Re: [PATCH v2 3/5] media: v4l2-core: Add v4l2-stats interface

Hi Hans,

On 6/19/26 09:05, Hans Verkuil wrote:

On 17/06/2026 20:10, Detlev Casanova wrote:

quoted

Provide helpers for media drivers to set fdinfo data and print the
key:value pairs in a standard way.

User drivers can set stats values with helpers like:
  - v4l2_stats_update_hw_usage
  - v4l2_stats_set_media_dev_type

And also call the show helpers from their show_fdinfo callback with:
  - v4l2_stats_show -- Shows the values set previously
  - v4l2_stats_show_clock -- Shows the main clock state.

The show_clock helper is used instead of updating a clock value in
v4l2_stats for the following reasons:
  - Clocks are at the device level, this is not a per-fd information
  - This avoids having clock references in v4l2-core
  - Drivers can use different approaches to manage clocks
    (e.g.: bulk_data or not: A set helper wouldn't please all drivers)
  - Arguably, clocks could be exposed elsewhere (like a debugfs), but we
    want something close to what DRM does and centralizing information has
    its advantages for userspace tooling.

In DRM the key:value pair format for clocks is documented and each driver
can write them directly based on that.
In this case, provide a helper and document the format.

Signed-off-by: Detlev Casanova <detlev.casanova@collabora.com>
---
  drivers/media/v4l2-core/Makefile     |  2 +-
  drivers/media/v4l2-core/v4l2-dev.c   |  2 ++
  drivers/media/v4l2-core/v4l2-fh.c    |  3 ++
  drivers/media/v4l2-core/v4l2-stats.c | 65 ++++++++++++++++++++++++++++++++++++
  include/media/v4l2-fh.h              |  2 ++
  include/media/v4l2-stats.h           | 44 ++++++++++++++++++++++++
  6 files changed, 117 insertions(+), 1 deletion(-)

diff --git a/drivers/media/v4l2-core/Makefile b/drivers/media/v4l2-core/Makefile
index 329f0eadce99..20e1ab74ac09 100644
--- a/drivers/media/v4l2-core/Makefile
+++ b/drivers/media/v4l2-core/Makefile

@@ -9,7 +9,7 @@ ccflags-y += -I$(srctree)/drivers/media/tuners
  tuner-objs	:=	tuner-core.o
  
  videodev-objs	:=	v4l2-dev.o v4l2-ioctl.o v4l2-device.o v4l2-fh.o \
-			v4l2-event.o v4l2-subdev.o v4l2-common.o \
+			v4l2-event.o v4l2-subdev.o v4l2-common.o v4l2-stats.o \
  			v4l2-ctrls-core.o v4l2-ctrls-api.o \
  			v4l2-ctrls-request.o v4l2-ctrls-defs.o
  

diff --git a/drivers/media/v4l2-core/v4l2-dev.c b/drivers/media/v4l2-core/v4l2-dev.c
index 3878fa2ff73e..3e7a6876dffd 100644
--- a/drivers/media/v4l2-core/v4l2-dev.c
+++ b/drivers/media/v4l2-core/v4l2-dev.c

@@ -486,6 +486,8 @@ static void v4l2_show_fdinfo(struct seq_file *m, struct file *filp)
  {
  	struct video_device *vdev = video_devdata(filp);
  
+	seq_printf(m, "media-driver:\t%s\n", vdev->v4l2_dev->name);
+
  	if (vdev->fops->show_fdinfo)
  		vdev->fops->show_fdinfo(m, filp);
  }

diff --git a/drivers/media/v4l2-core/v4l2-fh.c b/drivers/media/v4l2-core/v4l2-fh.c
index b184bed8aca9..1b655672c718 100644
--- a/drivers/media/v4l2-core/v4l2-fh.c
+++ b/drivers/media/v4l2-core/v4l2-fh.c

@@ -17,6 +17,7 @@
  #include <media/v4l2-event.h>
  #include <media/v4l2-ioctl.h>
  #include <media/v4l2-mc.h>
+#include <media/v4l2-stats.h>
  
  void v4l2_fh_init(struct v4l2_fh *fh, struct video_device *vdev)
  {

@@ -38,6 +39,7 @@ void v4l2_fh_init(struct v4l2_fh *fh, struct video_device *vdev)
  	INIT_LIST_HEAD(&fh->subscribed);
  	fh->sequence = -1;
  	mutex_init(&fh->subscribe_lock);
+	v4l2_stats_init(&fh->stats);
  }
  EXPORT_SYMBOL_GPL(v4l2_fh_init);
  

@@ -88,6 +90,7 @@ void v4l2_fh_exit(struct v4l2_fh *fh)
  	v4l2_event_unsubscribe_all(fh);
  	mutex_destroy(&fh->subscribe_lock);
  	fh->vdev = NULL;
+	v4l2_stats_exit(&fh->stats);
  }
  EXPORT_SYMBOL_GPL(v4l2_fh_exit);
  

diff --git a/drivers/media/v4l2-core/v4l2-stats.c b/drivers/media/v4l2-core/v4l2-stats.c
new file mode 100644
index 000000000000..93e64ef2e7bb
--- /dev/null
+++ b/drivers/media/v4l2-core/v4l2-stats.c

@@ -0,0 +1,65 @@
+// SPDX-License-Identifier: GPL-2.0-only
+/*
+ * v4l2-stats.c
+ *
+ * V4L2 statistics management.
+ *
+ * Maintain a per-file handle list of statistics about the hardware and handle
+ * exposing it in the fdinfo.
+ *
+ * Copyright (C) 2026 Collabora.
+ *
+ * Contact: Detlev Casanova <detlev.casanova@collabora.com>
+ */
+
+#include <linux/types.h>
+#include <linux/seq_file.h>
+#include <linux/clk.h>
+#include <media/v4l2-stats.h>
+
+static const char * const dev_type_name[] = {
+	[MEDIA_DEV_TYPE_V4L2] = "media",
+	[MEDIA_DEV_TYPE_V4L2_STATELESS_ENCODER] = "encoder",
+	[MEDIA_DEV_TYPE_V4L2_STATELESS_DECODER] = "decoder",
+};
+
+void v4l2_stats_init(struct v4l2_stats *stats)
+{
+	stats->hw_usage_time = 0;
+	stats->media_dev_type = MEDIA_DEV_TYPE_V4L2;
+}
+
+void v4l2_stats_exit(struct v4l2_stats *stats)
+{
+}
+
+void v4l2_stats_update_hw_usage(struct v4l2_stats *stats, u64 usage_time)
+{
+	stats->hw_usage_time += usage_time;
+}
+EXPORT_SYMBOL_GPL(v4l2_stats_update_hw_usage);
+
+void v4l2_stats_set_media_dev_type(struct v4l2_stats *stats, enum v4l2_media_dev_type type)
+{
+	if (type >= MEDIA_DEV_TYPE_COUNT)
+		return;
+
+	stats->media_dev_type = type;
+}
+EXPORT_SYMBOL_GPL(v4l2_stats_set_media_dev_type);
+
+void v4l2_stats_show(struct v4l2_stats *stats, struct seq_file *m)
+{
+	seq_printf(m, "media-type:\t%s\n", dev_type_name[stats->media_dev_type]);
+	seq_printf(m, "media-engine-usage:\t%llu ns\n", stats->hw_usage_time);
+}
+EXPORT_SYMBOL_GPL(v4l2_stats_show);
+
+void v4l2_stats_show_clock(struct seq_file *m, struct clk *clk)
+{
+	seq_printf(m, "media-maxfreq:\t%lu Hz\n",
+		   clk_get_rate(clk));
+	seq_printf(m, "media-curfreq:\t%lu Hz\n",
+		   clk_get_rate(clk));
+}
+EXPORT_SYMBOL_GPL(v4l2_stats_show_clock);

diff --git a/include/media/v4l2-fh.h b/include/media/v4l2-fh.h
index aad4b3689d7e..ae6688722bee 100644
--- a/include/media/v4l2-fh.h
+++ b/include/media/v4l2-fh.h

@@ -17,6 +17,7 @@
  #include <linux/kconfig.h>
  #include <linux/list.h>
  #include <linux/videodev2.h>
+#include <media/v4l2-stats.h>
  
  struct video_device;
  struct v4l2_ctrl_handler;

@@ -43,6 +44,7 @@ struct v4l2_fh {
  	struct list_head	list;
  	struct video_device	*vdev;
  	struct v4l2_ctrl_handler *ctrl_handler;
+	struct v4l2_stats	stats;
  	enum v4l2_priority	prio;
  
  	/* Events */

diff --git a/include/media/v4l2-stats.h b/include/media/v4l2-stats.h
new file mode 100644
index 000000000000..d580933c4181
--- /dev/null
+++ b/include/media/v4l2-stats.h

@@ -0,0 +1,44 @@
+/* SPDX-License-Identifier: GPL-2.0-only */
+/*
+ * v4l2-stats.h
+ *
+ * V4L2 statistics management.
+ *
+ * Maintain a per-file handle list of statistics about the hardware and handle
+ * exposing it in the fdinfo.
+ *
+ * Copyright (C) 2026 Collabora.
+ *
+ * Contact: Detlev Casanova <detlev.casanova@collabora.com>
+ */
+#ifndef V4L2_STATS_H
+#define V4L2_STATS_H
+
+#include <linux/types.h>
+
+struct clk;
+struct seq_file;
+
+enum v4l2_media_dev_type {
+	MEDIA_DEV_TYPE_V4L2 = 0,
+	MEDIA_DEV_TYPE_V4L2_STATELESS_ENCODER,
+	MEDIA_DEV_TYPE_V4L2_STATELESS_DECODER,
+
+	MEDIA_DEV_TYPE_COUNT,

I'm a bit unhappy about introducing yet another type. Do we need it?

I didn't want it either, but this will extend to other kind of drivers 
and I couldn't find a type enum to rule them all.
Maybe just using Device Capabilities Flags ?

It still won't differentiate encoder from decoder.
I wouldn't super mad if we loose that information (userspace can simply 
see it as a capture device) but it'd still be nice to have it.

quoted

+};
+
+struct v4l2_stats {

Poor name, and it conflicts with ISP statistics.

How about v4l2_pdinfo? And v4l2-fdinfo.h etc. That avoids any confusion and
also clearly says what this is about.

As mentioned in the other patch, "v4l2_metrics" is more accurate.

Detlev.

quoted

+	u64 hw_usage_time;
+	enum v4l2_media_dev_type media_dev_type;

This should be the first field.

quoted

+};
+
+void v4l2_stats_init(struct v4l2_stats *stats);
+void v4l2_stats_exit(struct v4l2_stats *stats);
+
+void v4l2_stats_update_hw_usage(struct v4l2_stats *stats, u64 usage_time);
+void v4l2_stats_set_media_dev_type(struct v4l2_stats *stats, enum v4l2_media_dev_type type);
+
+void v4l2_stats_show(struct v4l2_stats *stats, struct seq_file *m);
+void v4l2_stats_show_clock(struct seq_file *m, struct clk *clk);
+
+#endif /* V4L2_STATS_H */

Regards,

	Hans