchwork.freedesktop.org/patch/msgid/20230217162151.59996-2-contact@emersion.fr --- include/uapi/drm/drm.h | 30 ++++++++++++++++++++++++++++++ 1 file changed, 30 insertions(+) (limited to 'include/uapi/drm') diff --git a/include/uapi/drm/drm.h b/include/uapi/drm/drm.h index 292e4778a2f4..a87bbbbca2d4 100644 --- a/include/uapi/drm/drm.h +++ b/include/uapi/drm/drm.h @@ -1025,7 +1025,37 @@ extern "C" { #define DRM_IOCTL_UNLOCK DRM_IOW( 0x2b, struct drm_lock) #define DRM_IOCTL_FINISH DRM_IOW( 0x2c, struct drm_lock) +/** + * DRM_IOCTL_PRIME_HANDLE_TO_FD - Convert a GEM handle to a DMA-BUF FD. + * + * User-space sets &drm_prime_handle.handle with the GEM handle to export and + * &drm_prime_handle.flags, and gets back a DMA-BUF file descriptor in + * &drm_prime_handle.fd. + * + * The export can fail for any driver-specific reason, e.g. because export is + * not supported for this specific GEM handle (but might be for others). + * + * Support for exporting DMA-BUFs is advertised via &DRM_PRIME_CAP_EXPORT. + */ #define DRM_IOCTL_PRIME_HANDLE_TO_FD DRM_IOWR(0x2d, struct drm_prime_handle) +/** + * DRM_IOCTL_PRIME_FD_TO_HANDLE - Convert a DMA-BUF FD to a GEM handle. + * + * User-space sets &drm_prime_handle.fd with a DMA-BUF file descriptor to + * import, and gets back a GEM handle in &drm_prime_handle.handle. + * &drm_prime_handle.flags is unused. + * + * If an existing GEM handle refers to the memory object backing the DMA-BUF, + * that GEM handle is returned. Therefore user-space which needs to handle + * arbitrary DMA-BUFs must have a user-space lookup data structure to manually + * reference-count duplicated GEM handles. For more information see + * &DRM_IOCTL_GEM_CLOSE. + * + * The import can fail for any driver-specific reason, e.g. because import is + * only supported for DMA-BUFs allocated on this DRM device. + * + * Support for importing DMA-BUFs is advertised via &DRM_PRIME_CAP_IMPORT. + */ #define DRM_IOCTL_PRIME_FD_TO_HANDLE DRM_IOWR(0x2e, struct drm_prime_handle) #define DRM_IOCTL_AGP_ACQUIRE DRM_IO( 0x30) -- cgit v1.2.3