- 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.
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 --- Documentation/dma-buf-sharing.txt | 47 ----------------------------------- Documentation/driver-api/dma-buf.rst | 48 ++++++++++++++++++++++++++++++++++++ drivers/dma-buf/dma-buf.c | 22 +++++++++++++++++ 3 files changed, 70 insertions(+), 47 deletions(-) delete mode 100644 Documentation/dma-buf-sharing.txt
diff --git a/Documentation/dma-buf-sharing.txt b/Documentation/dma-buf-sharing.txt deleted file mode 100644 index 74c99edb7976..000000000000 --- a/Documentation/dma-buf-sharing.txt +++ /dev/null @@ -1,47 +0,0 @@ - DMA Buffer Sharing API Guide - ~~~~~~~~~~~~~~~~~~~~~~~~~~~~ - - Sumit Semwal - <sumit dot semwal at linaro dot org> - <sumit dot semwal at ti dot com> - - -Other Interfaces Exposed to Userspace on the dma-buf FD ------------------------------------------------------- - -- Since kernel 3.12 the dma-buf FD supports the llseek system call, but only - with offset=0 and whence=SEEK_END|SEEK_SET. SEEK_SET is supported to allow - the usual size discover pattern size = SEEK_END(0); SEEK_SET(0). Every other - llseek operation will report -EINVAL. - - If llseek on dma-buf FDs isn't support the kernel will report -ESPIPE for all - cases. Userspace can use this to detect support for discovering the dma-buf - size using llseek. - -Miscellaneous notes -------------------- - -- Any exporters or users of the dma-buf buffer sharing framework must have - a 'select DMA_SHARED_BUFFER' in their respective Kconfigs. - -- In order to avoid fd leaks on exec, the FD_CLOEXEC flag must be set - on the file descriptor. This is not just a resource leak, but a - potential security hole. It could give the newly exec'd application - access to buffers, via the leaked fd, to which it should otherwise - not be permitted access. - - The problem with doing this via a separate fcntl() call, versus doing it - atomically when the fd is created, is that this is inherently racy in a - multi-threaded app[3]. The issue is made worse when it is library code - opening/creating the file descriptor, as the application may not even be - aware of the fd's. - - To avoid this problem, userspace must have a way to request O_CLOEXEC - flag be set when the dma-buf fd is created. So any API provided by - the exporting driver to create a dmabuf fd must provide a way to let - userspace control setting of O_CLOEXEC flag passed in to dma_buf_fd(). - -References: -[1] struct dma_buf_ops in include/linux/dma-buf.h -[2] All interfaces mentioned above defined in include/linux/dma-buf.h -[3] https://lwn.net/Articles/236486/ diff --git a/Documentation/driver-api/dma-buf.rst b/Documentation/driver-api/dma-buf.rst index 92e417035e16..31671b469627 100644 --- a/Documentation/driver-api/dma-buf.rst +++ b/Documentation/driver-api/dma-buf.rst @@ -46,6 +46,48 @@ The buffer-user same area of memory. This interface is provided by :c:type:`struct dma_buf_attachment <dma_buf_attachment>`.
+Any exporters or users of the dma-buf buffer sharing framework must have a +'select DMA_SHARED_BUFFER' in their respective Kconfigs. + +Userspace Interface Notes +~~~~~~~~~~~~~~~~~~~~~~~~~ + +Mostly a DMA buffer file descriptor is simply an opaque object for userspace, +and hence the generic interface exposed is very minimal. There's a few things to +consider though: + +- Since kernel 3.12 the dma-buf FD supports the llseek system call, but only + with offset=0 and whence=SEEK_END|SEEK_SET. SEEK_SET is supported to allow + the usual size discover pattern size = SEEK_END(0); SEEK_SET(0). Every other + llseek operation will report -EINVAL. + + If llseek on dma-buf FDs isn't support the kernel will report -ESPIPE for all + cases. Userspace can use this to detect support for discovering the dma-buf + size using llseek. + +- In order to avoid fd leaks on exec, the FD_CLOEXEC flag must be set + on the file descriptor. This is not just a resource leak, but a + potential security hole. It could give the newly exec'd application + access to buffers, via the leaked fd, to which it should otherwise + not be permitted access. + + The problem with doing this via a separate fcntl() call, versus doing it + atomically when the fd is created, is that this is inherently racy in a + multi-threaded app[3]. The issue is made worse when it is library code + opening/creating the file descriptor, as the application may not even be + aware of the fd's. + + To avoid this problem, userspace must have a way to request O_CLOEXEC + flag be set when the dma-buf fd is created. So any API provided by + the exporting driver to create a dmabuf fd must provide a way to let + userspace control setting of O_CLOEXEC flag passed in to dma_buf_fd(). + +- Memory mapping the contents of the DMA buffer is also supported. See the + discussion below on `CPU Access to DMA Buffer Objects`_ for the full details. + +- The DMA buffer FD is also pollable, see `Fence Poll Support`_ below for + details. + Basic Operation and Device DMA Access ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
@@ -58,6 +100,12 @@ CPU Access to DMA Buffer Objects .. kernel-doc:: drivers/dma-buf/dma-buf.c :doc: cpu access
+Fence Poll Support +~~~~~~~~~~~~~~~~~~ + +.. kernel-doc:: drivers/dma-buf/dma-buf.c + :doc: fence polling + Kernel Functions and Structures Reference ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
diff --git a/drivers/dma-buf/dma-buf.c b/drivers/dma-buf/dma-buf.c index 0933c01e6d8c..0775fc947d73 100644 --- a/drivers/dma-buf/dma-buf.c +++ b/drivers/dma-buf/dma-buf.c @@ -124,6 +124,28 @@ static loff_t dma_buf_llseek(struct file *file, loff_t offset, int whence) return base + offset; }
+/** + * DOC: fence polling + * + * To support cross-device and cross-driver synchronization of buffer access + * implicit fences (represented internally in the kernel with struct &fence) can + * be attached to a &dma_buf. The glue for that and a few related things are + * provided in the &reservation_object structure. + * + * Userspace can query the state of these implicitly tracked fences using poll() + * and related system calls: + * + * - Checking for POLLIN, i.e. read access, can be use to query the state of the + * most recent write or exclusive fence. + * + * - Checking for POLLOUT, i.e. write access, can be used to query the state of + * all attached fences, shared and exclusive ones. + * + * Note that this only signals the completion of the respective fences, i.e. the + * DMA transfers are complete. Cache flushing and any other necessary + * preparations before CPU access can begin still need to happen. + */ + static void dma_buf_poll_cb(struct dma_fence *fence, struct dma_fence_cb *cb) { struct dma_buf_poll_cb_t *dcb = (struct dma_buf_poll_cb_t *)cb;