Thanks for reviewing, David. Please take a look in my comments in-line.
On 02/09/2016 07:26 AM, David Herrmann wrote:
On Tue, Dec 22, 2015 at 10:36 PM, Tiago Vignatti tiago.vignatti@intel.com wrote:
From: Daniel Vetter daniel.vetter@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. v7 (Tiago): Alex' nit on flags definition and being even more wording in the doc about sync usage.
Cc: Sumit Semwal sumit.semwal@linaro.org Signed-off-by: Daniel Vetter daniel.vetter@intel.com Signed-off-by: Tiago Vignatti tiago.vignatti@intel.com
Documentation/dma-buf-sharing.txt | 21 ++++++++++++++++++- drivers/dma-buf/dma-buf.c | 43 +++++++++++++++++++++++++++++++++++++++ include/uapi/linux/dma-buf.h | 38 ++++++++++++++++++++++++++++++++++ 3 files changed, 101 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..32ac32e 100644 --- a/Documentation/dma-buf-sharing.txt +++ b/Documentation/dma-buf-sharing.txt @@ -350,7 +350,26 @@ 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, making
sure that the cache synchronization ioctl (DMA_BUF_IOCTL_SYNC) is *always*
used when the access happens. This is discussed next paragraphs.
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/writeto mmap area 3. SYNC_END ioctl. This can be repeated as often as youwant (with the new data being consumed by the GPU or say scanout device)- munmap once you don't need the buffer any moreTherefore, for correctness and optimal performance, systems with the memory
cache shared by the GPU and CPU i.e. the "coherent" and also the
"incoherent" are always required to use SYNC_START and SYNC_END before and
after, respectively, when accessing the mapped address.
- 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;Why? This can never happen, and you better not use dma_buf_ioctl() outside of dma_buf_fops.. I guess it's simply copied from the other fop callbacks, but I don't see why. dma_buf_poll() doesn't do it, neither should this, or one of the other 3 callbacks.
yup. I fixed now.
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;elsereturn -EINVAL;This looks bogus. It always ends up being "DMA_BIDIRECTIONAL" or EINVAL. I recommend changing it to:
switch (sync.flags & DMA_BUF_SYNC_RW) { case DMA_BUF_SYNC_READ: direction = DMA_FROM_DEVICE; break; case DMA_BUF_SYNC_WRITE: direction = DMA_TO_DEVICE; break; case DMA_BUF_SYNC_READ: direction = DMA_BIDIRECTIONAL; break; default: return -EINVAL; }
hmm I can't really get what's wrong with my snip. Why bogus? Can you double-check actually your suggestion, cause that's wrong with _READ being repeated.
if (sync.flags & ~DMA_BUF_SYNC_VALID_FLAGS_MASK)return -EINVAL;Why isn't this done immediately after copy_from_user()?
done.
if (sync.flags & DMA_BUF_SYNC_END)dma_buf_end_cpu_access(dmabuf, direction);elsedma_buf_begin_cpu_access(dmabuf, direction);Why are SYNC_START and SYNC_END exclusive? It feels very natural to me to invoke both at the same time (especially if two objects are stored in the same dma-buf).
Can you open a bit and teach how two objects would be stored in the same dma-buf? I didn't care about this case and if we want that, we'd need also to change the sequence of accesses as described in the dma-buf-sharing.txt I'm proposing in this patch.
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..4a9b36b --- /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;+};
Please add '#include <linux/types.h>', otherwise this header cannot be compiled on its own (missing __u64).
done.
+#define DMA_BUF_SYNC_READ (1 << 0) +#define DMA_BUF_SYNC_WRITE (2 << 0) +#define DMA_BUF_SYNC_RW (DMA_BUF_SYNC_READ | DMA_BUF_SYNC_WRITE) +#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)
Why _IOW? A read-only ioctl should be able to call DMA_BUF_SYNC_READ, right?
yup. I've changed it to _IOWR now.
Tiago