Commit c11e391d authored by Daniel Vetter's avatar Daniel Vetter
Browse files

dma-buf: Add ioctls to allow userspace to flush 



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.
v9 (Tiago): remove useless is_dma_buf_file check. Fix sync.flags conditionals
and its mask order check. Add <linux/types.h> include in dma-buf.h.

Cc: Ville Syrjälä <ville.syrjala@linux.intel.com>
Cc: David Herrmann <dh.herrmann@gmail.com>
Cc: Sumit Semwal <sumit.semwal@linaro.org>
Reviewed-by: default avatarStéphane Marchesin <marcheu@chromium.org>
Signed-off-by: default avatarDaniel Vetter <daniel.vetter@intel.com>
Signed-off-by: default avatarTiago Vignatti <tiago.vignatti@intel.com>
Reviewed-by: default avatarDavid Herrmann <dh.herrmann@gmail.com>
Signed-off-by: default avatarDaniel Vetter <daniel.vetter@ffwll.ch>
Link: http://patchwork.freedesktop.org/patch/msgid/1455228291-29640-1-git-send-email-tiago.vignatti@intel.com
parent e94cb37b

Documentation/dma-buf-sharing.txt

+20 −1
Original line number Diff line number Diff line
@@ -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/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



    Therefore, 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.



2. Supporting existing mmap interfaces in importers

drivers/dma-buf/dma-buf.c

+45 −0
Original line number Diff line number Diff line
@@ -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 +253,54 @@ 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;



	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_VALID_FLAGS_MASK)

			return -EINVAL;



		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_RW:

			direction = DMA_BIDIRECTIONAL;

			break;

		default:

			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,

};



/*

include/uapi/linux/dma-buf.h

0 → 100644
+40 −0
Original line number Diff line number Diff line 
/*

 * 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_



#include <linux/types.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        (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)



#endif

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