On Fri, Mar 2, 2012 at 6:23 PM, Sakari Ailus <sakari.ailus@xxxxxx> wrote: > Hi Daniel, > > Thanks for the patch. > > On Thu, Mar 01, 2012 at 04:36:01PM +0100, Daniel Vetter wrote: >> Signed-off-by: Daniel Vetter <daniel.vetter@xxxxxxxx> >> --- >> 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..f12542b 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 transperancy 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. > > Where it should be decided which operations are being done to the buffer > when it is passed to user space and back to kernel space? > > How about spliting these operations to those done on the first time the > buffer is passed to the user space (mapping to kernel address space, for > example) and those required every time buffer is passed from kernel to user > and back (cache flusing)? > > I'm asking since any unnecessary time-consuming operations, especially as > heavy as mapping the buffer, should be avoidable in subsystems dealing > with streaming video, cameras etc., i.e. non-GPU users. Well, this is really something for the buffer exporter to deal with.. since there is no way for an importer to create a userspace mmap'ing of the buffer. A lot of these expensive operations go away if you don't even create a userspace virtual mapping in the first place ;-) BR, -R > >> +1. Prepare acces >> + >> + Before an importer can acces a dma_buf object with the cpu from the kernel >> + context, it needs to notice 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 temporaray 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 unmmapped, 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_acces, >> + 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. > > Kind regards, > > -- > Sakari Ailus > e-mail: sakari.ailus@xxxxxx jabber/XMPP/Gmail: sailus@xxxxxxxxxxxxxx > > _______________________________________________ > Linaro-mm-sig mailing list > Linaro-mm-sig@xxxxxxxxxxxxxxxx > http://lists.linaro.org/mailman/listinfo/linaro-mm-sig -- 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
