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-rw-r--r--drivers/dma-buf/dma-buf.c207
1 files changed, 202 insertions, 5 deletions
diff --git a/drivers/dma-buf/dma-buf.c b/drivers/dma-buf/dma-buf.c
index e72e64484131..718f832a5c71 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;
@@ -314,19 +336,52 @@ static inline int is_dma_buf_file(struct file *file)
}
/**
+ * DOC: dma buf device access
+ *
+ * For device DMA access to a shared DMA buffer the usual sequence of operations
+ * is fairly simple:
+ *
+ * 1. The exporter defines his exporter instance using
+ * DEFINE_DMA_BUF_EXPORT_INFO() and calls dma_buf_export() to wrap a private
+ * buffer object into a &dma_buf. It then exports that &dma_buf to userspace
+ * as a file descriptor by calling dma_buf_fd().
+ *
+ * 2. Userspace passes this file-descriptors to all drivers it wants this buffer
+ * to share with: First the filedescriptor is converted to a &dma_buf using
+ * dma_buf_get(). The the buffer is attached to the device using
+ * dma_buf_attach().
+ *
+ * Up to this stage the exporter is still free to migrate or reallocate the
+ * backing storage.
+ *
+ * 3. Once the buffer is attached to all devices userspace can inniate DMA
+ * access to the shared buffer. In the kernel this is done by calling
+ * dma_buf_map_attachment() and dma_buf_unmap_attachment().
+ *
+ * 4. Once a driver is done with a shared buffer it needs to call
+ * dma_buf_detach() (after cleaning up any mappings) and then release the
+ * reference acquired with dma_buf_get by calling dma_buf_put().
+ *
+ * For the detailed semantics exporters are expected to implement see
+ * &dma_buf_ops.
+ */
+
+/**
* dma_buf_export - Creates a new dma_buf, and associates an anon file
* with this buffer, so it can be exported.
* Also connect the allocator specific data and ops to the buffer.
* Additionally, provide a name string for exporter; useful in debugging.
*
* @exp_info: [in] holds all the export related information provided
- * by the exporter. see struct dma_buf_export_info
+ * by the exporter. see &struct dma_buf_export_info
* for further details.
*
* Returns, on success, a newly created dma_buf object, which wraps the
* supplied private data and operations for dma_buf_ops. On either missing
* ops, or error in allocating struct dma_buf, will return negative error.
*
+ * For most cases the easiest way to create @exp_info is through the
+ * %DEFINE_DMA_BUF_EXPORT_INFO macro.
*/
struct dma_buf *dma_buf_export(const struct dma_buf_export_info *exp_info)
{
@@ -458,7 +513,11 @@ EXPORT_SYMBOL_GPL(dma_buf_get);
* dma_buf_put - decreases refcount of the buffer
* @dmabuf: [in] buffer to reduce refcount of
*
- * Uses file's refcounting done implicitly by fput()
+ * Uses file's refcounting done implicitly by fput().
+ *
+ * If, as a result of this call, the refcount becomes 0, the 'release' file
+ * operation related to this fd is called. It calls &dma_buf_ops.release vfunc
+ * in turn, and frees the memory allocated for dmabuf when exported.
*/
void dma_buf_put(struct dma_buf *dmabuf)
{
@@ -475,8 +534,17 @@ EXPORT_SYMBOL_GPL(dma_buf_put);
* @dmabuf: [in] buffer to attach device to.
* @dev: [in] device to be attached.
*
- * Returns struct dma_buf_attachment * for this attachment; returns ERR_PTR on
- * error.
+ * Returns struct dma_buf_attachment pointer for this attachment. Attachments
+ * must be cleaned up by calling dma_buf_detach().
+ *
+ * Returns:
+ *
+ * A pointer to newly created &dma_buf_attachment on success, or a negative
+ * error code wrapped into a pointer on failure.
+ *
+ * Note that this can fail if the backing storage of @dmabuf is in a place not
+ * accessible to @dev, and cannot be moved to a more suitable place. This is
+ * indicated with the error code -EBUSY.
*/
struct dma_buf_attachment *dma_buf_attach(struct dma_buf *dmabuf,
struct device *dev)
@@ -519,6 +587,7 @@ EXPORT_SYMBOL_GPL(dma_buf_attach);
* @dmabuf: [in] buffer to detach from.
* @attach: [in] attachment to be detached; is free'd after this call.
*
+ * Clean up a device attachment obtained by calling dma_buf_attach().
*/
void dma_buf_detach(struct dma_buf *dmabuf, struct dma_buf_attachment *attach)
{
@@ -543,7 +612,12 @@ EXPORT_SYMBOL_GPL(dma_buf_detach);
* @direction: [in] direction of DMA transfer
*
* Returns sg_table containing the scatterlist to be returned; returns ERR_PTR
- * on error.
+ * on error. May return -EINTR if it is interrupted by a signal.
+ *
+ * A mapping must be unmapped again using dma_buf_map_attachment(). Note that
+ * the underlying backing storage is pinned for as long as a mapping exists,
+ * therefore users/importers should not hold onto a mapping for undue amounts of
+ * time.
*/
struct sg_table *dma_buf_map_attachment(struct dma_buf_attachment *attach,
enum dma_data_direction direction)
@@ -571,6 +645,7 @@ EXPORT_SYMBOL_GPL(dma_buf_map_attachment);
* @sg_table: [in] scatterlist info of the buffer to unmap
* @direction: [in] direction of DMA transfer
*
+ * This unmaps a DMA mapping for @attached obtained by dma_buf_map_attachment().
*/
void dma_buf_unmap_attachment(struct dma_buf_attachment *attach,
struct sg_table *sg_table,
@@ -586,6 +661,122 @@ void dma_buf_unmap_attachment(struct dma_buf_attachment *attach,
}
EXPORT_SYMBOL_GPL(dma_buf_unmap_attachment);
+/**
+ * DOC: cpu access
+ *
+ * There are mutliple reasons for supporting CPU access to a dma buffer object:
+ *
+ * - Fallback operations in the kernel, for example when a device is connected
+ * over USB and the kernel needs to shuffle the data around first before
+ * sending it away. Cache coherency is handled by braketing any transactions
+ * with calls to dma_buf_begin_cpu_access() and dma_buf_end_cpu_access()
+ * access.
+ *
+ * To support dma_buf objects residing in highmem cpu access is page-based
+ * using an api similar to kmap. Accessing a dma_buf is done in aligned chunks
+ * of PAGE_SIZE size. Before accessing a chunk it needs to be mapped, which
+ * returns a pointer in kernel virtual address space. Afterwards the chunk
+ * needs to be unmapped again. There is no limit on how often a given chunk
+ * can be mapped and unmapped, i.e. the importer does not need to call
+ * begin_cpu_access again before mapping the same chunk again.
+ *
+ * Interfaces::
+ * void \*dma_buf_kmap(struct dma_buf \*, unsigned long);
+ * void dma_buf_kunmap(struct dma_buf \*, unsigned long, void \*);
+ *
+ * There are also atomic variants of these interfaces. Like for kmap they
+ * facilitate non-blocking fast-paths. Neither the importer nor the exporter
+ * (in the callback) is allowed to block when using these.
+ *
+ * Interfaces::
+ * void \*dma_buf_kmap_atomic(struct dma_buf \*, unsigned long);
+ * void dma_buf_kunmap_atomic(struct dma_buf \*, unsigned long, void \*);
+ *
+ * For importers all the restrictions of using kmap apply, like the limited
+ * supply of kmap_atomic slots. Hence an importer shall only hold onto at
+ * max 2 atomic dma_buf kmaps at the same time (in any given process context).
+ *
+ * dma_buf kmap calls outside of the range specified in begin_cpu_access are
+ * undefined. If the range is not PAGE_SIZE aligned, kmap needs to succeed on
+ * the partial chunks at the beginning and end but may return stale or bogus
+ * data outside of the range (in these partial chunks).
+ *
+ * Note that these calls need to always succeed. The exporter needs to
+ * complete any preparations that might fail in begin_cpu_access.
+ *
+ * For some cases the overhead of kmap can be too high, a vmap interface
+ * is introduced. This interface should be used very carefully, as vmalloc
+ * space is a limited resources on many architectures.
+ *
+ * Interfaces::
+ * void \*dma_buf_vmap(struct dma_buf \*dmabuf)
+ * void dma_buf_vunmap(struct dma_buf \*dmabuf, void \*vaddr)
+ *
+ * The vmap call can fail if there is no vmap support in the exporter, or if
+ * it runs out of vmalloc space. Fallback to kmap should be implemented. Note
+ * that the dma-buf layer keeps a reference count for all vmap access and
+ * calls down into the exporter's vmap function only when no vmapping exists,
+ * and only unmaps it once. Protection against concurrent vmap/vunmap calls is
+ * provided by taking the dma_buf->lock mutex.
+ *
+ * - For full compatibility on the importer side with existing userspace
+ * interfaces, which might already support mmap'ing buffers. This is needed in
+ * many processing pipelines (e.g. feeding a software rendered image into a
+ * hardware pipeline, thumbnail creation, snapshots, ...). Also, Android's ION
+ * framework already supported this and for DMA buffer file descriptors to
+ * replace ION buffers mmap support was needed.
+ *
+ * There is no special interfaces, userspace simply calls mmap on the dma-buf
+ * fd. But like for CPU access there's a need to braket the actual access,
+ * which is handled by the ioctl (DMA_BUF_IOCTL_SYNC). Note that
+ * DMA_BUF_IOCTL_SYNC can fail with -EAGAIN or -EINTR, in which case it must
+ * be restarted.
+ *
+ * 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 say the GPU or the scanout
+ * device)
+ * - munmap once you don't need the buffer any more
+ *
+ * For correctness and optimal performance, it is always required to use
+ * SYNC_START and SYNC_END before and after, respectively, when accessing the
+ * mapped address. Userspace cannot rely on coherent access, even when there
+ * are systems where it just works without calling these ioctls.
+ *
+ * - And as a CPU fallback in userspace processing pipelines.
+ *
+ * Similar to the motivation for kernel cpu access it is again important that
+ * the userspace code of a given importing subsystem can use the same
+ * interfaces with a imported dma-buf buffer object as with a native buffer
+ * object. This is especially important for drm where the userspace part of
+ * contemporary OpenGL, X, and other drivers is huge, and reworking them to
+ * use a different way to mmap a buffer rather invasive.
+ *
+ * The assumption in the current dma-buf interfaces is that redirecting the
+ * initial mmap is all that's needed. A survey of some of the existing
+ * subsystems shows that no driver seems to do any nefarious thing like
+ * syncing up with outstanding asynchronous processing on the device or
+ * allocating special resources at fault time. So hopefully this is good
+ * enough, since adding interfaces to intercept pagefaults and allow pte
+ * shootdowns would increase the complexity quite a bit.
+ *
+ * Interface::
+ * int dma_buf_mmap(struct dma_buf \*, struct vm_area_struct \*,
+ * unsigned long);
+ *
+ * If the importing subsystem simply provides a special-purpose mmap call to
+ * set up a mapping in userspace, calling do_mmap with dma_buf->file will
+ * equally achieve that for a dma-buf object.
+ */
+
static int __dma_buf_begin_cpu_access(struct dma_buf *dmabuf,
enum dma_data_direction direction)
{
@@ -611,6 +802,10 @@ static int __dma_buf_begin_cpu_access(struct dma_buf *dmabuf,
* @dmabuf: [in] buffer to prepare cpu access for.
* @direction: [in] length of range for cpu access.
*
+ * After the cpu access is complete the caller should call
+ * dma_buf_end_cpu_access(). Only when cpu access is braketed by both calls is
+ * it guaranteed to be coherent with other DMA access.
+ *
* Can return negative error values, returns 0 on success.
*/
int dma_buf_begin_cpu_access(struct dma_buf *dmabuf,
@@ -643,6 +838,8 @@ EXPORT_SYMBOL_GPL(dma_buf_begin_cpu_access);
* @dmabuf: [in] buffer to complete cpu access for.
* @direction: [in] length of range for cpu access.
*
+ * This terminates CPU access started with dma_buf_begin_cpu_access().
+ *
* Can return negative error values, returns 0 on success.
*/
int dma_buf_end_cpu_access(struct dma_buf *dmabuf,