[PATCH] initramfs-framework: add opt-in root-only udev trigger

[PATCH] initramfs-framework: add opt-in root-only udev trigger

[Wenwen Fu]

The initramfs udev module replays add events for the complete device tree
and waits for every resulting event. An initramfs that only mounts a root
partition does not need to coldplug unrelated devices.

Add an opt-in initramfs.udev-root-only kernel parameter. When root uses
PARTLABEL or PARTUUID, trigger only the matching block device. For
PARTLABEL roots, match the PARTNAME property exposed by udev. Keep the
existing full trigger as the default so current users and other initramfs
modules retain their existing behaviour.

If the requested root cannot be matched safely, or a targeted trigger
fails, fall back to the original full coldplug.

Signed-off-by: Wenwen Fu <wenwfu@qti.qualcomm.com>
---
 .../initrdscripts/initramfs-framework/udev    | 30 ++++++++++++++++++-
 1 file changed, 29 insertions(+), 1 deletion(-)

diff --git a/meta/recipes-core/initrdscripts/initramfs-framework/udev b/meta/recipes-core/initrdscripts/initramfs-framework/udev
index 4898b89246..7fa26ef807 100644
--- a/meta/recipes-core/initrdscripts/initramfs-framework/udev
+++ b/meta/recipes-core/initrdscripts/initramfs-framework/udev
@@ -35,6 +35,29 @@ udev_enabled() {
 	return 0
 }
 
+udev_trigger_root_device() {
+	case "${bootparam_root:-}" in
+		PARTLABEL=?*)
+			root_match="PARTNAME=${bootparam_root#PARTLABEL=}"
+			;;
+		PARTUUID=?*)
+			root_match="PARTUUID=${bootparam_root#PARTUUID=}"
+			;;
+		*)
+			return 1
+			;;
+	esac
+
+	root_devices=$(udevadm trigger --dry-run --verbose \
+		--subsystem-match=block "--property-match=$root_match") || return 1
+
+	# udevadm succeeds even when no devices match, so check its dry-run output.
+	[ -n "$root_devices" ] || return 1
+
+	udevadm trigger --subsystem-match=block \
+		"--property-match=$root_match" --action=add
+}
+
 udev_run() {
 	add_module_pre_hook "udev_shutdown_hook_handler"
 
@@ -45,6 +68,11 @@ udev_run() {
 	sh -c "exec 4< /dev/console" || { exec 0> /dev/null; exec 1> /dev/null; exec 2> /dev/null; }
 
 	$_UDEV_DAEMON --daemon
-	udevadm trigger --action=add
+	if [ "${bootparam_initramfs_udev_root_only:-}" = "1" ] &&
+	   udev_trigger_root_device; then
+		debug "Triggered udev for root partition only"
+	else
+		udevadm trigger --action=add
+	fi
 	udevadm settle
 }
-- 
2.43.0

Re: [PATCH] initramfs-framework: add opt-in root-only udev trigger

[Paul Barker]

Hi,

Thanks for the patch, I have a couple of feedback comments.

On Wed, 2026-08-12 at 17:28 +0800, Wenwen Fu wrote:
> The initramfs udev module replays add events for the complete device tree
> and waits for every resulting event. An initramfs that only mounts a root
> partition does not need to coldplug unrelated devices.
> 
> Add an opt-in initramfs.udev-root-only kernel parameter. When root uses
> PARTLABEL or PARTUUID, trigger only the matching block device. For
> PARTLABEL roots, match the PARTNAME property exposed by udev. Keep the
> existing full trigger as the default so current users and other initramfs
> modules retain their existing behaviour.
> 
> If the requested root cannot be matched safely, or a targeted trigger
> fails, fall back to the original full coldplug.

This explains the mechanism, but not why we would want the option to
avoid the current behaviour. Does the current behaviour cause problems,
is it too slow, etc?

> 
> Signed-off-by: Wenwen Fu <wenwfu@qti.qualcomm.com>
> ---
>  .../initrdscripts/initramfs-framework/udev    | 30 ++++++++++++++++++-
>  1 file changed, 29 insertions(+), 1 deletion(-)
> 
> diff --git a/meta/recipes-core/initrdscripts/initramfs-framework/udev b/meta/recipes-core/initrdscripts/initramfs-framework/udev
> index 4898b89246..7fa26ef807 100644
> --- a/meta/recipes-core/initrdscripts/initramfs-framework/udev
> +++ b/meta/recipes-core/initrdscripts/initramfs-framework/udev
> @@ -35,6 +35,29 @@ udev_enabled() {
>  	return 0
>  }
>  
> +udev_trigger_root_device() {
> +	case "${bootparam_root:-}" in
> +		PARTLABEL=?*)
> +			root_match="PARTNAME=${bootparam_root#PARTLABEL=}"
> +			;;
> +		PARTUUID=?*)
> +			root_match="PARTUUID=${bootparam_root#PARTUUID=}"
> +			;;
> +		*)
> +			return 1
> +			;;
> +	esac
> +
> +	root_devices=$(udevadm trigger --dry-run --verbose \
> +		--subsystem-match=block "--property-match=$root_match") || return 1
> +
> +	# udevadm succeeds even when no devices match, so check its dry-run output.
> +	[ -n "$root_devices" ] || return 1
> +
> +	udevadm trigger --subsystem-match=block \
> +		"--property-match=$root_match" --action=add

The commit message says that there is a fallback to the original full
coldplug if the targeted trigger fails, but the return status of this
command is ignored.

> +}
> +
>  udev_run() {
>  	add_module_pre_hook "udev_shutdown_hook_handler"
>  
> @@ -45,6 +68,11 @@ udev_run() {
>  	sh -c "exec 4< /dev/console" || { exec 0> /dev/null; exec 1> /dev/null; exec 2> /dev/null; }
>  
>  	$_UDEV_DAEMON --daemon
> -	udevadm trigger --action=add
> +	if [ "${bootparam_initramfs_udev_root_only:-}" = "1" ] &&

I don't think this enabling condition isn't explained clearly in the
commit message.

> +	   udev_trigger_root_device; then
> +		debug "Triggered udev for root partition only"
> +	else
> +		udevadm trigger --action=add
> +	fi
>  	udevadm settle
>  }

Best regards,

-- 
Paul Barker

Re: [PATCH] initramfs-framework: add opt-in root-only udev trigger

[Wenwen Fu]

[-- Attachment #1: Type: text/plain, Size: 5176 bytes --]

Hi Paul,

Thanks for the review.

The motivation is to reduce initramfs boot latency. On the Qualcomm RB3
Gen 2 Core Kit, the full coldplug initializes unrelated devices, and
udevadm settle waits for their events although only the root partition
is needed.

I measured the following average boot times with this configuration:

Device: Qualcomm RB3 Gen 2 Core Kit
MACHINE: rb3gen2-core-kit
Kernel: Linux 6.18
KAS configuration:
       ci/rb3gen2-core-kit.yml:ci/qcom-distro.yml:ci/performance.yml
Image: initramfs-rootfs-image
Root: PARTLABEL=rootfs


Default
full coldplug
With path
root-only enabled
Improvement
udev trigger + settle
981.6 ms
109.8 ms
871.8 ms (88.8%)
/init to rootfs EXT4 mount
1340.2 ms
441.1 ms
899.1 ms (67.1%)


This reduces the udev trigger and settle time by 871.8 ms and the time
to mount the root filesystem by 899.1 ms, corresponding to improvements
of 88.8% and 67.1%, respectively. The measurements cover three boots
with the full trigger and two boots with the root-only trigger.

For the targeted trigger, udev_trigger_root_device() returns the status
of its final udevadm command, so failure already makes the condition
false and executes the full trigger. However, I agree this is not
obvious, so I will make the error handling explicit in v2.

I will also clarify that the optimization is enabled with:

    initramfs.udev-root-only=1

I will address these points in v2.

Best regards,
Wenwen

________________________________
From: Paul Barker <paul@pbarker.dev>
Sent: Tuesday, August 18, 2026 16:47
To: Wenwen Fu <wenwfu@qti.qualcomm.com>; openembedded-core@lists.openembedded.org <openembedded-core@lists.openembedded.org>
Subject: Re: [PATCH] initramfs-framework: add opt-in root-only udev trigger

WARNING: This email originated from outside of Qualcomm. Please be wary of any links or attachments, and do not enable macros.

Hi,

Thanks for the patch, I have a couple of feedback comments.

On Wed, 2026-08-12 at 17:28 +0800, Wenwen Fu wrote:
> The initramfs udev module replays add events for the complete device tree
> and waits for every resulting event. An initramfs that only mounts a root
> partition does not need to coldplug unrelated devices.
>
> Add an opt-in initramfs.udev-root-only kernel parameter. When root uses
> PARTLABEL or PARTUUID, trigger only the matching block device. For
> PARTLABEL roots, match the PARTNAME property exposed by udev. Keep the
> existing full trigger as the default so current users and other initramfs
> modules retain their existing behaviour.
>
> If the requested root cannot be matched safely, or a targeted trigger
> fails, fall back to the original full coldplug.

This explains the mechanism, but not why we would want the option to
avoid the current behaviour. Does the current behaviour cause problems,
is it too slow, etc?

>
> Signed-off-by: Wenwen Fu <wenwfu@qti.qualcomm.com>
> ---
>  .../initrdscripts/initramfs-framework/udev    | 30 ++++++++++++++++++-
>  1 file changed, 29 insertions(+), 1 deletion(-)
>
> diff --git a/meta/recipes-core/initrdscripts/initramfs-framework/udev b/meta/recipes-core/initrdscripts/initramfs-framework/udev
> index 4898b89246..7fa26ef807 100644
> --- a/meta/recipes-core/initrdscripts/initramfs-framework/udev
> +++ b/meta/recipes-core/initrdscripts/initramfs-framework/udev
> @@ -35,6 +35,29 @@ udev_enabled() {
>       return 0
>  }
>
> +udev_trigger_root_device() {
> +     case "${bootparam_root:-}" in
> +             PARTLABEL=?*)
> +                     root_match="PARTNAME=${bootparam_root#PARTLABEL=}"
> +                     ;;
> +             PARTUUID=?*)
> +                     root_match="PARTUUID=${bootparam_root#PARTUUID=}"
> +                     ;;
> +             *)
> +                     return 1
> +                     ;;
> +     esac
> +
> +     root_devices=$(udevadm trigger --dry-run --verbose \
> +             --subsystem-match=block "--property-match=$root_match") || return 1
> +
> +     # udevadm succeeds even when no devices match, so check its dry-run output.
> +     [ -n "$root_devices" ] || return 1
> +
> +     udevadm trigger --subsystem-match=block \
> +             "--property-match=$root_match" --action=add

The commit message says that there is a fallback to the original full
coldplug if the targeted trigger fails, but the return status of this
command is ignored.

> +}
> +
>  udev_run() {
>       add_module_pre_hook "udev_shutdown_hook_handler"
>
> @@ -45,6 +68,11 @@ udev_run() {
>       sh -c "exec 4< /dev/console" || { exec 0> /dev/null; exec 1> /dev/null; exec 2> /dev/null; }
>
>       $_UDEV_DAEMON --daemon
> -     udevadm trigger --action=add
> +     if [ "${bootparam_initramfs_udev_root_only:-}" = "1" ] &&

I don't think this enabling condition isn't explained clearly in the
commit message.

> +        udev_trigger_root_device; then
> +             debug "Triggered udev for root partition only"
> +     else
> +             udevadm trigger --action=add
> +     fi
>       udevadm settle
>  }

Best regards,

--
Paul Barker


[-- Attachment #2: Type: text/html, Size: 19489 bytes --]

[PATCH v2] initramfs-framework: add opt-in root-only udev trigger

[Wenwen Fu]

The initramfs udev module replays add events for the complete device
tree and waits for every resulting event. This can significantly delay
mounting the root filesystem when unrelated devices take time to
initialize.

On a Qualcomm RB3 Gen 2 Core Kit (MACHINE=rb3gen2-core-kit) running
Linux 6.18, using initramfs-rootfs-image with root=PARTLABEL=rootfs,
the following average times were measured:

                              Full coldplug    Root-only
udev trigger + settle           981.6 ms        109.8 ms
/init to rootfs EXT4 mount     1340.2 ms        441.1 ms

The root-only trigger reduced these times by 88.8% and 67.1%,
respectively. The values are averages of three full-coldplug boots and
two root-only boots.

Add the opt-in initramfs.udev-root-only=1 kernel parameter. When root
uses PARTLABEL or PARTUUID, trigger only the matching block device. For
PARTLABEL roots, match the PARTNAME property exposed by udev.

Keep the existing full trigger as the default so current users and
other initramfs modules retain their existing behaviour. If the root
device cannot be matched safely or the targeted trigger fails, fall
back to the original full coldplug.

Signed-off-by: Wenwen Fu <wenwfu@qti.qualcomm.com>
---
Changes in v2:
- Explain the boot-time motivation and include performance measurements.
- Clarify that initramfs.udev-root-only=1 enables the optimization.
- Make targeted-trigger error handling explicit.
---
 .../initrdscripts/initramfs-framework/udev    | 32 ++++++++++++++++++-
 1 file changed, 31 insertions(+), 1 deletion(-)

diff --git a/meta/recipes-core/initrdscripts/initramfs-framework/udev b/meta/recipes-core/initrdscripts/initramfs-framework/udev
index 4898b89246..8f67e78a7c 100644
--- a/meta/recipes-core/initrdscripts/initramfs-framework/udev
+++ b/meta/recipes-core/initrdscripts/initramfs-framework/udev
@@ -35,6 +35,31 @@ udev_enabled() {
 	return 0
 }
 
+udev_trigger_root_device() {
+	case "${bootparam_root:-}" in
+		PARTLABEL=?*)
+			root_match="PARTNAME=${bootparam_root#PARTLABEL=}"
+			;;
+		PARTUUID=?*)
+			root_match="PARTUUID=${bootparam_root#PARTUUID=}"
+			;;
+		*)
+			return 1
+			;;
+	esac
+
+	root_devices=$(udevadm trigger --dry-run --verbose \
+		--subsystem-match=block "--property-match=$root_match") || return 1
+
+	# udevadm succeeds even when no devices match, so check its dry-run output.
+	[ -n "$root_devices" ] || return 1
+
+	udevadm trigger --subsystem-match=block \
+		"--property-match=$root_match" --action=add || return 1
+
+	return 0
+}
+
 udev_run() {
 	add_module_pre_hook "udev_shutdown_hook_handler"
 
@@ -45,6 +70,11 @@ udev_run() {
 	sh -c "exec 4< /dev/console" || { exec 0> /dev/null; exec 1> /dev/null; exec 2> /dev/null; }
 
 	$_UDEV_DAEMON --daemon
-	udevadm trigger --action=add
+	if [ "${bootparam_initramfs_udev_root_only:-}" = "1" ] &&
+	   udev_trigger_root_device; then
+		debug "Triggered udev for root partition only"
+	else
+		udevadm trigger --action=add
+	fi
 	udevadm settle
 }
-- 
2.43.0