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Diffstat (limited to 'Documentation')
| -rw-r--r-- | Documentation/dma-buf-sharing.txt | 120 |
1 files changed, 117 insertions, 3 deletions
diff --git a/Documentation/dma-buf-sharing.txt b/Documentation/dma-buf-sharing.txt index 225f96d88f5..3bbd5c51605 100644 --- a/Documentation/dma-buf-sharing.txt +++ b/Documentation/dma-buf-sharing.txt @@ -32,8 +32,12 @@ The buffer-user *IMPORTANT*: [see https://lkml.org/lkml/2011/12/20/211 for more details] For this first version, A buffer shared using the dma_buf sharing API: - *may* be exported to user space using "mmap" *ONLY* by exporter, outside of - this framework. -- may be used *ONLY* by importers that do not need CPU access to the buffer. + this framework. +- with this new iteration of the dma-buf api cpu access from the kernel has been + enable, see below for the details. + +dma-buf operations for device dma only +-------------------------------------- The dma_buf buffer sharing API usage contains the following steps: @@ -219,10 +223,120 @@ NOTES: If the exporter chooses not to allow an attach() operation once a map_dma_buf() API has been called, it simply returns an error. -Miscellaneous notes: +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, + size_t start, size_t len, + 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 given range and access direction. The range and access + direction can be used by the exporter to optimize the cache flushing, i.e. + access outside of the range or 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. + +3. Finish access + + When the importer is done accessing the range specified in begin_cpu_access, + it needs to announce this to the exporter (to facilitate cache flushing and + unpinning of any pinned resources). The result of of any dma_buf kmap calls + after end_cpu_access is undefined. + + Interface: + void dma_buf_end_cpu_access(struct dma_buf *dma_buf, + size_t start, size_t len, + enum dma_data_direction dir); + + +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/ |