[PATCH RFC 05/11] media: v4l2-core: Move the H.264 encode CBR rate control to a helper

[Arun Iyengar <aiyengar1990@gmail.com>]

The constant bitrate mode of the H.264 encode core does not depend on the
codec: it plans picture sizes over the GOP from the measured I/P size
ratio and IDR period, and moves one QP state by the QP error 6 *
log2(size / target), which holds for HEVC as well. Only the QP delta of
the I pictures differs between the codecs.

Move it to v4l2-enc-cbr, with the I picture QP delta as a parameter, so
that the HEVC encode core can use the same model. The H.264 core keeps
its mode callbacks and uses a QP delta of 2, as before. No functional
change.

Signed-off-by: Arun Iyengar <aiyengar1990@gmail.com>
---
 drivers/media/v4l2-core/Kconfig            |   5 +
 drivers/media/v4l2-core/Makefile           |   1 +
 drivers/media/v4l2-core/v4l2-enc-cbr.c     | 300 +++++++++++++++++++++++++++++
 drivers/media/v4l2-core/v4l2-h264-enc-rc.c | 246 ++---------------------
 include/media/v4l2-enc-cbr.h               |  64 ++++++
 include/media/v4l2-h264-enc-rc.h           |  35 +---
 6 files changed, 387 insertions(+), 264 deletions(-)

diff --git a/drivers/media/v4l2-core/Kconfig b/drivers/media/v4l2-core/Kconfig
index a88eaf8af48c..b8b849a44abd 100644
--- a/drivers/media/v4l2-core/Kconfig
+++ b/drivers/media/v4l2-core/Kconfig
@@ -44,9 +44,14 @@ config V4L2_JPEG_HELPER
 config V4L2_H264
 	tristate
 
+# Used by the stateless encode cores that need v4l2-enc-cbr.ko
+config V4L2_ENC_CBR
+	tristate
+
 # Used by drivers that need v4l2-h264-enc.ko
 config V4L2_H264_ENC
 	tristate
+	select V4L2_ENC_CBR
 
 # Used by drivers that need v4l2-vp9.ko
 config V4L2_VP9
diff --git a/drivers/media/v4l2-core/Makefile b/drivers/media/v4l2-core/Makefile
index 81c5582d6a34..3aa0855d7be7 100644
--- a/drivers/media/v4l2-core/Makefile
+++ b/drivers/media/v4l2-core/Makefile
@@ -26,6 +26,7 @@ videodev-$(CONFIG_VIDEO_V4L2_I2C) += v4l2-i2c.o
 
 obj-$(CONFIG_V4L2_ASYNC) += v4l2-async.o
 obj-$(CONFIG_V4L2_CCI) += v4l2-cci.o
+obj-$(CONFIG_V4L2_ENC_CBR) += v4l2-enc-cbr.o
 obj-$(CONFIG_V4L2_FLASH_LED_CLASS) += v4l2-flash-led-class.o
 obj-$(CONFIG_V4L2_FWNODE) += v4l2-fwnode.o
 obj-$(CONFIG_V4L2_H264) += v4l2-h264.o
diff --git a/drivers/media/v4l2-core/v4l2-enc-cbr.c b/drivers/media/v4l2-core/v4l2-enc-cbr.c
new file mode 100644
index 000000000000..5bd10d494054
--- /dev/null
+++ b/drivers/media/v4l2-core/v4l2-enc-cbr.c
@@ -0,0 +1,300 @@
+// SPDX-License-Identifier: GPL-2.0-or-later
+/*
+ * V4L2 stateless encoder constant bitrate rate control
+ *
+ * One QP state is kept for the stream: it is the QP of the P (and B)
+ * pictures, and I pictures are coded a fixed number of steps below it, so
+ * that the quality of the reference pictures stays even across GOPs. Each
+ * picture gets a size target from a GOP plan, in which an I picture is
+ * "ratio" times the size of a P picture, with the ratio and the IDR period
+ * measured on the stream. The difference between the coded size so far and
+ * the size the plan expects at this point of the GOP is spread over the next
+ * pictures, so that the bitrate converges to the target.
+ *
+ * After each picture the QP state moves by a fraction of the QP error
+ * 6 * log2(size / target) (a QP step of 6 halves the size): slowly for small
+ * errors, so that the QP does not follow the size noise from picture to
+ * picture, and quickly for errors of several QP steps (a scene change, a
+ * poor start).
+ *
+ * The QP to size relation (6 QP steps per halving) is the same for H.264
+ * and HEVC, so the model does not depend on the codec. Only the QP delta of
+ * the I pictures is a parameter.
+ */
+
+#include <linux/export.h>
+#include <linux/int_log.h>
+#include <linux/math64.h>
+#include <linux/minmax.h>
+#include <linux/module.h>
+#include <linux/printk.h>
+#include <linux/string.h>
+#include <media/v4l2-enc-cbr.h>
+
+/* The accumulated size error is spread over this many pictures. */
+#define CBR_WINDOW		8
+/* Limit of the accumulated size error, in seconds of the bitrate */
+#define CBR_BUFFER_SECONDS	3
+/* I/P size ratio, 1/16 units: initial value (10x) and limits (1.5x .. 30x) */
+#define CBR_RATIO_INIT		160
+#define CBR_RATIO_MIN		24
+#define CBR_RATIO_MAX		480
+/* P pictures after an I picture when the I/P size ratio is measured */
+#define CBR_RATIO_PICS		4
+/* QP errors up to this are corrected by 1/8 per picture, 1/256 units */
+#define CBR_ERR_SLOW		(3 * 256)
+/*
+ * Largest QP increase and decrease per picture, 1/256 units. Decreases are
+ * slower, so that pictures that are much smaller than their target, such as
+ * repeated pictures, don't pull the QP down far.
+ */
+#define CBR_STEP_UP_MAX		(4 * 256)
+#define CBR_STEP_DOWN_MAX	(2 * 256)
+
+/* log2(x) in 8.8 fixed point */
+static int cbr_log2_q8(u64 x)
+{
+	unsigned int shift = 0;
+
+	if (!x)
+		return 0;
+
+	while (x > U32_MAX) {
+		x >>= 1;
+		shift++;
+	}
+
+	return (int)(intlog2(x) >> 16) + shift * 256;
+}
+
+static unsigned int cbr_fps(const struct v4l2_fract *timeperframe)
+{
+	if (!timeperframe->numerator || !timeperframe->denominator)
+		return 30;
+
+	return max(DIV_ROUND_CLOSEST(timeperframe->denominator,
+				     timeperframe->numerator), 1U);
+}
+
+static u64 cbr_nominal(u32 bitrate, const struct v4l2_fract *timeperframe)
+{
+	if (!timeperframe->numerator || !timeperframe->denominator)
+		return max(bitrate / 30, 1U);
+
+	return max_t(u64, div_u64((u64)bitrate * timeperframe->numerator,
+				  timeperframe->denominator), 1);
+}
+
+/* Planned size of an I or P picture of the current GOP */
+static u64 cbr_plan(const struct v4l2_enc_cbr *cbr, unsigned int fps,
+		    bool intra)
+{
+	unsigned int gop = cbr->gop;
+	u64 p;
+
+	/* Until the second I picture, assume at least two seconds. */
+	if (!gop)
+		gop = max(cbr->since_i + 1, 2 * fps);
+	if (gop <= 1)
+		return cbr->nominal;
+
+	p = div_u64(cbr->nominal * gop * 16, cbr->ratio + 16 * (gop - 1));
+	if (intra)
+		p = div_u64(p * cbr->ratio, 16);
+
+	return max_t(u64, p, 1);
+}
+
+/*
+ * Start QP of an I picture from its size in bits per luma sample:
+ * 20.3 - 6 * log2(bits per sample), clipped to 15..45. This is the relation
+ * the Rockchip MPP library uses for its first picture at 1080p, for H.264
+ * and HEVC alike.
+ */
+static int cbr_qp_initial(u64 bits, unsigned int samples)
+{
+	int qp_q8 = 20 * 256 + 77 +
+		    6 * (cbr_log2_q8(samples) - cbr_log2_q8(max_t(u64, bits, 1)));
+
+	return clamp(DIV_ROUND_CLOSEST(qp_q8, 256), 15, 45);
+}
+
+/**
+ * v4l2_enc_cbr_reset() - reset the rate control state
+ * @cbr: the rate control state
+ * @qp_delta_intra: QP of the I pictures below the QP of the P pictures
+ *
+ * The next picture starts the rate control afresh.
+ */
+void v4l2_enc_cbr_reset(struct v4l2_enc_cbr *cbr, int qp_delta_intra)
+{
+	memset(cbr, 0, sizeof(*cbr));
+	cbr->qp_delta_intra = qp_delta_intra;
+}
+EXPORT_SYMBOL_GPL(v4l2_enc_cbr_reset);
+
+/**
+ * v4l2_enc_cbr_begin() - set up the size target of the next picture
+ * @cbr: the rate control state
+ * @bitrate: target bitrate in bits per second
+ * @timeperframe: frame interval
+ * @samples: number of luma samples of a picture
+ * @intra: the picture is an I picture
+ * @qp_min: lowest QP
+ * @qp_max: highest QP
+ *
+ * Sets @cbr->target_bits. The QP of the picture is then given by
+ * v4l2_enc_cbr_qp(). A change of the bitrate keeps the QP state and the
+ * measured statistics.
+ *
+ * Return: 0, or -EINVAL if @bitrate is 0.
+ */
+int v4l2_enc_cbr_begin(struct v4l2_enc_cbr *cbr, u32 bitrate,
+		       const struct v4l2_fract *timeperframe,
+		       unsigned int samples, bool intra, int qp_min,
+		       int qp_max)
+{
+	unsigned int fps = cbr_fps(timeperframe);
+	u64 nominal = cbr_nominal(bitrate, timeperframe);
+	s64 target, expected = 0;
+	int qp;
+
+	if (!bitrate)
+		return -EINVAL;
+
+	if (!cbr->bitrate) {
+		cbr->ratio = CBR_RATIO_INIT;
+		cbr->qp_q8 = -1;
+	}
+
+	if (cbr->bitrate != bitrate || cbr->nominal != nominal) {
+		cbr->bitrate = bitrate;
+		cbr->nominal = nominal;
+		cbr->buffer_bits = 0;
+	}
+
+	if (intra) {
+		/* An I picture closes the GOP: its length is the IDR period. */
+		if (cbr->seen_i)
+			cbr->gop = cbr->since_i + 1;
+		cbr->since_i = 0;
+		cbr->seen_i = true;
+	}
+
+	cbr->plan_bits = cbr_plan(cbr, fps, intra);
+
+	if (cbr->qp_q8 < 0) {
+		/* First picture: start QP from the bits per sample. */
+		qp = cbr_qp_initial(cbr_plan(cbr, fps, true), samples) +
+		     cbr->qp_delta_intra;
+		cbr->qp_q8 = clamp_t(int, qp, qp_min, qp_max) * 256;
+	}
+
+	/*
+	 * The buffer holds the coded minus the nominal sizes. Within a GOP the
+	 * plan expects it to go up by the excess of the I picture and back
+	 * down to zero by the savings of the P pictures until the next I
+	 * picture. Spread the difference to that over the next pictures.
+	 */
+	if (!intra && cbr->seen_i)
+		expected = (s64)cbr_plan(cbr, fps, true) - (s64)nominal +
+			   (s64)cbr->since_i *
+			   ((s64)cbr->plan_bits - (s64)nominal);
+
+	target = (s64)cbr->plan_bits -
+		 div_s64(cbr->buffer_bits - max_t(s64, expected, 0),
+			 CBR_WINDOW);
+	cbr->target_bits = clamp_t(s64, target,
+				   max_t(u64, cbr->plan_bits / 4, 8),
+				   cbr->plan_bits * 2);
+	cbr->intra = intra;
+
+	pr_debug("+ v4l2-enc-cbr: begin");
+	pr_debug("  nominal: %llu bits, plan: %llu bits, target: %llu bits",
+		 cbr->nominal, cbr->plan_bits, cbr->target_bits);
+	pr_debug("  buffer: %lld bits, expected: %lld bits, gop: %u, ratio: %u",
+		 cbr->buffer_bits, expected, cbr->gop, cbr->ratio);
+
+	return 0;
+}
+EXPORT_SYMBOL_GPL(v4l2_enc_cbr_begin);
+
+/**
+ * v4l2_enc_cbr_qp() - QP of the picture set up by v4l2_enc_cbr_begin()
+ * @cbr: the rate control state
+ *
+ * Return: the QP, not limited to the QP range of the codec.
+ */
+int v4l2_enc_cbr_qp(const struct v4l2_enc_cbr *cbr)
+{
+	int qp = DIV_ROUND_CLOSEST(cbr->qp_q8, 256);
+
+	if (cbr->intra)
+		qp -= cbr->qp_delta_intra;
+
+	return qp;
+}
+EXPORT_SYMBOL_GPL(v4l2_enc_cbr_qp);
+
+/**
+ * v4l2_enc_cbr_end() - account for a coded picture
+ * @cbr: the rate control state
+ * @bits: size of the coded picture in bits
+ * @timeperframe: frame interval
+ * @qp_min: lowest QP
+ * @qp_max: highest QP
+ */
+void v4l2_enc_cbr_end(struct v4l2_enc_cbr *cbr, u64 bits,
+		      const struct v4l2_fract *timeperframe, int qp_min,
+		      int qp_max)
+{
+	s64 limit = cbr->nominal * CBR_BUFFER_SECONDS * cbr_fps(timeperframe);
+	unsigned int ratio;
+	int err, step;
+
+	if (!cbr->bitrate || !cbr->target_bits)
+		return;
+
+	cbr->buffer_bits = clamp_t(s64, cbr->buffer_bits + (s64)bits -
+				   (s64)cbr->nominal, -limit, limit);
+
+	/* QP error of the picture, 1/256 units */
+	err = 6 * (cbr_log2_q8(bits) - cbr_log2_q8(cbr->target_bits));
+
+	step = err / 8;
+	if (err > CBR_ERR_SLOW)
+		step += (err - CBR_ERR_SLOW) / 2;
+	else if (err < -CBR_ERR_SLOW)
+		step += (err + CBR_ERR_SLOW) / 2;
+	step = clamp(step, -CBR_STEP_DOWN_MAX, CBR_STEP_UP_MAX);
+
+	if (!cbr->intra) {
+		cbr->qp_q8 += step;
+		cbr->p_bits = cbr->p_bits ? (7 * cbr->p_bits + bits) / 8 : bits;
+		cbr->since_i++;
+
+		/* I/P size ratio, once a few P pictures were coded */
+		if (cbr->since_i == CBR_RATIO_PICS && cbr->i_bits) {
+			ratio = div64_u64(cbr->i_bits * 16, cbr->p_bits);
+			ratio = clamp(ratio, CBR_RATIO_MIN, CBR_RATIO_MAX);
+			cbr->ratio = (cbr->ratio + ratio + 1) / 2;
+		}
+	} else {
+		/*
+		 * The size of an I picture mostly reflects the I/P ratio
+		 * estimate: only all-intra streams steer the QP fully with it.
+		 */
+		cbr->i_bits = bits;
+		cbr->qp_q8 += cbr->gop == 1 ? step : step / 4;
+	}
+
+	cbr->qp_q8 = clamp_t(int, cbr->qp_q8, qp_min * 256, qp_max * 256);
+
+	pr_debug("+ v4l2-enc-cbr: end");
+	pr_debug("  size: %llu bits, target: %llu bits, qp error: %d/256, qp: %d/256",
+		 bits, cbr->target_bits, err, cbr->qp_q8);
+}
+EXPORT_SYMBOL_GPL(v4l2_enc_cbr_end);
+
+MODULE_DESCRIPTION("V4L2 stateless encoder constant bitrate rate control");
+MODULE_LICENSE("GPL");
diff --git a/drivers/media/v4l2-core/v4l2-h264-enc-rc.c b/drivers/media/v4l2-core/v4l2-h264-enc-rc.c
index bbfe03febac8..c4f1718ead0e 100644
--- a/drivers/media/v4l2-core/v4l2-h264-enc-rc.c
+++ b/drivers/media/v4l2-core/v4l2-h264-enc-rc.c
@@ -5,7 +5,6 @@
  * Copyright (C) 2025-2026 Paul Kocialkowski <paulk@sys-base.io>
  */
 
-#include <linux/int_log.h>
 #include <linux/math64.h>
 #include <linux/minmax.h>
 #include <linux/module.h>
@@ -162,116 +161,15 @@ static const struct v4l2_h264_enc_rc_mode cq_mode = {
 };
 
 /*
- * Constant bitrate mode
- *
- * One QP state is kept for the stream: it is the QP of the P (and B)
- * pictures, and I pictures are coded a fixed number of steps below it, so
- * that the quality of the reference pictures stays even across GOPs. Each
- * picture gets a size target from a GOP plan, in which an I picture is
- * "ratio" times the size of a P picture, with the ratio and the IDR period
- * measured on the stream. The difference between the coded size so far and
- * the size the plan expects at this point of the GOP is spread over the next
- * pictures, so that the bitrate converges to the target.
- *
- * After each picture the QP state moves by a fraction of the QP error
- * 6 * log2(size / target) (a QP step of 6 halves the size): slowly for small
- * errors, so that the QP does not follow the size noise from picture to
- * picture, and quickly for errors of several QP steps (a scene change, a
- * poor start).
+ * Constant bitrate mode, with the codec independent rate control of
+ * v4l2-enc-cbr: I pictures 2 QP below the P pictures (the default of the
+ * Rockchip MPP library for H.264).
  */
-
-/* The accumulated size error is spread over this many pictures. */
-#define CBR_WINDOW		8
-/* Limit of the accumulated size error, in seconds of the bitrate */
-#define CBR_BUFFER_SECONDS	3
-/* I/P size ratio, 1/16 units: initial value (10x) and limits (1.5x .. 30x) */
-#define CBR_RATIO_INIT		160
-#define CBR_RATIO_MIN		24
-#define CBR_RATIO_MAX		480
-/* P pictures after an I picture when the I/P size ratio is measured */
-#define CBR_RATIO_PICS		4
-/* QP of the I pictures below the QP of the P pictures */
 #define CBR_QP_DELTA_INTRA	2
-/* QP errors up to this are corrected by 1/8 per picture, 1/256 units */
-#define CBR_ERR_SLOW		(3 * 256)
-/*
- * Largest QP increase and decrease per picture, 1/256 units. Decreases are
- * slower, so that pictures that are much smaller than their target, such as
- * repeated pictures, don't pull the QP down far.
- */
-#define CBR_STEP_UP_MAX		(4 * 256)
-#define CBR_STEP_DOWN_MAX	(2 * 256)
-
-/* log2(x) in 8.8 fixed point */
-static int cbr_log2_q8(u64 x)
-{
-	unsigned int shift = 0;
-
-	if (!x)
-		return 0;
-
-	while (x > U32_MAX) {
-		x >>= 1;
-		shift++;
-	}
-
-	return (int)(intlog2(x) >> 16) + shift * 256;
-}
-
-static unsigned int cbr_fps(const struct v4l2_fract *timeperframe)
-{
-	if (!timeperframe->numerator || !timeperframe->denominator)
-		return 30;
-
-	return max(DIV_ROUND_CLOSEST(timeperframe->denominator,
-				     timeperframe->numerator), 1U);
-}
-
-static u64 cbr_nominal(u32 bitrate, const struct v4l2_fract *timeperframe)
-{
-	if (!timeperframe->numerator || !timeperframe->denominator)
-		return max(bitrate / 30, 1U);
-
-	return max_t(u64, div_u64((u64)bitrate * timeperframe->numerator,
-				  timeperframe->denominator), 1);
-}
-
-/* Planned size of an I or P picture of the current GOP */
-static u64 cbr_plan(const struct v4l2_h264_enc_rc_cbr *cbr, unsigned int fps,
-		    bool intra)
-{
-	unsigned int gop = cbr->gop;
-	u64 p;
-
-	/* Until the second I picture, assume at least two seconds. */
-	if (!gop)
-		gop = max(cbr->since_i + 1, 2 * fps);
-	if (gop <= 1)
-		return cbr->nominal;
-
-	p = div_u64(cbr->nominal * gop * 16, cbr->ratio + 16 * (gop - 1));
-	if (intra)
-		p = div_u64(p * cbr->ratio, 16);
-
-	return max_t(u64, p, 1);
-}
-
-/*
- * Start QP of an I picture from its size in bits per luma sample:
- * 20.3 - 6 * log2(bits per sample), clipped to 15..45. This is the relation
- * the Rockchip MPP library uses for its first picture at 1080p.
- */
-static int cbr_qp_initial(u64 bits, unsigned int samples)
-{
-	int qp_q8 = 20 * 256 + 77 +
-		    6 * (cbr_log2_q8(samples) - cbr_log2_q8(max_t(u64, bits, 1)));
-
-	return clamp(DIV_ROUND_CLOSEST(qp_q8, 256), 15, 45);
-}
 
 static int cbr_init(struct v4l2_h264_enc_rc *rc)
 {
-	memset(&rc->cbr, 0, sizeof(rc->cbr));
+	v4l2_enc_cbr_reset(&rc->cbr, CBR_QP_DELTA_INTRA);
 
 	return 0;
 }
@@ -279,148 +177,34 @@ static int cbr_init(struct v4l2_h264_enc_rc *rc)
 static int cbr_measure(struct v4l2_h264_enc_rc *rc, unsigned long *size)
 {
 	struct v4l2_h264_enc_state *state = rc->state;
-	struct v4l2_h264_enc_rc_cbr *cbr = &rc->cbr;
-	struct v4l2_fract *timeperframe = &state->timeperframe;
 	bool intra = state->encode.slice_type == V4L2_H264_SLICE_TYPE_I;
-	unsigned int fps = cbr_fps(timeperframe);
-	u64 nominal = cbr_nominal(state->bitrate, timeperframe);
 	unsigned int samples = state->width_mbs * state->height_mbs * 256;
-	s64 target, expected = 0;
-	int qp;
-
-	if (!state->bitrate)
-		return -EINVAL;
-
-	if (!cbr->bitrate) {
-		cbr->ratio = CBR_RATIO_INIT;
-		cbr->qp_q8 = -1;
-	}
-
-	/* A new bitrate keeps the QP state and the measured statistics. */
-	if (cbr->bitrate != state->bitrate || cbr->nominal != nominal) {
-		cbr->bitrate = state->bitrate;
-		cbr->nominal = nominal;
-		cbr->buffer_bits = 0;
-	}
-
-	if (intra) {
-		/* An I picture closes the GOP: its length is the IDR period. */
-		if (cbr->seen_i)
-			cbr->gop = cbr->since_i + 1;
-		cbr->since_i = 0;
-		cbr->seen_i = true;
-	}
-
-	cbr->plan_bits = cbr_plan(cbr, fps, intra);
+	int ret;
 
-	if (cbr->qp_q8 < 0) {
-		/* First picture: start QP from the bits per sample. */
-		qp = cbr_qp_initial(cbr_plan(cbr, fps, true), samples) +
-		     CBR_QP_DELTA_INTRA;
-		cbr->qp_q8 = clamp_t(int, qp, state->qp_min, state->qp_max) *
-			     256;
-	}
+	ret = v4l2_enc_cbr_begin(&rc->cbr, state->bitrate,
+				 &state->timeperframe, samples, intra,
+				 state->qp_min, state->qp_max);
+	if (ret)
+		return ret;
 
-	/*
-	 * The buffer holds the coded minus the nominal sizes. Within a GOP the
-	 * plan expects it to go up by the excess of the I picture and back
-	 * down to zero by the savings of the P pictures until the next I
-	 * picture. Spread the difference to that over the next pictures.
-	 */
-	if (!intra && cbr->seen_i)
-		expected = (s64)cbr_plan(cbr, fps, true) - (s64)nominal +
-			   (s64)cbr->since_i *
-			   ((s64)cbr->plan_bits - (s64)nominal);
-
-	target = (s64)cbr->plan_bits -
-		 div_s64(cbr->buffer_bits - max_t(s64, expected, 0),
-			 CBR_WINDOW);
-	cbr->target_bits = clamp_t(s64, target,
-				   max_t(u64, cbr->plan_bits / 4, 8),
-				   cbr->plan_bits * 2);
-	cbr->intra = intra;
-
-	pr_debug("+ v4l2-h264-enc-rc: cbr measure");
-	pr_debug("  nominal: %llu bits, plan: %llu bits, target: %llu bits",
-		 cbr->nominal, cbr->plan_bits, cbr->target_bits);
-	pr_debug("  buffer: %lld bits, expected: %lld bits, gop: %u, ratio: %u",
-		 cbr->buffer_bits, expected, cbr->gop, cbr->ratio);
-
-	*size = max_t(unsigned long, cbr->target_bits / 8, 1);
+	*size = max_t(unsigned long, rc->cbr.target_bits / 8, 1);
 
 	return 0;
 }
 
 static int cbr_estimate(struct v4l2_h264_enc_rc *rc, unsigned int *qp)
 {
-	struct v4l2_h264_enc_rc_cbr *cbr = &rc->cbr;
-	int value;
-
-	value = DIV_ROUND_CLOSEST(cbr->qp_q8, 256);
-	if (cbr->intra)
-		value -= CBR_QP_DELTA_INTRA;
-
-	*qp = clamp(value, 0, 51);
+	*qp = clamp(v4l2_enc_cbr_qp(&rc->cbr), 0, 51);
 
 	return 0;
 }
 
-static void cbr_update(struct v4l2_h264_enc_rc *rc, u64 bits)
+static int cbr_complete(struct v4l2_h264_enc_rc *rc, unsigned long bytesused)
 {
 	struct v4l2_h264_enc_state *state = rc->state;
-	struct v4l2_h264_enc_rc_cbr *cbr = &rc->cbr;
-	s64 limit = cbr->nominal * CBR_BUFFER_SECONDS *
-		    cbr_fps(&state->timeperframe);
-	unsigned int ratio;
-	int err, step;
-
-	if (!cbr->bitrate || !cbr->target_bits)
-		return;
-
-	cbr->buffer_bits = clamp_t(s64, cbr->buffer_bits + (s64)bits -
-				   (s64)cbr->nominal, -limit, limit);
-
-	/* QP error of the picture, 1/256 units */
-	err = 6 * (cbr_log2_q8(bits) - cbr_log2_q8(cbr->target_bits));
-
-	step = err / 8;
-	if (err > CBR_ERR_SLOW)
-		step += (err - CBR_ERR_SLOW) / 2;
-	else if (err < -CBR_ERR_SLOW)
-		step += (err + CBR_ERR_SLOW) / 2;
-	step = clamp(step, -CBR_STEP_DOWN_MAX, CBR_STEP_UP_MAX);
-
-	if (!cbr->intra) {
-		cbr->qp_q8 += step;
-		cbr->p_bits = cbr->p_bits ? (7 * cbr->p_bits + bits) / 8 : bits;
-		cbr->since_i++;
-
-		/* I/P size ratio, once a few P pictures were coded */
-		if (cbr->since_i == CBR_RATIO_PICS && cbr->i_bits) {
-			ratio = div64_u64(cbr->i_bits * 16, cbr->p_bits);
-			ratio = clamp(ratio, CBR_RATIO_MIN, CBR_RATIO_MAX);
-			cbr->ratio = (cbr->ratio + ratio + 1) / 2;
-		}
-	} else {
-		/*
-		 * The size of an I picture mostly reflects the I/P ratio
-		 * estimate: only all-intra streams steer the QP fully with it.
-		 */
-		cbr->i_bits = bits;
-		cbr->qp_q8 += cbr->gop == 1 ? step : step / 4;
-	}
 
-	cbr->qp_q8 = clamp_t(int, cbr->qp_q8, state->qp_min * 256,
-			     state->qp_max * 256);
-
-	pr_debug("+ v4l2-h264-enc-rc: cbr update");
-	pr_debug("  size: %llu bits, target: %llu bits, qp error: %d/256, qp: %d/256",
-		 bits, cbr->target_bits, err, cbr->qp_q8);
-}
-
-static int cbr_complete(struct v4l2_h264_enc_rc *rc, unsigned long bytesused)
-{
-	cbr_update(rc, (u64)bytesused * 8);
+	v4l2_enc_cbr_end(&rc->cbr, (u64)bytesused * 8, &state->timeperframe,
+			 state->qp_min, state->qp_max);
 
 	return v4l2_h264_enc_rc_stats_collect(rc, bytesused);
 }
diff --git a/include/media/v4l2-enc-cbr.h b/include/media/v4l2-enc-cbr.h
new file mode 100644
index 000000000000..6b66d121b0cc
--- /dev/null
+++ b/include/media/v4l2-enc-cbr.h
@@ -0,0 +1,64 @@
+/* SPDX-License-Identifier: GPL-2.0-or-later */
+/*
+ * V4L2 stateless encoder constant bitrate rate control
+ *
+ * A codec independent frame-level rate control for the constant bitrate
+ * mode of the stateless encode cores.
+ */
+
+#ifndef _MEDIA_V4L2_ENC_CBR_H
+#define _MEDIA_V4L2_ENC_CBR_H
+
+#include <linux/types.h>
+#include <linux/videodev2.h>
+
+/**
+ * struct v4l2_enc_cbr - constant bitrate rate control state
+ * @qp_delta_intra: QP of the I pictures below the QP of the P pictures
+ * @bitrate: bitrate the state was set up for, 0 before the first picture
+ * @nominal: nominal picture size in bits (bitrate times frame interval)
+ * @qp_q8: QP of the P pictures in 1/256 units, -1 before the first picture
+ * @ratio: I/P picture size ratio in 1/16 units
+ * @gop: measured IDR period in pictures, 0 until two I pictures were seen
+ * @since_i: pictures coded since the last I picture
+ * @seen_i: an I picture was coded
+ * @intra: the current picture is an I picture
+ * @p_bits: running mean of the P picture sizes
+ * @i_bits: size of the last I picture
+ * @buffer_bits: accumulated difference between the coded and nominal sizes
+ * @plan_bits: planned size of the current picture
+ * @target_bits: size target of the current picture
+ *
+ * Only @qp_delta_intra is set by the user, after v4l2_enc_cbr_reset().
+ * @target_bits is valid from v4l2_enc_cbr_begin() of a picture until its
+ * v4l2_enc_cbr_end().
+ */
+struct v4l2_enc_cbr {
+	int qp_delta_intra;
+
+	u32 bitrate;
+	u64 nominal;
+	int qp_q8;
+	unsigned int ratio;
+	unsigned int gop;
+	unsigned int since_i;
+	bool seen_i;
+	bool intra;
+	u64 p_bits;
+	u64 i_bits;
+	s64 buffer_bits;
+	u64 plan_bits;
+	u64 target_bits;
+};
+
+void v4l2_enc_cbr_reset(struct v4l2_enc_cbr *cbr, int qp_delta_intra);
+int v4l2_enc_cbr_begin(struct v4l2_enc_cbr *cbr, u32 bitrate,
+		       const struct v4l2_fract *timeperframe,
+		       unsigned int samples, bool intra, int qp_min,
+		       int qp_max);
+int v4l2_enc_cbr_qp(const struct v4l2_enc_cbr *cbr);
+void v4l2_enc_cbr_end(struct v4l2_enc_cbr *cbr, u64 bits,
+		      const struct v4l2_fract *timeperframe, int qp_min,
+		      int qp_max);
+
+#endif
diff --git a/include/media/v4l2-h264-enc-rc.h b/include/media/v4l2-h264-enc-rc.h
index 72d78a594e40..76781fe2d844 100644
--- a/include/media/v4l2-h264-enc-rc.h
+++ b/include/media/v4l2-h264-enc-rc.h
@@ -10,6 +10,7 @@
 
 #include <linux/types.h>
 #include <linux/v4l2-controls.h>
+#include <media/v4l2-enc-cbr.h>
 #include <media/v4l2-h264.h>
 
 #define V4L2_H264_ENC_RC_STATS_TYPE_COUNT	10
@@ -83,38 +84,6 @@ struct v4l2_h264_enc_rc_stats {
 	unsigned int index;
 };
 
-/**
- * struct v4l2_h264_enc_rc_cbr - constant bitrate mode state
- * @bitrate: bitrate the state was set up for, 0 before the first picture
- * @nominal: nominal picture size in bits (bitrate times frame interval)
- * @qp_q8: QP of the P pictures in 1/256 units, -1 before the first picture
- * @ratio: I/P picture size ratio in 1/16 units
- * @gop: measured IDR period in pictures, 0 until two I pictures were seen
- * @since_i: pictures coded since the last I picture
- * @seen_i: an I picture was coded
- * @intra: the current picture is an I picture
- * @p_bits: running mean of the P picture sizes
- * @i_bits: size of the last I picture
- * @buffer_bits: accumulated difference between the coded and nominal sizes
- * @plan_bits: planned size of the current picture
- * @target_bits: size target of the current picture
- */
-struct v4l2_h264_enc_rc_cbr {
-	u32 bitrate;
-	u64 nominal;
-	int qp_q8;
-	unsigned int ratio;
-	unsigned int gop;
-	unsigned int since_i;
-	bool seen_i;
-	bool intra;
-	u64 p_bits;
-	u64 i_bits;
-	s64 buffer_bits;
-	u64 plan_bits;
-	u64 target_bits;
-};
-
 struct v4l2_h264_enc_rc {
 	const struct v4l2_h264_enc_rc_ops *ops;
 	void *private_data;
@@ -124,7 +93,7 @@ struct v4l2_h264_enc_rc {
 	void *mode_data;
 
 	struct v4l2_h264_enc_rc_stats stats;
-	struct v4l2_h264_enc_rc_cbr cbr;
+	struct v4l2_enc_cbr cbr;
 	unsigned long size;
 	unsigned int qp;
 };

-- 
2.55.0