dma-buf: Final bits of doc polish
- Put all the remaing bits of the old doc into suitable places in the new sphinx world. - Also document the poll support, we forgot to do that. - Delete dma-buf-sharing.txt. v2: Don't forget to update MAINTAINERS. Cc: linux-doc@vger.kernel.org Cc: Jonathan Corbet <corbet@lwn.net> Cc: Sumit Semwal <sumit.semwal@linaro.org> Signed-off-by: Daniel Vetter <daniel.vetter@intel.com> Signed-off-by: Sumit Semwal <sumit.semwal@linaro.org> Link: http://patchwork.freedesktop.org/patch/msgid/20161209215055.3492-1-daniel.vetter@ffwll.ch
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Daniel Vetter
Sumit Semwal
parent
0959a1683d
commit
e7e21c72b1
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@ -1,47 +0,0 @@
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DMA Buffer Sharing API Guide
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~~~~~~~~~~~~~~~~~~~~~~~~~~~~
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Sumit Semwal
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<sumit dot semwal at linaro dot org>
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<sumit dot semwal at ti dot com>
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Other Interfaces Exposed to Userspace on the dma-buf FD
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------------------------------------------------------
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- Since kernel 3.12 the dma-buf FD supports the llseek system call, but only
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with offset=0 and whence=SEEK_END|SEEK_SET. SEEK_SET is supported to allow
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the usual size discover pattern size = SEEK_END(0); SEEK_SET(0). Every other
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llseek operation will report -EINVAL.
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If llseek on dma-buf FDs isn't support the kernel will report -ESPIPE for all
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cases. Userspace can use this to detect support for discovering the dma-buf
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size using llseek.
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Miscellaneous notes
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-------------------
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- Any exporters or users of the dma-buf buffer sharing framework must have
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a 'select DMA_SHARED_BUFFER' in their respective Kconfigs.
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- In order to avoid fd leaks on exec, the FD_CLOEXEC flag must be set
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on the file descriptor. This is not just a resource leak, but a
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potential security hole. It could give the newly exec'd application
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access to buffers, via the leaked fd, to which it should otherwise
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not be permitted access.
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The problem with doing this via a separate fcntl() call, versus doing it
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atomically when the fd is created, is that this is inherently racy in a
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multi-threaded app[3]. The issue is made worse when it is library code
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opening/creating the file descriptor, as the application may not even be
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aware of the fd's.
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To avoid this problem, userspace must have a way to request O_CLOEXEC
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flag be set when the dma-buf fd is created. So any API provided by
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the exporting driver to create a dmabuf fd must provide a way to let
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userspace control setting of O_CLOEXEC flag passed in to dma_buf_fd().
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References:
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[1] struct dma_buf_ops in include/linux/dma-buf.h
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[2] All interfaces mentioned above defined in include/linux/dma-buf.h
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[3] https://lwn.net/Articles/236486/
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@ -46,6 +46,48 @@ The buffer-user
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same area of memory. This interface is provided by :c:type:`struct
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dma_buf_attachment <dma_buf_attachment>`.
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Any exporters or users of the dma-buf buffer sharing framework must have a
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'select DMA_SHARED_BUFFER' in their respective Kconfigs.
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Userspace Interface Notes
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~~~~~~~~~~~~~~~~~~~~~~~~~
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Mostly a DMA buffer file descriptor is simply an opaque object for userspace,
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and hence the generic interface exposed is very minimal. There's a few things to
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consider though:
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- Since kernel 3.12 the dma-buf FD supports the llseek system call, but only
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with offset=0 and whence=SEEK_END|SEEK_SET. SEEK_SET is supported to allow
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the usual size discover pattern size = SEEK_END(0); SEEK_SET(0). Every other
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llseek operation will report -EINVAL.
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If llseek on dma-buf FDs isn't support the kernel will report -ESPIPE for all
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cases. Userspace can use this to detect support for discovering the dma-buf
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size using llseek.
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- In order to avoid fd leaks on exec, the FD_CLOEXEC flag must be set
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on the file descriptor. This is not just a resource leak, but a
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potential security hole. It could give the newly exec'd application
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access to buffers, via the leaked fd, to which it should otherwise
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not be permitted access.
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The problem with doing this via a separate fcntl() call, versus doing it
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atomically when the fd is created, is that this is inherently racy in a
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multi-threaded app[3]. The issue is made worse when it is library code
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opening/creating the file descriptor, as the application may not even be
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aware of the fd's.
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To avoid this problem, userspace must have a way to request O_CLOEXEC
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flag be set when the dma-buf fd is created. So any API provided by
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the exporting driver to create a dmabuf fd must provide a way to let
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userspace control setting of O_CLOEXEC flag passed in to dma_buf_fd().
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- Memory mapping the contents of the DMA buffer is also supported. See the
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discussion below on `CPU Access to DMA Buffer Objects`_ for the full details.
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- The DMA buffer FD is also pollable, see `Fence Poll Support`_ below for
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details.
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Basic Operation and Device DMA Access
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~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
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@ -58,6 +100,12 @@ CPU Access to DMA Buffer Objects
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.. kernel-doc:: drivers/dma-buf/dma-buf.c
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:doc: cpu access
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Fence Poll Support
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~~~~~~~~~~~~~~~~~~
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.. kernel-doc:: drivers/dma-buf/dma-buf.c
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:doc: fence polling
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Kernel Functions and Structures Reference
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~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
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@ -3917,7 +3917,7 @@ F: drivers/dma-buf/
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F: include/linux/dma-buf*
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F: include/linux/reservation.h
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F: include/linux/*fence.h
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F: Documentation/dma-buf-sharing.txt
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F: Documentation/driver-api/dma-buf.rst
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T: git git://anongit.freedesktop.org/drm/drm-misc
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SYNC FILE FRAMEWORK
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@ -124,6 +124,28 @@ static loff_t dma_buf_llseek(struct file *file, loff_t offset, int whence)
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return base + offset;
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}
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/**
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* DOC: fence polling
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*
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* To support cross-device and cross-driver synchronization of buffer access
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* implicit fences (represented internally in the kernel with struct &fence) can
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* be attached to a &dma_buf. The glue for that and a few related things are
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* provided in the &reservation_object structure.
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*
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* Userspace can query the state of these implicitly tracked fences using poll()
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* and related system calls:
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*
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* - Checking for POLLIN, i.e. read access, can be use to query the state of the
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* most recent write or exclusive fence.
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*
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* - Checking for POLLOUT, i.e. write access, can be used to query the state of
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* all attached fences, shared and exclusive ones.
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*
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* Note that this only signals the completion of the respective fences, i.e. the
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* DMA transfers are complete. Cache flushing and any other necessary
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* preparations before CPU access can begin still need to happen.
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*/
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static void dma_buf_poll_cb(struct dma_fence *fence, struct dma_fence_cb *cb)
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{
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struct dma_buf_poll_cb_t *dcb = (struct dma_buf_poll_cb_t *)cb;
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