media: core: Report the maximum possible number of buffers for the queue

Use one of the struct v4l2_create_buffers reserved bytes to report
the maximum possible number of buffers for the queue.
V4l2 framework set V4L2_BUF_CAP_SUPPORTS_MAX_NUM_BUFFERS flags in queue
capabilities so userland can know when the field is valid.
Does the same change in v4l2_create_buffers32 structure.

Signed-off-by: Benjamin Gaignard <[e-mail]>
Signed-off-by: Hans Verkuil <[e-mail]>
Signed-off-by: Mauro Carvalho Chehab <[e-mail]>

Documentation/userspace-api/media/v4l/vidioc-create-bufs.rst

···
       - ``flags``
       - Specifies additional buffer management attributes.
 	See :ref:`memory-flags`.
-
     * - __u32
-      - ``reserved``\ [6]
+      - ``max_num_buffers``
+      - If the V4L2_BUF_CAP_SUPPORTS_MAX_NUM_BUFFERS capability flag is set
+        this field indicates the maximum possible number of buffers
+        for this queue.
+    * - __u32
+      - ``reserved``\ [5]
       - A place holder for future extensions. Drivers and applications
 	must set the array to zero.
 

Documentation/userspace-api/media/v4l/vidioc-reqbufs.rst

···
 .. _V4L2-BUF-CAP-SUPPORTS-ORPHANED-BUFS:
 .. _V4L2-BUF-CAP-SUPPORTS-M2M-HOLD-CAPTURE-BUF:
 .. _V4L2-BUF-CAP-SUPPORTS-MMAP-CACHE-HINTS:
+.. _V4L2-BUF-CAP-SUPPORTS-MAX-NUM-BUFFERS:
 
 .. raw:: latex
 

drivers/media/common/videobuf2/videobuf2-v4l2.c

···
 	fill_buf_caps(q, &create->capabilities);
 	validate_memory_flags(q, create->memory, &create->flags);
 	create->index = vb2_get_num_buffers(q);
+	create->max_num_buffers = q->max_num_buffers;
+	create->capabilities |= V4L2_BUF_CAP_SUPPORTS_MAX_NUM_BUFFERS;
 	if (create->count == 0)
 		return ret != -EBUSY ? ret : 0;
 

drivers/media/v4l2-core/v4l2-compat-ioctl32.c

···
  * @flags:	additional buffer management attributes (ignored unless the
  *		queue has V4L2_BUF_CAP_SUPPORTS_MMAP_CACHE_HINTS capability and
  *		configured for MMAP streaming I/O).
+ * @max_num_buffers: if V4L2_BUF_CAP_SUPPORTS_MAX_NUM_BUFFERS capability flag is set
+ *		this field indicate the maximum possible number of buffers
+ *		for this queue.
  * @reserved:	future extensions
  */
 struct v4l2_create_buffers32 {
···
 	struct v4l2_format32	format;
 	__u32			capabilities;
 	__u32			flags;
-	__u32			reserved[6];
+	__u32			max_num_buffers;
+	__u32			reserved[5];
 };
 
 static int get_v4l2_format32(struct v4l2_format *p64,
···
 		return -EFAULT;
 	if (copy_from_user(&p64->flags, &p32->flags, sizeof(p32->flags)))
 		return -EFAULT;
+	if (copy_from_user(&p64->max_num_buffers, &p32->max_num_buffers,
+			   sizeof(p32->max_num_buffers)))
+		return -EFAULT;
 	return get_v4l2_format32(&p64->format, &p32->format);
 }
 
···
 			 offsetof(struct v4l2_create_buffers32, format)) ||
 	    put_user(p64->capabilities, &p32->capabilities) ||
 	    put_user(p64->flags, &p32->flags) ||
+	    put_user(p64->max_num_buffers, &p32->max_num_buffers) ||
 	    copy_to_user(p32->reserved, p64->reserved, sizeof(p64->reserved)))
 		return -EFAULT;
 	return put_v4l2_format32(&p64->format, &p32->format);

drivers/media/v4l2-core/v4l2-ioctl.c

···
 {
 	const struct v4l2_create_buffers *p = arg;
 
-	pr_cont("index=%d, count=%d, memory=%s, capabilities=0x%08x, ",
+	pr_cont("index=%d, count=%d, memory=%s, capabilities=0x%08x, max num buffers=%u",
 		p->index, p->count, prt_names(p->memory, v4l2_memory_names),
-		p->capabilities);
+		p->capabilities, p->max_num_buffers);
 	v4l_print_format(&p->format, write_only);
 }
 

include/uapi/linux/videodev2.h

···
 #define V4L2_BUF_CAP_SUPPORTS_ORPHANED_BUFS		(1 << 4)
 #define V4L2_BUF_CAP_SUPPORTS_M2M_HOLD_CAPTURE_BUF	(1 << 5)
 #define V4L2_BUF_CAP_SUPPORTS_MMAP_CACHE_HINTS		(1 << 6)
+#define V4L2_BUF_CAP_SUPPORTS_MAX_NUM_BUFFERS		(1 << 7)
 
 /**
  * struct v4l2_plane - plane info for multi-planar buffers
···
  * @flags:	additional buffer management attributes (ignored unless the
  *		queue has V4L2_BUF_CAP_SUPPORTS_MMAP_CACHE_HINTS capability
  *		and configured for MMAP streaming I/O).
+ * @max_num_buffers: if V4L2_BUF_CAP_SUPPORTS_MAX_NUM_BUFFERS capability flag is set
+ *		this field indicate the maximum possible number of buffers
+ *		for this queue.
  * @reserved:	future extensions
  */
 struct v4l2_create_buffers {
···
 	struct v4l2_format	format;
 	__u32			capabilities;
 	__u32			flags;
-	__u32			reserved[6];
+	__u32			max_num_buffers;
+	__u32			reserved[5];
 };
 
 /*