diff --git a/doc/APIchanges b/doc/APIchanges
index 29ddaa31bd..44ba3ad634 100644
--- a/doc/APIchanges
+++ b/doc/APIchanges
@@ -2,6 +2,11 @@ The last version increases of all libraries were on 2023-02-09
 
 API changes, most recent first:
 
+2023-04-04 - xxxxxxxxxx - lavu 58.6.100 - hdr_dynamic_metadata.h
+  Add AV_HDR_PLUS_MAX_PAYLOAD_SIZE.
+  av_dynamic_hdr_plus_create_side_data() now accepts a user provided
+  buffer.
+
 2023-03-xx - xxxxxxxxxx - lavfi 9.5.100 - avfilter.h
   Add AVFILTER_FLAG_HWDEVICE.
 
diff --git a/libavutil/hdr_dynamic_metadata.c b/libavutil/hdr_dynamic_metadata.c
index d458788c32..7033f060c0 100644
--- a/libavutil/hdr_dynamic_metadata.c
+++ b/libavutil/hdr_dynamic_metadata.c
@@ -18,14 +18,13 @@
  * Foundation, Inc., 51 Franklin Street, Fifth Floor, Boston, MA 02110-1301 USA
  */
 
+#include "avassert.h"
 #include "hdr_dynamic_metadata.h"
 #include "mem.h"
 #include "libavcodec/defs.h"
 #include "libavcodec/get_bits.h"
 #include "libavcodec/put_bits.h"
 
-#define T35_PAYLOAD_MAX_SIZE 907
-
 static const int64_t luminance_den = 1;
 static const int32_t peak_luminance_den = 15;
 static const int64_t rgb_den = 100000;
@@ -62,14 +61,14 @@ AVDynamicHDRPlus *av_dynamic_hdr_plus_create_side_data(AVFrame *frame)
 int av_dynamic_hdr_plus_from_t35(AVDynamicHDRPlus *s, const uint8_t *data,
                                  size_t size)
 {
-    uint8_t padded_buf[T35_PAYLOAD_MAX_SIZE + AV_INPUT_BUFFER_PADDING_SIZE];
+    uint8_t padded_buf[AV_HDR_PLUS_MAX_PAYLOAD_SIZE + AV_INPUT_BUFFER_PADDING_SIZE];
     GetBitContext gbc, *gb = &gbc;
     int ret;
 
     if (!s)
         return AVERROR(ENOMEM);
 
-    if (size > T35_PAYLOAD_MAX_SIZE)
+    if (size > AV_HDR_PLUS_MAX_PAYLOAD_SIZE)
         return AVERROR(EINVAL);
 
     memcpy(padded_buf, data, size);
@@ -243,8 +242,10 @@ int av_dynamic_hdr_plus_to_t35(const AVDynamicHDRPlus *s, uint8_t **data, size_t
     size_t size_bits, size_bytes;
     PutBitContext pbc, *pb = &pbc;
 
-    if (!s || !data)
+    if (!s)
         return AVERROR(EINVAL);
+    if ((!data || *data) && !size)
+       return AVERROR(EINVAL);
 
     /**
      * Buffer size per CTA-861-H p.253-254:
@@ -296,9 +297,20 @@ int av_dynamic_hdr_plus_to_t35(const AVDynamicHDRPlus *s, uint8_t **data, size_t
 
     size_bytes = (size_bits + 7) / 8;
 
-    buf = av_mallocz(size_bytes);
-    if (!buf)
-        return AVERROR(ENOMEM);
+    av_assert0(size_bytes <= AV_HDR_PLUS_MAX_PAYLOAD_SIZE);
+
+    if (!data) {
+        *size = size_bytes;
+        return 0;
+    } else if (*data) {
+        if (*size < size_bytes)
+            return AVERROR_BUFFER_TOO_SMALL;
+        buf = *data;
+    } else {
+        buf = av_malloc(size_bytes);
+        if (!buf)
+            return AVERROR(ENOMEM);
+    }
 
     init_put_bits(pb, buf, size_bytes);
 
diff --git a/libavutil/hdr_dynamic_metadata.h b/libavutil/hdr_dynamic_metadata.h
index 771bb8f468..09e9d8bbcc 100644
--- a/libavutil/hdr_dynamic_metadata.h
+++ b/libavutil/hdr_dynamic_metadata.h
@@ -353,13 +353,21 @@ AVDynamicHDRPlus *av_dynamic_hdr_plus_create_side_data(AVFrame *frame);
 int av_dynamic_hdr_plus_from_t35(AVDynamicHDRPlus *s, const uint8_t *data,
                                  size_t size);
 
+#define AV_HDR_PLUS_MAX_PAYLOAD_SIZE 907
+
 /**
  * Serialize dynamic HDR10+ metadata to a user data registered ITU-T T.35 buffer,
  * excluding the first 48 bytes of the header, and beginning with the application mode.
  * @param s A pointer containing the decoded AVDynamicHDRPlus structure.
- * @param data A pointer to a byte buffer to be allocated and filled
- *             with the serialized metadata.
- * @param size A pointer to a size to be set to the returned buffer's size (optional).
+ * @param data[in,out] A pointer to pointer to a byte buffer to be filled with the
+ *                     serialized metadata.
+ *                     If *data is NULL, a buffer be will be allocated and a pointer to
+ *                     it stored in its place. The caller assumes ownership of the buffer.
+ *                     May be NULL, in which case the function will only store the
+ *                     required buffer size in *size.
+ * @param size[in,out] A pointer to a size to be set to the returned buffer's size.
+ *                     If *data is not NULL, *size must contain the size of the input
+ *                     buffer. May be NULL only if *data is NULL.
  *
  * @return >= 0 on success. Otherwise, returns the appropriate AVERROR.
  */
diff --git a/libavutil/version.h b/libavutil/version.h
index a232381ba5..40f92af055 100644
--- a/libavutil/version.h
+++ b/libavutil/version.h
@@ -79,7 +79,7 @@
  */
 
 #define LIBAVUTIL_VERSION_MAJOR  58
-#define LIBAVUTIL_VERSION_MINOR   5
+#define LIBAVUTIL_VERSION_MINOR   6
 #define LIBAVUTIL_VERSION_MICRO 100
 
 #define LIBAVUTIL_VERSION_INT   AV_VERSION_INT(LIBAVUTIL_VERSION_MAJOR, \