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-rw-r--r--Documentation/dma-buf-sharing.txt109
1 files changed, 102 insertions, 7 deletions
diff --git a/Documentation/dma-buf-sharing.txt b/Documentation/dma-buf-sharing.txt
index 3bbd5c5..ad86fb8 100644
--- a/Documentation/dma-buf-sharing.txt
+++ b/Documentation/dma-buf-sharing.txt
@@ -29,13 +29,6 @@ The buffer-user
in memory, mapped into its own address space, so it can access the same area
of memory.
-*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.
-- 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
--------------------------------------
@@ -300,6 +293,17 @@ Access to a dma_buf from the kernel context involves three steps:
Note that these calls need to always succeed. The exporter needs to complete
any preparations that might fail in begin_cpu_access.
+ For some cases the overhead of kmap can be too high, a vmap interface
+ is introduced. This interface should be used very carefully, as vmalloc
+ space is a limited resources on many architectures.
+
+ Interfaces:
+ void *dma_buf_vmap(struct dma_buf *dmabuf)
+ void dma_buf_vunmap(struct dma_buf *dmabuf, void *vaddr)
+
+ The vmap call can fail if there is no vmap support in the exporter, or if it
+ runs out of vmalloc space. Fallback to kmap should be implemented.
+
3. Finish access
When the importer is done accessing the range specified in begin_cpu_access,
@@ -313,6 +317,83 @@ Access to a dma_buf from the kernel context involves three steps:
enum dma_data_direction dir);
+Direct Userspace Access/mmap Support
+------------------------------------
+
+Being able to mmap an export dma-buf buffer object has 2 main use-cases:
+- CPU fallback processing in a pipeline and
+- supporting existing mmap interfaces in importers.
+
+1. CPU fallback processing in a pipeline
+
+ In many processing pipelines it is sometimes required that the cpu can access
+ the data in a dma-buf (e.g. for thumbnail creation, snapshots, ...). To avoid
+ the need to handle this specially in userspace frameworks for buffer sharing
+ it's ideal if the dma_buf fd itself can be used to access the backing storage
+ from userspace using mmap.
+
+ Furthermore Android's ION framework already supports this (and is otherwise
+ rather similar to dma-buf from a userspace consumer side with using fds as
+ handles, too). So it's beneficial to support this in a similar fashion on
+ dma-buf to have a good transition path for existing Android userspace.
+
+ No special interfaces, userspace simply calls mmap on the dma-buf fd.
+
+2. Supporting existing mmap interfaces in exporters
+
+ Similar to the motivation for kernel cpu access it is again important that
+ the userspace code of a given importing subsystem can use the same interfaces
+ with a imported dma-buf buffer object as with a native buffer object. This is
+ especially important for drm where the userspace part of contemporary OpenGL,
+ X, and other drivers is huge, and reworking them to use a different way to
+ mmap a buffer rather invasive.
+
+ The assumption in the current dma-buf interfaces is that redirecting the
+ initial mmap is all that's needed. A survey of some of the existing
+ subsystems shows that no driver seems to do any nefarious thing like syncing
+ up with outstanding asynchronous processing on the device or allocating
+ special resources at fault time. So hopefully this is good enough, since
+ adding interfaces to intercept pagefaults and allow pte shootdowns would
+ increase the complexity quite a bit.
+
+ Interface:
+ int dma_buf_mmap(struct dma_buf *, struct vm_area_struct *,
+ unsigned long);
+
+ If the importing subsystem simply provides a special-purpose mmap call to set
+ up a mapping in userspace, calling do_mmap with dma_buf->file will equally
+ achieve that for a dma-buf object.
+
+3. Implementation notes for exporters
+
+ Because dma-buf buffers have invariant size over their lifetime, the dma-buf
+ core checks whether a vma is too large and rejects such mappings. The
+ exporter hence does not need to duplicate this check.
+
+ Because existing importing subsystems might presume coherent mappings for
+ userspace, the exporter needs to set up a coherent mapping. If that's not
+ possible, it needs to fake coherency by manually shooting down ptes when
+ leaving the cpu domain and flushing caches at fault time. Note that all the
+ dma_buf files share the same anon inode, hence the exporter needs to replace
+ the dma_buf file stored in vma->vm_file with it's own if pte shootdown is
+ requred. This is because the kernel uses the underlying inode's address_space
+ for vma tracking (and hence pte tracking at shootdown time with
+ unmap_mapping_range).
+
+ If the above shootdown dance turns out to be too expensive in certain
+ scenarios, we can extend dma-buf with a more explicit cache tracking scheme
+ for userspace mappings. But the current assumption is that using mmap is
+ always a slower path, so some inefficiencies should be acceptable.
+
+ Exporters that shoot down mappings (for any reasons) shall not do any
+ synchronization at fault time with outstanding device operations.
+ Synchronization is an orthogonal issue to sharing the backing storage of a
+ buffer and hence should not be handled by dma-buf itself. This is explictly
+ mentioned here because many people seem to want something like this, but if
+ different exporters handle this differently, buffer sharing can fail in
+ interesting ways depending upong the exporter (if userspace starts depending
+ upon this implicit synchronization).
+
Miscellaneous notes
-------------------
@@ -336,6 +417,20 @@ Miscellaneous notes
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().
+- If an exporter needs to manually flush caches and hence needs to fake
+ coherency for mmap support, it needs to be able to zap all the ptes pointing
+ at the backing storage. Now linux mm needs a struct address_space associated
+ with the struct file stored in vma->vm_file to do that with the function
+ unmap_mapping_range. But the dma_buf framework only backs every dma_buf fd
+ with the anon_file struct file, i.e. all dma_bufs share the same file.
+
+ Hence exporters need to setup their own file (and address_space) association
+ by setting vma->vm_file and adjusting vma->vm_pgoff in the dma_buf mmap
+ callback. In the specific case of a gem driver the exporter could use the
+ shmem file already provided by gem (and set vm_pgoff = 0). Exporters can then
+ zap ptes by unmapping the corresponding range of the struct address_space
+ associated with their own file.
+
References:
[1] struct dma_buf_ops in include/linux/dma-buf.h
[2] All interfaces mentioned above defined in include/linux/dma-buf.h