dma-buf: Add ioctls to allow userspace to flush
The userspace might need some sort of cache coherency management e.g. when CPU
and GPU domains are being accessed through dma-buf at the same time. To
circumvent this problem there are begin/end coherency markers, that forward
directly to existing dma-buf device drivers vfunc hooks. Userspace can make use
of those markers through the DMA_BUF_IOCTL_SYNC ioctl. The sequence would be
used like following:
- mmap dma-buf fd
- for each drawing/upload cycle in CPU 1. SYNC_START ioctl, 2. read/write
to mmap area 3. SYNC_END ioctl. This can be repeated as often as you
want (with the new data being consumed by the GPU or say scanout device)
- munmap once you don't need the buffer any more
v2 (Tiago): Fix header file type names (u64 -> __u64)
v3 (Tiago): Add documentation. Use enum dma_buf_sync_flags to the begin/end
dma-buf functions. Check for overflows in start/length.
v4 (Tiago): use 2d regions for sync.
v5 (Tiago): forget about 2d regions (v4); use _IOW in DMA_BUF_IOCTL_SYNC and
remove range information from struct dma_buf_sync.
v6 (Tiago): use __u64 structured padded flags instead enum. Adjust
documentation about the recommendation on using sync ioctls.
v7 (Tiago): Alex' nit on flags definition and being even more wording in the
doc about sync usage.
v9 (Tiago): remove useless is_dma_buf_file check. Fix sync.flags conditionals
and its mask order check. Add <linux/types.h> include in dma-buf.h.
Cc: Ville Syrjälä <ville.syrjala@linux.intel.com>
Cc: David Herrmann <dh.herrmann@gmail.com>
Cc: Sumit Semwal <sumit.semwal@linaro.org>
Reviewed-by: Stéphane Marchesin <marcheu@chromium.org>
Signed-off-by: Daniel Vetter <daniel.vetter@intel.com>
Signed-off-by: Tiago Vignatti <tiago.vignatti@intel.com>
Reviewed-by: David Herrmann <dh.herrmann@gmail.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Link: http://patchwork.freedesktop.org/patch/msgid/1455228291-29640-1-git-send-email-tiago.vignatti@intel.com
diff --git a/Documentation/dma-buf-sharing.txt b/Documentation/dma-buf-sharing.txt
index 4f4a84b..32ac32e 100644
--- a/Documentation/dma-buf-sharing.txt
+++ b/Documentation/dma-buf-sharing.txt
@@ -350,7 +350,26 @@
handles, too). So it's beneficial to support this in a similar fashion on
dma-buf to have a good transition path for existing Android userspace.
- No special interfaces, userspace simply calls mmap on the dma-buf fd.
+ No special interfaces, userspace simply calls mmap on the dma-buf fd, making
+ sure that the cache synchronization ioctl (DMA_BUF_IOCTL_SYNC) is *always*
+ used when the access happens. This is discussed next paragraphs.
+
+ Some systems might need some sort of cache coherency management e.g. when
+ CPU and GPU domains are being accessed through dma-buf at the same time. To
+ circumvent this problem there are begin/end coherency markers, that forward
+ directly to existing dma-buf device drivers vfunc hooks. Userspace can make
+ use of those markers through the DMA_BUF_IOCTL_SYNC ioctl. The sequence
+ would be used like following:
+ - mmap dma-buf fd
+ - for each drawing/upload cycle in CPU 1. SYNC_START ioctl, 2. read/write
+ to mmap area 3. SYNC_END ioctl. This can be repeated as often as you
+ want (with the new data being consumed by the GPU or say scanout device)
+ - munmap once you don't need the buffer any more
+
+ Therefore, for correctness and optimal performance, systems with the memory
+ cache shared by the GPU and CPU i.e. the "coherent" and also the
+ "incoherent" are always required to use SYNC_START and SYNC_END before and
+ after, respectively, when accessing the mapped address.
2. Supporting existing mmap interfaces in importers