| Message ID | 20210617194258.579011-1-jason@jlekstrand.net (mailing list archive) |
|---|---|
| State | New, archived |
| Headers | show |
| Series | dma-buf: Document DMA_BUF_IOCTL_SYNC (v3) | expand |
On Thu, Jun 17, 2021 at 02:42:58PM -0500, Jason Ekstrand wrote: > This adds a new "DMA Buffer ioctls" section to the dma-buf docs and adds > documentation for DMA_BUF_IOCTL_SYNC. > > v2 (Daniel Vetter): > - Fix a couple typos > - Add commentary about synchronization with other devices > - Use item list format for describing flags > > v3 (Pekka Paalanen): > - Clarify stalling requirements. > - Be more clear that that DMA_BUF_IOCTL_SYNC with SINC_END has to be > called before more GPU work happens. > > Signed-off-by: Jason Ekstrand <jason@jlekstrand.net> > Reviewed-by: Daniel Vetter <daniel.vetter@ffwll.ch> > Acked-by: Christian König <christian.koenig@amd.com> > Acked-by: Pekka Paalanen <pekka.paalanen@collabora.com> > Cc: Sumit Semwal <sumit.semwal@linaro.org> Merged to drm-misc-next, thanks. -Daniel > --- > Documentation/driver-api/dma-buf.rst | 8 +++++ > include/uapi/linux/dma-buf.h | 50 +++++++++++++++++++++++++++- > 2 files changed, 57 insertions(+), 1 deletion(-) > > diff --git a/Documentation/driver-api/dma-buf.rst b/Documentation/driver-api/dma-buf.rst > index 7f21425d9435a..0d4c13ec1a800 100644 > --- a/Documentation/driver-api/dma-buf.rst > +++ b/Documentation/driver-api/dma-buf.rst > @@ -88,6 +88,9 @@ consider though: > - The DMA buffer FD is also pollable, see `Implicit Fence Poll Support`_ below for > details. > > +- The DMA buffer FD also supports a few dma-buf-specific ioctls, see > + `DMA Buffer ioctls`_ below for details. > + > Basic Operation and Device DMA Access > ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ > > @@ -106,6 +109,11 @@ Implicit Fence Poll Support > .. kernel-doc:: drivers/dma-buf/dma-buf.c > :doc: implicit fence polling > > +DMA Buffer ioctls > +~~~~~~~~~~~~~~~~~ > + > +.. kernel-doc:: include/uapi/linux/dma-buf.h > + > Kernel Functions and Structures Reference > ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ > > diff --git a/include/uapi/linux/dma-buf.h b/include/uapi/linux/dma-buf.h > index 7f30393b92c3b..8e4a2ca0bcbf7 100644 > --- a/include/uapi/linux/dma-buf.h > +++ b/include/uapi/linux/dma-buf.h > @@ -22,8 +22,56 @@ > > #include <linux/types.h> > > -/* begin/end dma-buf functions used for userspace mmap. */ > +/** > + * struct dma_buf_sync - Synchronize with CPU access. > + * > + * When a DMA buffer is accessed from the CPU via mmap, it is not always > + * possible to guarantee coherency between the CPU-visible map and underlying > + * memory. To manage coherency, DMA_BUF_IOCTL_SYNC must be used to bracket > + * any CPU access to give the kernel the chance to shuffle memory around if > + * needed. > + * > + * Prior to accessing the map, the client must call DMA_BUF_IOCTL_SYNC > + * with DMA_BUF_SYNC_START and the appropriate read/write flags. Once the > + * access is complete, the client should call DMA_BUF_IOCTL_SYNC with > + * DMA_BUF_SYNC_END and the same read/write flags. > + * > + * The synchronization provided via DMA_BUF_IOCTL_SYNC only provides cache > + * coherency. It does not prevent other processes or devices from > + * accessing the memory at the same time. If synchronization with a GPU or > + * other device driver is required, it is the client's responsibility to > + * wait for buffer to be ready for reading or writing before calling this > + * ioctl with DMA_BUF_SYNC_START. Likewise, the client must ensure that > + * follow-up work is not submitted to GPU or other device driver until > + * after this ioctl has been called with DMA_BUF_SYNC_END? > + * > + * If the driver or API with which the client is interacting uses implicit > + * synchronization, waiting for prior work to complete can be done via > + * poll() on the DMA buffer file descriptor. If the driver or API requires > + * explicit synchronization, the client may have to wait on a sync_file or > + * other synchronization primitive outside the scope of the DMA buffer API. > + */ > struct dma_buf_sync { > + /** > + * @flags: Set of access flags > + * > + * DMA_BUF_SYNC_START: > + * Indicates the start of a map access session. > + * > + * DMA_BUF_SYNC_END: > + * Indicates the end of a map access session. > + * > + * DMA_BUF_SYNC_READ: > + * Indicates that the mapped DMA buffer will be read by the > + * client via the CPU map. > + * > + * DMA_BUF_SYNC_WRITE: > + * Indicates that the mapped DMA buffer will be written by the > + * client via the CPU map. > + * > + * DMA_BUF_SYNC_RW: > + * An alias for DMA_BUF_SYNC_READ | DMA_BUF_SYNC_WRITE. > + */ > __u64 flags; > }; > > -- > 2.31.1 >
diff --git a/Documentation/driver-api/dma-buf.rst b/Documentation/driver-api/dma-buf.rst index 7f21425d9435a..0d4c13ec1a800 100644 --- a/Documentation/driver-api/dma-buf.rst +++ b/Documentation/driver-api/dma-buf.rst @@ -88,6 +88,9 @@ consider though: - The DMA buffer FD is also pollable, see `Implicit Fence Poll Support`_ below for details. +- The DMA buffer FD also supports a few dma-buf-specific ioctls, see + `DMA Buffer ioctls`_ below for details. + Basic Operation and Device DMA Access ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ @@ -106,6 +109,11 @@ Implicit Fence Poll Support .. kernel-doc:: drivers/dma-buf/dma-buf.c :doc: implicit fence polling +DMA Buffer ioctls +~~~~~~~~~~~~~~~~~ + +.. kernel-doc:: include/uapi/linux/dma-buf.h + Kernel Functions and Structures Reference ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ diff --git a/include/uapi/linux/dma-buf.h b/include/uapi/linux/dma-buf.h index 7f30393b92c3b..8e4a2ca0bcbf7 100644 --- a/include/uapi/linux/dma-buf.h +++ b/include/uapi/linux/dma-buf.h @@ -22,8 +22,56 @@ #include <linux/types.h> -/* begin/end dma-buf functions used for userspace mmap. */ +/** + * struct dma_buf_sync - Synchronize with CPU access. + * + * When a DMA buffer is accessed from the CPU via mmap, it is not always + * possible to guarantee coherency between the CPU-visible map and underlying + * memory. To manage coherency, DMA_BUF_IOCTL_SYNC must be used to bracket + * any CPU access to give the kernel the chance to shuffle memory around if + * needed. + * + * Prior to accessing the map, the client must call DMA_BUF_IOCTL_SYNC + * with DMA_BUF_SYNC_START and the appropriate read/write flags. Once the + * access is complete, the client should call DMA_BUF_IOCTL_SYNC with + * DMA_BUF_SYNC_END and the same read/write flags. + * + * The synchronization provided via DMA_BUF_IOCTL_SYNC only provides cache + * coherency. It does not prevent other processes or devices from + * accessing the memory at the same time. If synchronization with a GPU or + * other device driver is required, it is the client's responsibility to + * wait for buffer to be ready for reading or writing before calling this + * ioctl with DMA_BUF_SYNC_START. Likewise, the client must ensure that + * follow-up work is not submitted to GPU or other device driver until + * after this ioctl has been called with DMA_BUF_SYNC_END? + * + * If the driver or API with which the client is interacting uses implicit + * synchronization, waiting for prior work to complete can be done via + * poll() on the DMA buffer file descriptor. If the driver or API requires + * explicit synchronization, the client may have to wait on a sync_file or + * other synchronization primitive outside the scope of the DMA buffer API. + */ struct dma_buf_sync { + /** + * @flags: Set of access flags + * + * DMA_BUF_SYNC_START: + * Indicates the start of a map access session. + * + * DMA_BUF_SYNC_END: + * Indicates the end of a map access session. + * + * DMA_BUF_SYNC_READ: + * Indicates that the mapped DMA buffer will be read by the + * client via the CPU map. + * + * DMA_BUF_SYNC_WRITE: + * Indicates that the mapped DMA buffer will be written by the + * client via the CPU map. + * + * DMA_BUF_SYNC_RW: + * An alias for DMA_BUF_SYNC_READ | DMA_BUF_SYNC_WRITE. + */ __u64 flags; };