Re: [Linaro-mm-sig] [RFC/PATCH] fb: Add dma-buf support

Add support for the dma-buf exporter role to the frame buffer API. The
importer role isn't meaningful for frame buffer devices, as the frame
buffer device model doesn't allow using externally allocated memory.

Signed-off-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
---
 Documentation/fb/api.txt |   36 ++++++++++++++++++++++++++++++++++++
 drivers/video/fbmem.c    |   36 ++++++++++++++++++++++++++++++++++++
 include/linux/fb.h       |   12 ++++++++++++
 3 files changed, 84 insertions(+), 0 deletions(-)

diff --git a/Documentation/fb/api.txt b/Documentation/fb/api.txt
index d4ff7de..f0b2173 100644
--- a/Documentation/fb/api.txt
+++ b/Documentation/fb/api.txt

@@ -304,3 +304,39 @@ extensions.

 Upon successful format configuration, drivers update the fb_fix_screeninfo
 type, visual and line_length fields depending on the selected format. The type
 and visual fields are set to FB_TYPE_FOURCC and FB_VISUAL_FOURCC respectively.
+
+
+5. DMA buffer sharing
+---------------------
+
+The dma-buf kernel framework allows DMA buffers to be shared across devices
+and applications. Sharing buffers across display devices and video capture or
+video decoding devices allow zero-copy operation when displaying video content
+produced by a hardware device such as a camera or a hardware codec. This is
+crucial to achieve optimal system performances during video display.
+
+While dma-buf supports both exporting internally allocated memory as a dma-buf
+object (known as the exporter role) and importing a dma-buf object to be used
+as device memory (known as the importer role), the frame buffer API only
+supports the exporter role, as the frame buffer device model doesn't support
+using externally-allocated memory.
+
+The export a frame buffer as a dma-buf file descriptors, applications call the

+FBIOGET_DMABUF ioctl. The ioctl takes a pointer to a fb_dmabuf_export
+structure.
+
+struct fb_dmabuf_export {
+       __u32 fd;
+       __u32 flags;
+};
+
+The flag field specifies the flags to be used when creating the dma-buf file
+descriptor. The only supported flag is O_CLOEXEC. If the call is successful,
+the driver will set the fd field to a file descriptor corresponding to the
+dma-buf object.
+
+Applications can then pass the file descriptors to another application or
+another device driver. The dma-buf object is automatically reference-counted,
+applications can and should close the file descriptor as soon as they don't
+need it anymore. The underlying dma-buf object will not be freed before the
+last device that uses the dma-buf object releases it.

diff --git a/drivers/video/fbmem.c b/drivers/video/fbmem.c
index 0dff12a..400e449 100644
--- a/drivers/video/fbmem.c
+++ b/drivers/video/fbmem.c

@@ -15,6 +15,7 @@

 #include <linux/compat.h>
 #include <linux/types.h>
+#include <linux/dma-buf.h>
 #include <linux/errno.h>
 #include <linux/kernel.h>
 #include <linux/major.h>

@@ -1074,6 +1075,23 @@ fb_blank(struct fb_info *info, int blank)

       return ret;
 }

+#ifdef CONFIG_DMA_SHARED_BUFFER
+int
+fb_get_dmabuf(struct fb_info *info, int flags)
+{
+       struct dma_buf *dmabuf;
+
+       if (info->fbops->fb_dmabuf_export = NULL)
+               return -ENOTTY;
+
+       dmabuf = info->fbops->fb_dmabuf_export(info);
+       if (IS_ERR(dmabuf))
+               return PTR_ERR(dmabuf);
+
+       return dma_buf_fd(dmabuf, flags);
+}
+#endif
+
 static long do_fb_ioctl(struct fb_info *info, unsigned int cmd,
                       unsigned long arg)
 {

@@ -1084,6 +1102,7 @@ static long do_fb_ioctl(struct fb_info *info, unsigned int cmd,

       struct fb_cmap cmap_from;
       struct fb_cmap_user cmap;
       struct fb_event event;
+       struct fb_dmabuf_export dmaexp;
       void __user *argp = (void __user *)arg;
       long ret = 0;

@@ -1191,6 +1210,23 @@ static long do_fb_ioctl(struct fb_info *info, unsigned int cmd,

               console_unlock();
               unlock_fb_info(info);
               break;
+#ifdef CONFIG_DMA_SHARED_BUFFER
+       case FBIOGET_DMABUF:
+               if (copy_from_user(&dmaexp, argp, sizeof(dmaexp)))
+                       return -EFAULT;
+
+               if (!lock_fb_info(info))
+                       return -ENODEV;
+               dmaexp.fd = fb_get_dmabuf(info, dmaexp.flags);
+               unlock_fb_info(info);
+
+               if (dmaexp.fd < 0)
+                       return dmaexp.fd;
+
+               ret = copy_to_user(argp, &dmaexp, sizeof(dmaexp))
+                   ? -EFAULT : 0;
+               break;
+#endif
       default:
               if (!lock_fb_info(info))
                       return -ENODEV;

diff --git a/include/linux/fb.h b/include/linux/fb.h
index ac3f1c6..c9fee75 100644
--- a/include/linux/fb.h
+++ b/include/linux/fb.h

@@ -39,6 +39,7 @@

 #define FBIOPUT_MODEINFO        0x4617
 #define FBIOGET_DISPINFO        0x4618
 #define FBIO_WAITFORVSYNC      _IOW('F', 0x20, __u32)
+#define FBIOGET_DMABUF         _IOR('F', 0x21, struct fb_dmabuf_export)

 #define FB_TYPE_PACKED_PIXELS          0       /* Packed Pixels        */
 #define FB_TYPE_PLANES                 1       /* Non interleaved planes */

@@ -403,6 +404,11 @@ struct fb_cursor {

 #define FB_BACKLIGHT_MAX       0xFF
 #endif

+struct fb_dmabuf_export {
+       __u32 fd;
+       __u32 flags;
+};
+
 #ifdef __KERNEL__

 #include <linux/fs.h>

@@ -418,6 +424,7 @@ struct vm_area_struct;

 struct fb_info;
 struct device;
 struct file;
+struct dma_buf;

 /* Definitions below are used in the parsed monitor specs */
 #define FB_DPMS_ACTIVE_OFF     1

@@ -701,6 +708,11 @@ struct fb_ops {

       /* called at KDB enter and leave time to prepare the console */
       int (*fb_debug_enter)(struct fb_info *info);
       int (*fb_debug_leave)(struct fb_info *info);
+
+#ifdef CONFIG_DMA_SHARED_BUFFER
+       /* Export the frame buffer as a dmabuf object */
+       struct dma_buf *(*fb_dmabuf_export)(struct fb_info *info);
+#endif
 };

 #ifdef CONFIG_FB_TILEBLITTING
--
Regards,

Laurent Pinchart


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