From 46bfd438c57c197485b3757f6328bcf4a05fdc13 Mon Sep 17 00:00:00 2001
From: Jason Ekstrand <jason@jlekstrand.net>
Date: Thu, 17 Jun 2021 14:42:58 -0500
Subject: dma-buf: Document DMA_BUF_IOCTL_SYNC (v3)
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Git-commit: 51f52547df700819db0d0e2b17b677cb209212b4
Patch-mainline: v5.15-rc1
References: jsc#PED-1166 jsc#PED-1168 jsc#PED-1170 jsc#PED-1218 jsc#PED-1220 jsc#PED-1222 jsc#PED-1223 jsc#PED-1225
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>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Link: https://patchwork.freedesktop.org/patch/msgid/20210617194258.579011-1-jason@jlekstrand.net
Acked-by: Patrik Jakobsson <pjakobsson@suse.de>
---
include/uapi/linux/dma-buf.h | 50 +++++++++++++++++++++++++++++++++++-
1 file changed, 49 insertions(+), 1 deletion(-)
diff --git a/include/uapi/linux/dma-buf.h b/include/uapi/linux/dma-buf.h
index 7f30393b92c3..8e4a2ca0bcbf 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;
};
--
2.38.1