On Sun, Mar 18, 2012 at 6:34 PM, Daniel Vetter <daniel.vetter@xxxxxxxx> wrote: > v2: Fix spelling issues noticed by Rob Clark. > > Signed-off-by: Daniel Vetter <daniel.vetter@xxxxxxxx> Signed-off-by: Rob Clark <rob@xxxxxx> > --- > Documentation/dma-buf-sharing.txt | 102 +++++++++++++++++++++++++++++++++++- > 1 files changed, 99 insertions(+), 3 deletions(-) > > diff --git a/Documentation/dma-buf-sharing.txt b/Documentation/dma-buf-sharing.txt > index 225f96d..9f3aeef 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: > > @@ -219,7 +223,99 @@ 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. > > -- > 1.7.7.5 > > -- > To unsubscribe from this list: send the line "unsubscribe linux-media" in > the body of a message to majordomo@xxxxxxxxxxxxxxx > More majordomo info at http://vger.kernel.org/majordomo-info.html -- To unsubscribe from this list: send the line "unsubscribe linux-media" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
