diff options
Diffstat (limited to 'Documentation/dma-buf-sharing.txt')
| -rw-r--r-- | Documentation/dma-buf-sharing.txt | 213 |
1 files changed, 0 insertions, 213 deletions
diff --git a/Documentation/dma-buf-sharing.txt b/Documentation/dma-buf-sharing.txt index dca2fb7ac3b4..74c99edb7976 100644 --- a/Documentation/dma-buf-sharing.txt +++ b/Documentation/dma-buf-sharing.txt @@ -6,205 +6,6 @@ <sumit dot semwal at ti dot com> -Kernel cpu access to a dma-buf buffer object --------------------------------------------- - -The motivation to allow cpu access from the kernel to a dma-buf object from the -importers side are: -- fallback operations, e.g. if the devices is connected to a usb bus and the - kernel needs to shuffle the data around first before sending it away. -- full transparency for existing users on the importer side, i.e. userspace - should not notice the difference between a normal object from that subsystem - and an imported one backed by a dma-buf. This is really important for drm - opengl drivers that expect to still use all the existing upload/download - paths. - -Access to a dma_buf from the kernel context involves three steps: - -1. Prepare access, which invalidate any necessary caches and make the object - available for cpu access. -2. Access the object page-by-page with the dma_buf map apis -3. Finish access, which will flush any necessary cpu caches and free reserved - resources. - -1. Prepare access - - Before an importer can access a dma_buf object with the cpu from the kernel - context, it needs to notify the exporter of the access that is about to - happen. - - Interface: - int dma_buf_begin_cpu_access(struct dma_buf *dmabuf, - enum dma_data_direction direction) - - This allows the exporter to ensure that the memory is actually available for - cpu access - the exporter might need to allocate or swap-in and pin the - backing storage. The exporter also needs to ensure that cpu access is - coherent for the access direction. The direction can be used by the exporter - to optimize the cache flushing, i.e. access with a different direction (read - instead of write) might return stale or even bogus data (e.g. when the - exporter needs to copy the data to temporary storage). - - This step might fail, e.g. in oom conditions. - -2. Accessing the buffer - - 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 most 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. - -3. Finish access - - When the importer is done accessing the CPU, it needs to announce this to - the exporter (to facilitate cache flushing and unpinning of any pinned - resources). The result of any dma_buf kmap calls after end_cpu_access is - undefined. - - Interface: - void dma_buf_end_cpu_access(struct dma_buf *dma_buf, - enum dma_data_direction dir); - - -Direct Userspace Access/mmap Support ------------------------------------- - -Being able to mmap an export dma-buf buffer object has 2 main use-cases: -- CPU fallback processing in a pipeline and -- supporting existing mmap interfaces in importers. - -1. CPU fallback processing in a pipeline - - In many processing pipelines it is sometimes required that the cpu can access - the data in a dma-buf (e.g. for thumbnail creation, snapshots, ...). To avoid - the need to handle this specially in userspace frameworks for buffer sharing - it's ideal if the dma_buf fd itself can be used to access the backing storage - from userspace using mmap. - - Furthermore Android's ION framework already supports this (and is otherwise - rather similar to dma-buf from a userspace consumer side with using fds as - 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, making - sure that the cache synchronization ioctl (DMA_BUF_IOCTL_SYNC) is *always* - used when the access happens. 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 the GPU or say 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. - -2. Supporting existing mmap interfaces in importers - - 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. - -3. Implementation notes for exporters - - Because dma-buf buffers have invariant size over their lifetime, the dma-buf - core checks whether a vma is too large and rejects such mappings. The - exporter hence does not need to duplicate this check. - - Because existing importing subsystems might presume coherent mappings for - userspace, the exporter needs to set up a coherent mapping. If that's not - possible, it needs to fake coherency by manually shooting down ptes when - leaving the cpu domain and flushing caches at fault time. Note that all the - dma_buf files share the same anon inode, hence the exporter needs to replace - the dma_buf file stored in vma->vm_file with it's own if pte shootdown is - required. This is because the kernel uses the underlying inode's address_space - for vma tracking (and hence pte tracking at shootdown time with - unmap_mapping_range). - - If the above shootdown dance turns out to be too expensive in certain - scenarios, we can extend dma-buf with a more explicit cache tracking scheme - for userspace mappings. But the current assumption is that using mmap is - always a slower path, so some inefficiencies should be acceptable. - - Exporters that shoot down mappings (for any reasons) shall not do any - synchronization at fault time with outstanding device operations. - Synchronization is an orthogonal issue to sharing the backing storage of a - buffer and hence should not be handled by dma-buf itself. This is explicitly - mentioned here because many people seem to want something like this, but if - different exporters handle this differently, buffer sharing can fail in - interesting ways depending upong the exporter (if userspace starts depending - upon this implicit synchronization). - Other Interfaces Exposed to Userspace on the dma-buf FD ------------------------------------------------------ @@ -240,20 +41,6 @@ Miscellaneous notes 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(). -- If an exporter needs to manually flush caches and hence needs to fake - coherency for mmap support, it needs to be able to zap all the ptes pointing - at the backing storage. Now linux mm needs a struct address_space associated - with the struct file stored in vma->vm_file to do that with the function - unmap_mapping_range. But the dma_buf framework only backs every dma_buf fd - with the anon_file struct file, i.e. all dma_bufs share the same file. - - Hence exporters need to setup their own file (and address_space) association - by setting vma->vm_file and adjusting vma->vm_pgoff in the dma_buf mmap - callback. In the specific case of a gem driver the exporter could use the - shmem file already provided by gem (and set vm_pgoff = 0). Exporters can then - zap ptes by unmapping the corresponding range of the struct address_space - associated with their own file. - References: [1] struct dma_buf_ops in include/linux/dma-buf.h [2] All interfaces mentioned above defined in include/linux/dma-buf.h |
