[PATCH v6 3/5] dma-buf: Add ioctls to allow userspace to flush

[Tiago Vignatti]

From: Daniel Vetter <daniel.vetter at ffwll.ch>

The userspace might need some sort of cache coherency management e.g. when CPU
and GPU domains are being accessed through dma-buf at the same time. To
circumvent this problem there are begin/end coherency markers, that forward
directly to existing dma-buf device drivers vfunc hooks. Userspace can make use
of those markers through the DMA_BUF_IOCTL_SYNC ioctl. The sequence would be
used like following:
     - mmap dma-buf fd
     - for each drawing/upload cycle in CPU 1. SYNC_START ioctl, 2. read/write
       to mmap area 3. SYNC_END ioctl. This can be repeated as often as you
       want (with the new data being consumed by the GPU or say scanout device)
     - munmap once you don't need the buffer any more

v2 (Tiago): Fix header file type names (u64 -> __u64)
v3 (Tiago): Add documentation. Use enum dma_buf_sync_flags to the begin/end
dma-buf functions. Check for overflows in start/length.
v4 (Tiago): use 2d regions for sync.
v5 (Tiago): forget about 2d regions (v4); use _IOW in DMA_BUF_IOCTL_SYNC and
remove range information from struct dma_buf_sync.
v6 (Tiago): use __u64 structured padded flags instead enum. Adjust
documentation about the recommendation on using sync ioctls.

Cc: Sumit Semwal <sumit.semwal at linaro.org>
Signed-off-by: Daniel Vetter <daniel.vetter at intel.com>
Signed-off-by: Tiago Vignatti <tiago.vignatti at intel.com>
---
 Documentation/dma-buf-sharing.txt | 22 +++++++++++++++++++-
 drivers/dma-buf/dma-buf.c         | 43 +++++++++++++++++++++++++++++++++++++++
 include/uapi/linux/dma-buf.h      | 38 ++++++++++++++++++++++++++++++++++
 3 files changed, 102 insertions(+), 1 deletion(-)
 create mode 100644 include/uapi/linux/dma-buf.h

diff --git a/Documentation/dma-buf-sharing.txt b/Documentation/dma-buf-sharing.txt
index 4f4a84b..2ddd4b2 100644
--- a/Documentation/dma-buf-sharing.txt
+++ b/Documentation/dma-buf-sharing.txt
@@ -350,7 +350,27 @@ Being able to mmap an export dma-buf buffer object has 2 main use-cases:
    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.
+   No special interfaces, userspace simply calls mmap on the dma-buf fd. Very
+   important to note though is that, even if it is not mandatory, the userspace
+   is strongly recommended to always use the cache synchronization ioctl
+   (DMA_BUF_IOCTL_SYNC) discussed next.
+
+   Some systems might need some sort of cache coherency management e.g. when
+   CPU and GPU domains are being accessed through dma-buf at the same time. To
+   circumvent this problem there are begin/end coherency markers, that forward
+   directly to existing dma-buf device drivers vfunc hooks. Userspace can make
+   use of those markers through the DMA_BUF_IOCTL_SYNC ioctl. The sequence
+   would be used like following:
+     - mmap dma-buf fd
+     - for each drawing/upload cycle in CPU 1. SYNC_START ioctl, 2. read/write
+       to mmap area 3. SYNC_END ioctl. This can be repeated as often as you
+       want (with the new data being consumed by the GPU or say scanout device)
+     - munmap once you don't need the buffer any more
+
+    In principle systems with the memory cache shared by the GPU and CPU may
+    not need SYNC_START and SYNC_END but still, userspace is always encouraged
+    to use these ioctls before and after, respectively, when accessing the
+    mapped address.
 
 2. Supporting existing mmap interfaces in importers
 
diff --git a/drivers/dma-buf/dma-buf.c b/drivers/dma-buf/dma-buf.c
index b2ac13b..9a298bd 100644
--- a/drivers/dma-buf/dma-buf.c
+++ b/drivers/dma-buf/dma-buf.c
@@ -34,6 +34,8 @@
 #include <linux/poll.h>
 #include <linux/reservation.h>
 
+#include <uapi/linux/dma-buf.h>
+
 static inline int is_dma_buf_file(struct file *);
 
 struct dma_buf_list {
@@ -251,11 +253,52 @@ out:
 	return events;
 }
 
+static long dma_buf_ioctl(struct file *file,
+			  unsigned int cmd, unsigned long arg)
+{
+	struct dma_buf *dmabuf;
+	struct dma_buf_sync sync;
+	enum dma_data_direction direction;
+
+	dmabuf = file->private_data;
+
+	if (!is_dma_buf_file(file))
+		return -EINVAL;
+
+	switch (cmd) {
+	case DMA_BUF_IOCTL_SYNC:
+		if (copy_from_user(&sync, (void __user *) arg, sizeof(sync)))
+			return -EFAULT;
+
+		if (sync.flags & DMA_BUF_SYNC_RW)
+			direction = DMA_BIDIRECTIONAL;
+		else if (sync.flags & DMA_BUF_SYNC_READ)
+			direction = DMA_FROM_DEVICE;
+		else if (sync.flags & DMA_BUF_SYNC_WRITE)
+			direction = DMA_TO_DEVICE;
+		else
+			return -EINVAL;
+
+		if (sync.flags & ~DMA_BUF_SYNC_VALID_FLAGS_MASK)
+			return -EINVAL;
+
+		if (sync.flags & DMA_BUF_SYNC_END)
+			dma_buf_end_cpu_access(dmabuf, direction);
+		else
+			dma_buf_begin_cpu_access(dmabuf, direction);
+
+		return 0;
+	default:
+		return -ENOTTY;
+	}
+}
+
 static const struct file_operations dma_buf_fops = {
 	.release	= dma_buf_release,
 	.mmap		= dma_buf_mmap_internal,
 	.llseek		= dma_buf_llseek,
 	.poll		= dma_buf_poll,
+	.unlocked_ioctl	= dma_buf_ioctl,
 };
 
 /*
diff --git a/include/uapi/linux/dma-buf.h b/include/uapi/linux/dma-buf.h
new file mode 100644
index 0000000..bd195f2
--- /dev/null
+++ b/include/uapi/linux/dma-buf.h
@@ -0,0 +1,38 @@
+/*
+ * Framework for buffer objects that can be shared across devices/subsystems.
+ *
+ * Copyright(C) 2015 Intel Ltd
+ *
+ * This program is free software; you can redistribute it and/or modify it
+ * under the terms of the GNU General Public License version 2 as published by
+ * the Free Software Foundation.
+ *
+ * This program is distributed in the hope that it will be useful, but WITHOUT
+ * ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or
+ * FITNESS FOR A PARTICULAR PURPOSE.  See the GNU General Public License for
+ * more details.
+ *
+ * You should have received a copy of the GNU General Public License along with
+ * this program.  If not, see <http://www.gnu.org/licenses/>.
+ */
+
+#ifndef _DMA_BUF_UAPI_H_
+#define _DMA_BUF_UAPI_H_
+
+/* begin/end dma-buf functions used for userspace mmap. */
+struct dma_buf_sync {
+	__u64 flags;
+};
+
+#define DMA_BUF_SYNC_READ      (1 << 0)
+#define DMA_BUF_SYNC_WRITE     (2 << 0)
+#define DMA_BUF_SYNC_RW        (3 << 0)
+#define DMA_BUF_SYNC_START     (0 << 2)
+#define DMA_BUF_SYNC_END       (1 << 2)
+#define DMA_BUF_SYNC_VALID_FLAGS_MASK \
+	(DMA_BUF_SYNC_RW | DMA_BUF_SYNC_END)
+
+#define DMA_BUF_BASE		'b'
+#define DMA_BUF_IOCTL_SYNC	_IOW(DMA_BUF_BASE, 0, struct dma_buf_sync)
+
+#endif
-- 
2.1.4