HDMI-TX Resolution Handling in Kernel DRM Driver

Introduction

The scope of this document is to provide information and instructions for HDMI-TX resolution handling in the syna_drm kernel DRM driver, including runtime force mode testing and persistent configuration.

HDMI-TX Resolution Control Modes

The syna_drm driver supports two main options to control output resolution:

Fixed mode disabled in DTS
Fixed mode enabled in DTS

Fixed mode disabled in DTS

If fixed mode is disabled, full control is delegated to the display managers/compositors in application or middleware layers.

Fixed mode enabled in DTS

If fixed mode is enabled, the kernel driver handles resolution selection, independent of display managers/compositors.

When fixed mode is enabled, two functional paths are used:

Default case
- Resolution is selected from sink EDID capability.
- Typical behavior:
  - 2K TV: 2K60
  - 4K TV: 4K30
Force mode
- Resolution is explicitly forced through the module parameter.
- Supported force values are listed in the next section.

HDMI Force Mode Testing Guide

Feature overview

The HDMI driver applies resolution selection in this priority order:

Force mode: user sets hdmi_force_mode through module parameter.
Auto-detect: driver chooses from sink (TV) EDID capabilities.

Supported Force Mode Values

The following strings are supported for hdmi_force_mode:

4K30
2K60
2K50
720p
576p
480p

Runtime Testing (Valid for Current Boot)

These steps apply mode changes on next hotplug event and do not persist across reboot.

Step 1: Set the mode

Use the module parameter interface:

For 4K at 30 Hz:

echo "4K30" > /sys/module/syna_drm/parameters/hdmi_force_mode

For 1080p at 60 Hz:

echo "2K60" > /sys/module/syna_drm/parameters/hdmi_force_mode

For legacy 480p:

echo "480p" > /sys/module/syna_drm/parameters/hdmi_force_mode

Step 2: Trigger hotplug

The new mode is applied on next HDMI connection event.

Physically unplug and replug the HDMI cable.

Step 3: Verification

Top-level check:

Verify the TV OSD/pop-up shows expected resolution.

Kernel log check:

dmesg | grep "syna_drm"

Success log examples:

Processing force mode: '4K30'
HDMI_HPD detected, setting res to resId:16

Fallback log examples:

Force 4K30 requested but Sink ... doesn't support it ... Fallback.
Fallback: setting res to resId:24

Step 4: Clear force mode

Clear the force mode setting:

echo > /sys/module/syna_drm/parameters/hdmi_force_mode

Persistent Configuration (Reboot Safe)

To ensure force mode remains active after reboot, configure the module parameter using modprobe.d.

Update syna_drm.conf on target device:

Open the configuration file:
```
vi /usr/lib/modprobe.d/syna_drm.conf
```
Add or update the option. Example to keep 2K60:
```
options syna_drm hdmi_force_mode=2K60
```
Reboot the board.
Verify that the configured resolution is maintained.

Sticky Mode Behavior Check

The driver remembers the last successfully applied mode from user request or force mode logic.

Expected behavior when switching TVs (hotplug):

Keep the same resolution if the new TV supports it.
If unsupported, fallback to the next best mode supported by the TV.