linux-kernel - Re: [PATCH v19 0/9] Add DELETE

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Message-ID: <63a46881-7fe8-4858-b0f7-cde33ffc7ea6@xs4all.nl>
Date: Wed, 7 Feb 2024 11:28:00 +0100
From: Hans Verkuil <hverkuil@...all.nl>
To: Benjamin Gaignard <benjamin.gaignard@...labora.com>, mchehab@...nel.org
Cc: linux-kernel@...r.kernel.org, linux-media@...r.kernel.org,
 kernel@...labora.com
Subject: Re: [PATCH v19 0/9] Add DELETE_BUF ioctl

On 06/02/2024 09:58, Hans Verkuil wrote:
> On 06/02/2024 09:02, Benjamin Gaignard wrote:
>> Unlike when resolution change on keyframes, dynamic resolution change
>> on inter frames doesn't allow to do a stream off/on sequence because
>> it is need to keep all previous references alive to decode inter frames.
>> This constraint have two main problems:
>> - more memory consumption.
>> - more buffers in use.
>> To solve these issue this series introduce DELETE_BUFS ioctl and remove
>> the 32 buffers limit per queue.
> 
> This v19 looks good. There are three outstanding issues that I need to take a
> look at:
> 
> 1) Can we still signal support for DELETE_BUFS in the V4L2_BUF_CAP_ caps?
>    It would be nice to have, but I'm not sure if and how that can be done.
> 
> 2) I want to take another look at providing a next version of the VIDIOC_CREATE_BUFS
>    ioctl and how that would possibly influence the design of DELETE_BUFS.
>    Just to make sure we're not blocking anything or making life harder.

So I just tried creating a new version of the VIDIOC_CREATE_BUFS ioctl
that is greatly simplified. This just updates the header, no real code yet:

Signed-off-by: Hans Verkuil <hverkuil-cisco@...all.nl>
---
diff --git a/include/uapi/linux/videodev2.h b/include/uapi/linux/videodev2.h
index 03443833aaaa..a7398e4c85e7 100644
--- a/include/uapi/linux/videodev2.h
+++ b/include/uapi/linux/videodev2.h
@@ -2624,6 +2624,39 @@ struct v4l2_create_buffers {
 	__u32			reserved[5];
 };

+/**
+ * struct v4l2_create_buffers_v2 - VIDIOC_CREATE_BUFFERS argument
+ * @type:	enum v4l2_buf_type
+ * @memory:	enum v4l2_memory; buffer memory type
+ * @count:	entry: number of requested buffers,
+ *		return: number of created buffers
+ * @num_planes:	requested number of planes for each buffer
+ * @sizes:	requested plane sizes for each buffer
+ * @start_index:on return, index of the first created buffer
+ * @total_count:on return, the total number of allocated buffers
+ * @capabilities: capabilities of this buffer type.
+ * @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_v2 {
+	__u32			type;
+	__u32			memory;
+	__u32			count;
+	__u32			num_planes;
+	__u32			size[VIDEO_MAX_PLANES];
+	__u32			start_index;
+	__u32			total_count;
+	__u32			capabilities;
+	__u32			flags;
+	__u32			max_num_buffers;
+	__u32			reserved[7];
+};
+
 /**
  * struct v4l2_delete_buffers - VIDIOC_DELETE_BUFS argument
  * @index:	the first buffer to be deleted
@@ -2738,6 +2771,7 @@ struct v4l2_delete_buffers {

 #define VIDIOC_QUERY_EXT_CTRL	_IOWR('V', 103, struct v4l2_query_ext_ctrl)
 #define VIDIOC_DELETE_BUFS	_IOWR('V', 104, struct v4l2_delete_buffers)
+#define VIDIOC_CREATE_BUFFERS	_IOWR('V', 105, struct v4l2_create_buffers_v2)


 /* Reminder: when adding new ioctls please add support for them to


Sadly, struct v4l2_create_buffers was already used for the existing
VIDIOC_CREATE_BUFS (I wish it was called v4l2_create_bufs...), so I did
have to add a _v2 suffix. Compared to the old struct it just moves the
type, num_planes and sizes from v4l2_format into the new struct and drops
the format field. Note that those fields are the only v4l2_format fields
that VIDIOC_CREATE_BUFS used, so getting rid of the format makes live
much easier. I also renamed 'index' to 'start_index' and added a new 'total_count'
field to report the total number of buffers.

The reason for adding 'total_count' is that VIDIOC_CREATE_BUFS with
count == 0 would report the total number of buffers in the 'index' field,
which was rather odd. Adding a specific field for this purpose is nicer.

One thing that might impact your series is the name of the ioctls.

We have:

VIDIOC_CREATE_BUFS/v4l2_create_buffers
VIDIOC_DELETE_BUFS/v4l2_delete_buffers
VIDIOC_CREATE_BUFFERS/v4l2_create_buffers_v2 (TBD)

I'm wondering if VIDIOC_DELETE_BUFS should be renamed to
VIDIOC_DELETE_BUFFERS, that would make it more consistent with
the proposed VIDIOC_CREATE_BUFFERS.

Alternatively, instead of calling it VIDIOC_CREATE_BUFFERS we
might call it VIDIOC_CREATE_BUFS_V2.

Or perhaps some other naming convention?

Ideas are welcome.

Regards,

	Hans