Title: i.MX8MP V4L2 H.264 Encoder Fails on Dynamic Runtime Update (Bitrate Mode / Resolution) — poll

[VishnuSidda]

IMX8MPLUS 

Platform Details:

• SoC: i.MX8MP

• BSP: (Linux imx8mpevk 6.12.20-lts-next-gdfaf2136deb2)

• GStreamer: 1.26.0 (NXP plugins)

• Encoder: v4l2h264enc

• Source: v4l2src (camera)

• OS: Linux (Yocto-based)

---

Use Case:
We are building a dynamic multi-stream video pipeline using GStreamer with the following architecture:

v4l2src → capsfilter → imxvideoconvert_g2d → tee
                                         ├── RTSP branch (v4l2h264enc)

We dynamically create/remove encoder branches from a tee (using request pads) and support runtime configuration updates via API.

---

Problem Description:

We are attempting to dynamically update encoder parameters at runtime (pipeline in PLAYING state), specifically:

• Bitrate mode (CBR / VBR)

• Bitrate

• Resolution (caps change)

This is done using:

• g_object_set() on encoder (extra-controls)

• Updating caps on downstream capsfilter

• Updating videorate (max-rate)

---

Relevant Code (Simplified):

enc_el.set_property("extra-controls", Gst.Structure.new_from_string(
    "controls,video_bitrate_mode=1,video_gop_size=25"))

rate_el.set_property("max-rate", fps)

res_el.set_property("caps", Gst.Caps.from_string(
    f"video/x-raw,width={w},height={h},format=BGRx"))

---

Observed Behavior:

Immediately after applying dynamic update (especially bitrate mode change), pipeline fails with:

gstvideoencoder.c:2506:
Can't copy metadata because input frame disappeared

v4l2object.c:6524: gst_v4l2_object_poll:
poll error 1: Invalid argument (22)

GST_ERROR:
Could not read from resource.

gstbasesrc.c:3187:
Internal data stream error.
streaming stopped, reason error (-5)

This results in:

• Encoder failure (v4l2h264enc)

• Queue failure

• Source (v4l2src) stopping

• Entire pipeline going to error state

---

Full Error Logs (Excerpt):

gst_v4l2_object_poll:<v4l2h264enc> error: poll error 1: Invalid argument (22)
gst_element_message_full_with_details: Could not read from resource
gst_base_src_loop:<v4l2src> error: Internal data stream error
streaming stopped, reason error (-5)

---

Key Observations:

1. Issue is reproducible when changing:

   • video_bitrate_mode (CBR ↔ VBR)

   • bitrate

2. Sometimes encoder accepts property but:

   • no visible effect OR

   • fails shortly after

3. FPS changes using videorate work fine

4. Resolution changes sometimes work but are unstable

---

Questions:

1. Is runtime update of v4l2h264enc controls supported?

   • Specifically:

     • video_bitrate_mode

     • video_bitrate

     • video_gop_size

2. Are these controls required to be set only in:

   • NULL / READY state?

3. Does the i.MX8MP VPU driver support:

   • dynamic bitrate adjustment in PLAYING state?

4. For resolution change:

   • Is dynamic caps renegotiation supported with imxvideoconvert_g2d + encoder?

   • Or is pipeline restart required?

5. What is the recommended approach for:

   • changing bitrate mode (CBR/VBR)

   • changing resolution

   • in a live streaming pipeline?

---

Additional Context:
In commercial IP cameras, bitrate appears to change dynamically without stream interruption.
We would like to understand whether:

• This is supported at driver level, or

• Requires encoder restart / pipeline rebuild.

---

Expected Guidance:

• Official recommendation from NXP for runtime encoder reconfiguration

• Supported vs unsupported controls in PLAYING state

• Best practice for dynamic streaming pipelines

---

Summary:
Dynamic updates to v4l2h264enc (especially bitrate mode) result in:

• V4L2 poll errors (EINVAL)

• Encoder failure

• Pipeline crash

We want to confirm whether this is expected behavior and how to correctly implement dynamic reconfiguration.

---

Thank you.

[joanxie]

I've already mailed to you for enabling VBR, then you can control it dynamically, pls check my mail