dma_buf: Add documentation for the new cpu access support

v2: Fix spelling issues noticed by Rob Clark.

Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Signed-off-by: Rob Clark <rob@ti.com>
Signed-off-by: Sumit Semwal <sumit.semwal@linaro.org>
This commit is contained in:
Daniel Vetter 2012-03-19 00:34:27 +01:00 committed by Sumit Semwal
parent fc13020e08
commit b0b40f2484

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@ -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.