diff options
Diffstat (limited to 'drivers/dma-buf/dma-buf.c')
| -rw-r--r-- | drivers/dma-buf/dma-buf.c | 208 |
1 files changed, 203 insertions, 5 deletions
diff --git a/drivers/dma-buf/dma-buf.c b/drivers/dma-buf/dma-buf.c index e72e64484131..91aff74ed092 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 +513,12 @@ 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 the release operation of + * struct &dma_buf_ops in turn, and frees the memory allocated for dmabuf when + * exported. */ void dma_buf_put(struct dma_buf *dmabuf) { @@ -475,8 +535,17 @@ 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 +588,7 @@ 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 +613,12 @@ 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 +646,7 @@ 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 +662,122 @@ 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 +803,10 @@ 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 +839,8 @@ 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, |