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The DMA Buffer synchronization API provides buffer synchronization
mechanism based on DMA buffer sharing machanism[1], dmafence and
reservation frameworks[2];
i.e., buffer access control to CPU and DMA, and easy-to-use interfaces
for device drivers and user application. And this API can be used
for all dma devices using system memory as dma buffer, especially
for most ARM based SoCs.
For more details, please refer to Documentation/dma-buf-syc.txt
[1] http://lwn.net/Articles/470339/
[2] https://lkml.org/lkml/2014/2/24/824
Change-Id: I3b2084a3c331fc06992fa8d2a4c71378e88b10b5
Signed-off-by: Inki Dae <inki.dae@samsung.com>
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This patch adds a buffer synchronization framework based on DMA BUF[1]
and and based on ww-mutexes[2] for lock mechanism.
The purpose of this framework is to provide not only buffer access control
to CPU and DMA but also easy-to-use interfaces for device drivers and
user application. This framework can be used for all dma devices using
system memory as dma buffer, especially for most ARM based SoCs.
Changelog v5:
- Rmove a dependence on reservation_object: the reservation_object is used
to hook up to ttm and dma-buf for easy sharing of reservations across
devices. However, the dmabuf sync can be used for all dma devices; v4l2
and drm based drivers, so doesn't need the reservation_object anymore.
With regared to this, it adds 'void *sync' to dma_buf structure.
- All patches are rebased on mainline, Linux v3.10.
Changelog v4:
- Add user side interface for buffer synchronization mechanism and update
descriptions related to the user side interface.
Changelog v3:
- remove cache operation relevant codes and update document file.
Changelog v2:
- use atomic_add_unless to avoid potential bug.
- add a macro for checking valid access type.
- code clean.
The mechanism of this framework has the following steps,
1. Register dmabufs to a sync object - A task gets a new sync object and
can add one or more dmabufs that the task wants to access.
This registering should be performed when a device context or an event
context such as a page flip event is created or before CPU accesses a shared
buffer.
dma_buf_sync_get(a sync object, a dmabuf);
2. Lock a sync object - A task tries to lock all dmabufs added in its own
sync object. Basically, the lock mechanism uses ww-mutex[1] to avoid dead
lock issue and for race condition between CPU and CPU, CPU and DMA, and DMA
and DMA. Taking a lock means that others cannot access all locked dmabufs
until the task that locked the corresponding dmabufs, unlocks all the locked
dmabufs.
This locking should be performed before DMA or CPU accesses these dmabufs.
dma_buf_sync_lock(a sync object);
3. Unlock a sync object - The task unlocks all dmabufs added in its own sync
object. The unlock means that the DMA or CPU accesses to the dmabufs have
been completed so that others may access them.
This unlocking should be performed after DMA or CPU has completed accesses
to the dmabufs.
dma_buf_sync_unlock(a sync object);
4. Unregister one or all dmabufs from a sync object - A task unregisters
the given dmabufs from the sync object. This means that the task dosen't
want to lock the dmabufs.
The unregistering should be performed after DMA or CPU has completed
accesses to the dmabufs or when dma_buf_sync_lock() is failed.
dma_buf_sync_put(a sync object, a dmabuf);
dma_buf_sync_put_all(a sync object);
The described steps may be summarized as:
get -> lock -> CPU or DMA access to a buffer/s -> unlock -> put
This framework includes the following two features.
1. read (shared) and write (exclusive) locks - A task is required to declare
the access type when the task tries to register a dmabuf;
READ, WRITE, READ DMA, or WRITE DMA.
The below is example codes,
struct dmabuf_sync *sync;
sync = dmabuf_sync_init(NULL, "test sync");
dmabuf_sync_get(sync, dmabuf, DMA_BUF_ACCESS_R);
...
And the below can be used as access types:
DMA_BUF_ACCESS_R - CPU will access a buffer for read.
DMA_BUF_ACCESS_W - CPU will access a buffer for read or write.
DMA_BUF_ACCESS_DMA_R - DMA will access a buffer for read
DMA_BUF_ACCESS_DMA_W - DMA will access a buffer for read or
write.
2. Mandatory resource releasing - a task cannot hold a lock indefinitely.
A task may never try to unlock a buffer after taking a lock to the buffer.
In this case, a timer handler to the corresponding sync object is called
in five (default) seconds and then the timed-out buffer is unlocked by work
queue handler to avoid lockups and to enforce resources of the buffer.
The below is how to use interfaces for device driver:
1. Allocate and Initialize a sync object:
struct dmabuf_sync *sync;
sync = dmabuf_sync_init(NULL, "test sync");
...
2. Add a dmabuf to the sync object when setting up dma buffer relevant
registers:
dmabuf_sync_get(sync, dmabuf, DMA_BUF_ACCESS_READ);
...
3. Lock all dmabufs of the sync object before DMA or CPU accesses
the dmabufs:
dmabuf_sync_lock(sync);
...
4. Now CPU or DMA can access all dmabufs locked in step 3.
5. Unlock all dmabufs added in a sync object after DMA or CPU access
to these dmabufs is completed:
dmabuf_sync_unlock(sync);
And call the following functions to release all resources,
dmabuf_sync_put_all(sync);
dmabuf_sync_fini(sync);
You can refer to actual example codes:
"drm/exynos: add dmabuf sync support for g2d driver" and
"drm/exynos: add dmabuf sync support for kms framework" from
https://git.kernel.org/cgit/linux/kernel/git/daeinki/
drm-exynos.git/log/?h=dmabuf-sync
And this framework includes fcntl system call[3] as interfaces exported
to user. As you know, user sees a buffer object as a dma-buf file descriptor.
So fcntl() call with the file descriptor means to lock some buffer region being
managed by the dma-buf object.
The below is how to use interfaces for user application:
struct flock filelock;
1. Lock a dma buf:
filelock.l_type = F_WRLCK or F_RDLCK;
/* lock entire region to the dma buf. */
filelock.lwhence = SEEK_CUR;
filelock.l_start = 0;
filelock.l_len = 0;
fcntl(dmabuf fd, F_SETLKW or F_SETLK, &filelock);
...
CPU access to the dma buf
2. Unlock a dma buf:
filelock.l_type = F_UNLCK;
fcntl(dmabuf fd, F_SETLKW or F_SETLK, &filelock);
close(dmabuf fd) call would also unlock the dma buf. And for more
detail, please refer to [3]
References:
[1] http://lwn.net/Articles/470339/
[2] https://patchwork.kernel.org/patch/2625361/
[3] http://linux.die.net/man/2/fcntl
Signed-off-by: Inki Dae <inki.dae@samsung.com>
Signed-off-by: Kyungmin Park <kyungmin.park@samsung.com>
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