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Subject[PATCH 3/4] dma_buf: Add documentation for the new cpu access support
Date
v2: Fix spelling issues noticed by Rob Clark.

Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>
---
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


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