[PATCH v4 0/2] Save Brightness on Macs

[PATCH v4 0/2] Save Brightness on Macs

[Atharva Tiwari]

Currently, when a Mac is rebooted, the display brightness does not reflect
the level used during the previous boot. Instead, the brightness is
reset to the value last stored by macOS, causing the system to
ignore any changes made since that time.

We fix this by writing the brightness value to efivar 'backlight-level'
at shutdown only, when it is changed since the last boot.

Changes in v4:
- Fixed build error, when apple-gmux is compiled without
  apple-brightness.
- Fix grammatical errors.
Changes in v3:
- Fixed a bug, in which apple_brightness_shutdown is still called on
  non-supported devices.
Changes in v2:
- Saved the brightness only when it is different than the previous boot.

Link to v3:
https://lore.kernel.org/all/20260315222020.24341-1-atharvatiwarilinuxdev@gmail.com/
Link to v2:
https://lore.kernel.org/all/20260315215302.24087-1-atharvatiwarilinuxdev@gmail.com/
Link to v1:
https://lore.kernel.org/all/20260206125641.12983-1-atharvatiwarilinuxdev@gmail.com/


Atharva Tiwari (2):
  efi: Save Brightness using EFI on Macs
  platform/apple-gmux: use apple_brightness to save brightness to EFI

 drivers/firmware/efi/Kconfig                  | 10 +++
 drivers/firmware/efi/Makefile                 |  1 +
 drivers/firmware/efi/apple-brightness.c       | 67 +++++++++++++++++++
 drivers/platform/x86/apple-gmux.c             | 21 ++++++
 .../linux/platform_data/apple-brightness.h    | 20 ++++++
 5 files changed, 119 insertions(+)
 create mode 100644 drivers/firmware/efi/apple-brightness.c
 create mode 100644 include/linux/platform_data/apple-brightness.h

-- 
2.43.0

[PATCH v4 1/2] efi: Save Brightness using EFI on Macs

[Atharva Tiwari]

Currently when a Mac reboots, the brightness is not the same
as the previous boot instead its the same as the last time the
Mac booted macOS.

We can fix this issue by saving the brightness level to the efivar
backlight-level.

(tested on iMac20,1)

Suggested-by: Lukas Wunner <lukas@wunner.de>
Signed-off-by: Atharva Tiwari <atharvatiwarilinuxdev@gmail.com>
---
 drivers/firmware/efi/Kconfig                  | 10 +++
 drivers/firmware/efi/Makefile                 |  1 +
 drivers/firmware/efi/apple-brightness.c       | 67 +++++++++++++++++++
 .../linux/platform_data/apple-brightness.h    | 20 ++++++
 4 files changed, 98 insertions(+)
 create mode 100644 drivers/firmware/efi/apple-brightness.c
 create mode 100644 include/linux/platform_data/apple-brightness.h

diff --git a/drivers/firmware/efi/Kconfig b/drivers/firmware/efi/Kconfig
index 29e0729299f5..243499a5d7b7 100644
--- a/drivers/firmware/efi/Kconfig
+++ b/drivers/firmware/efi/Kconfig
@@ -167,6 +167,16 @@ config APPLE_PROPERTIES
 
 	  If unsure, say Y if you have a Mac.  Otherwise N.
 
+config APPLE_BRIGHTNESS
+	bool "Apple Backlight control for EFI"
+	depends on X86
+	help
+	  This will save the brightness level to EFI, so brightness
+	  level is preserved across reboots and shutdowns. allowing
+	  for improved support of Apple hardware.
+
+	  If unsure, say Y if you have a Mac, otherwise N.
+
 config RESET_ATTACK_MITIGATION
 	bool "Reset memory attack mitigation"
 	depends on EFI_STUB
diff --git a/drivers/firmware/efi/Makefile b/drivers/firmware/efi/Makefile
index 8efbcf699e4f..1f5705cc87a2 100644
--- a/drivers/firmware/efi/Makefile
+++ b/drivers/firmware/efi/Makefile
@@ -26,6 +26,7 @@ obj-$(CONFIG_EFI_BOOTLOADER_CONTROL)	+= efibc.o
 obj-$(CONFIG_EFI_TEST)			+= test/
 obj-$(CONFIG_EFI_DEV_PATH_PARSER)	+= dev-path-parser.o
 obj-$(CONFIG_APPLE_PROPERTIES)		+= apple-properties.o
+obj-$(CONFIG_APPLE_BRIGHTNESS)		+= apple-brightness.o
 obj-$(CONFIG_EFI_RCI2_TABLE)		+= rci2-table.o
 obj-$(CONFIG_EFI_EMBEDDED_FIRMWARE)	+= embedded-firmware.o
 obj-$(CONFIG_LOAD_UEFI_KEYS)		+= mokvar-table.o
diff --git a/drivers/firmware/efi/apple-brightness.c b/drivers/firmware/efi/apple-brightness.c
new file mode 100644
index 000000000000..b060861b0795
--- /dev/null
+++ b/drivers/firmware/efi/apple-brightness.c
@@ -0,0 +1,67 @@
+// SPDX-License-Identifier: GPL-2.0-only OR MIT
+/*
+ * apple-brightness.c - EFI brightness saver on Macs
+ * Copyright (C) 2026 Atharva Tiwari <atharvatiwarilinuxdev@gmail.com>
+ */
+
+#define pr_fmt(fmt) KBUILD_MODNAME ": " fmt
+
+#include <linux/backlight.h>
+#include <linux/efi.h>
+#include <linux/platform_data/apple-brightness.h>
+
+static u32 efi_attr;
+static u16 last_saved_level;
+
+static int (*get_brightness)(struct backlight_device *bl);
+static struct backlight_device *bl_dev;
+
+void apple_brightness_shutdown(void)
+{
+	u16 level;
+	efi_status_t status;
+
+	level = (u16)get_brightness(bl_dev);
+
+	if (level == last_saved_level)
+		return;
+
+	status = efivar_set_variable(APPLE_BRIGHTNESS_NAME, &APPLE_BRIGHTNESS_GUID,
+				efi_attr, sizeof(level), &level);
+	if (status != EFI_SUCCESS)
+		pr_debug("Unable to set brightness: 0x%lx\n", status);
+}
+EXPORT_SYMBOL(apple_brightness_shutdown);
+
+int apple_brightness_probe(struct backlight_device *bl,
+	int (*get_brightnessfn)(struct backlight_device *bl))
+{
+	efi_status_t status;
+	unsigned long size = sizeof(last_saved_level);
+	int ret;
+
+	bl_dev = bl;
+	get_brightness = get_brightnessfn;
+
+	if (!efi_rt_services_supported(EFI_RT_SUPPORTED_SET_VARIABLE))
+		return -ENODEV;
+
+	ret = efivar_lock();
+	if (ret)
+		return ret;
+
+	status = efivar_get_variable(APPLE_BRIGHTNESS_NAME, &APPLE_BRIGHTNESS_GUID,
+				&efi_attr, &size, &last_saved_level);
+
+	efivar_unlock();
+
+	if (status != EFI_SUCCESS)
+		return -ENODEV;
+
+	return 0;
+}
+EXPORT_SYMBOL(apple_brightness_probe);
+
+MODULE_AUTHOR("Atharva Tiwari <atharvatiwarilinuxdev@gmail.com>");
+MODULE_DESCRIPTION("EFI Brightness saver for Macs");
+MODULE_LICENSE("Dual MIT/GPL");
diff --git a/include/linux/platform_data/apple-brightness.h b/include/linux/platform_data/apple-brightness.h
new file mode 100644
index 000000000000..54e3f3f8aa1c
--- /dev/null
+++ b/include/linux/platform_data/apple-brightness.h
@@ -0,0 +1,20 @@
+/* SPDX-License-Identifier: GPL-2.0-only OR MIT */
+/*
+ * apple-brightness.h - EFI brightness saver for Macs
+ * Copyright (C) 2026 Atharva Tiwari <atharvatiwarilinuxdev@gmail.com>
+ */
+
+#ifndef _APPLE_BL_H_
+#define _APPLE_BL_H_
+
+#include <linux/backlight.h>
+#include <linux/efi.h>
+
+#define APPLE_BRIGHTNESS_NAME           L"backlight-level"
+#define APPLE_BRIGHTNESS_GUID           EFI_GUID(0x7c436110, 0xab2a, 0x4bbb, 0xa8, 0x80, 0xfe, 0x41, 0x99, 0x5c, 0x9f, 0x82)
+
+int apple_brightness_probe(struct backlight_device *bl,
+	int (*get_brightnessfn)(struct backlight_device *bl));
+
+void apple_brightness_shutdown(void);
+#endif /* _APPLE_BL_H */
-- 
2.43.0

[PATCH v4 2/2] platform/apple-gmux: use apple_brightness to save brightness to EFI

[Atharva Tiwari]

use apple_brightness to save brightness to EFI.

(tested on iMac20,1)

Signed-off-by: Atharva Tiwari <atharvatiwarilinuxdev@gmail.com>
---
 drivers/platform/x86/apple-gmux.c | 21 +++++++++++++++++++++
 1 file changed, 21 insertions(+)

diff --git a/drivers/platform/x86/apple-gmux.c b/drivers/platform/x86/apple-gmux.c
index 1417e230edbd..cab3122c1a86 100644
--- a/drivers/platform/x86/apple-gmux.c
+++ b/drivers/platform/x86/apple-gmux.c
@@ -22,6 +22,7 @@
 #include <linux/pci.h>
 #include <linux/vga_switcheroo.h>
 #include <linux/debugfs.h>
+#include <linux/platform_data/apple-brightness.h>
 #include <acpi/video.h>
 #include <asm/io.h>
 
@@ -78,6 +79,8 @@ struct apple_gmux_data {
 	/* debugfs data */
 	u8 selected_port;
 	struct dentry *debug_dentry;
+
+	bool save_efi;
 };
 
 static struct apple_gmux_data *apple_gmux_data;
@@ -960,6 +963,13 @@ static int gmux_probe(struct pnp_dev *pnp, const struct pnp_device_id *id)
 	}
 
 	gmux_init_debugfs(gmux_data);
+	if (IS_ENABLED(CONFIG_APPLE_BRIGHTNESS)) {
+		ret = apple_brightness_probe(gmux_data->bdev, &gmux_get_brightness);
+		if (ret)
+			pr_warn("Unable to Enable EFI brightness save: %d\n", ret);
+		else
+			gmux_data->save_efi = true;
+	}
 	return 0;
 
 err_register_handler:
@@ -1012,6 +1022,16 @@ static void gmux_remove(struct pnp_dev *pnp)
 	kfree(gmux_data);
 }
 
+static void gmux_shutdown(struct pnp_dev *pnp)
+{
+	struct apple_gmux_data *gmux_data = pnp_get_drvdata(pnp);
+
+	if (IS_ENABLED(CONFIG_APPLE_BRIGHTNESS) && gmux_data->save_efi)
+		apple_brightness_shutdown();
+
+	gmux_remove(pnp);
+}
+
 static const struct pnp_device_id gmux_device_ids[] = {
 	{GMUX_ACPI_HID, 0},
 	{"", 0}
@@ -1026,6 +1046,7 @@ static struct pnp_driver gmux_pnp_driver = {
 	.name		= "apple-gmux",
 	.probe		= gmux_probe,
 	.remove		= gmux_remove,
+	.shutdown	= gmux_shutdown,
 	.id_table	= gmux_device_ids,
 	.driver		= {
 			.pm = &gmux_dev_pm_ops,
-- 
2.43.0

Re: [PATCH v4 1/2] efi: Save Brightness using EFI on Macs

[Aditya Garg]

> On 16 Mar 2026, at 9:03 PM, Atharva Tiwari <atharvatiwarilinuxdev@gmail.com> wrote:
> 
> Currently when a Mac reboots, the brightness is not the same
> as the previous boot instead its the same as the last time the
> Mac booted macOS.
> 
> We can fix this issue by saving the brightness level to the efivar
> backlight-level.
> 
> (tested on iMac20,1)
> 
> Suggested-by: Lukas Wunner <lukas@wunner.de>
> Signed-off-by: Atharva Tiwari <atharvatiwarilinuxdev@gmail.com>
> ---
> drivers/firmware/efi/Kconfig                  | 10 +++
> drivers/firmware/efi/Makefile                 |  1 +
> drivers/firmware/efi/apple-brightness.c       | 67 +++++++++++++++++++
> .../linux/platform_data/apple-brightness.h    | 20 ++++++
> 4 files changed, 98 insertions(+)
> create mode 100644 drivers/firmware/efi/apple-brightness.c
> create mode 100644 include/linux/platform_data/apple-brightness.h

I tested this patch on a MacBookPro16,1, which is a T2 Intel Mac, and the behaviour is very weird:

1. macOS will never respect any brightness set by Linux
2. The Max brightness set by macOS is not the max brightness on Linux. I get these values on Linux:

aditya@MacBook:~$ efivar -p -n '7c436110-ab2a-4bbb-a880-fe41995c9f82-backlight-level'
GUID: 7c436110-ab2a-4bbb-a880-fe41995c9f82
Name: "backlight-level"
Attributes:
	Non-Volatile
	Boot Service Access
	Runtime Service Access
Value:
00000000  f1 02                                             |..              |

aditya@MacBook:~$ cat /sys/class/backlight/gmux_backlight/brightness
48238

3. I set the Max brightness on Linux, and restarted. I get this:

aditya@MacBook:~$ efivar -p -n '7c436110-ab2a-4bbb-a880-fe41995c9f82-backlight-level'
GUID: 7c436110-ab2a-4bbb-a880-fe41995c9f82
Name: "backlight-level"
Attributes:
	Non-Volatile
	Boot Service Access
	Runtime Service Access
Value:
00000000  7a ff                                             |z.              |

aditya@MacBook:~$ cat /sys/class/backlight/gmux_backlight/brightness
65402


4. Finally, any attempt to reduce the brightness seems worthless as unless booted back to macOS, Linux always boots at max brightness, but the nvram variable is changed successfully:

aditya@MacBook:~$ efivar -p -n '7c436110-ab2a-4bbb-a880-fe41995c9f82-backlight-level'
GUID: 7c436110-ab2a-4bbb-a880-fe41995c9f82
Name: "backlight-level"
Attributes:
	Non-Volatile
	Boot Service Access
	Runtime Service Access
Value:
00000000  eb 41                                             |.A              |

aditya@MacBook:~$ cat /sys/class/backlight/gmux_backlight/brightness
65402


Overall, it’s very inconsistent. Also, I’ve noticed Windows doesn't mess around with efivars, rather stores the brightness locally and implements it on every boot, something which systemd should do on Linux.

Re: [PATCH v4 1/2] efi: Save Brightness using EFI on Macs

[Aditya Garg]

On 3/17/26 15:08, Aditya Garg wrote:
> 
> 
>> On 16 Mar 2026, at 9:03 PM, Atharva Tiwari <atharvatiwarilinuxdev@gmail.com> wrote:
>>
>> Currently when a Mac reboots, the brightness is not the same
>> as the previous boot instead its the same as the last time the
>> Mac booted macOS.
>>
>> We can fix this issue by saving the brightness level to the efivar
>> backlight-level.
>>
>> (tested on iMac20,1)
>>
>> Suggested-by: Lukas Wunner <lukas@wunner.de>
>> Signed-off-by: Atharva Tiwari <atharvatiwarilinuxdev@gmail.com>
>> ---
>> drivers/firmware/efi/Kconfig                  | 10 +++
>> drivers/firmware/efi/Makefile                 |  1 +
>> drivers/firmware/efi/apple-brightness.c       | 67 +++++++++++++++++++
>> .../linux/platform_data/apple-brightness.h    | 20 ++++++
>> 4 files changed, 98 insertions(+)
>> create mode 100644 drivers/firmware/efi/apple-brightness.c
>> create mode 100644 include/linux/platform_data/apple-brightness.h
> 
> I tested this patch on a MacBookPro16,1, which is a T2 Intel Mac, and the behaviour is very weird:
> 
> 1. macOS will never respect any brightness set by Linux
> 2. The Max brightness set by macOS is not the max brightness on Linux. I get these values on Linux:
> 
> aditya@MacBook:~$ efivar -p -n '7c436110-ab2a-4bbb-a880-fe41995c9f82-backlight-level'
> GUID: 7c436110-ab2a-4bbb-a880-fe41995c9f82
> Name: "backlight-level"
> Attributes:
> 	Non-Volatile
> 	Boot Service Access
> 	Runtime Service Access
> Value:
> 00000000  f1 02                                             |..              |
> 
> aditya@MacBook:~$ cat /sys/class/backlight/gmux_backlight/brightness
> 48238
> 
> 3. I set the Max brightness on Linux, and restarted. I get this:
> 
> aditya@MacBook:~$ efivar -p -n '7c436110-ab2a-4bbb-a880-fe41995c9f82-backlight-level'
> GUID: 7c436110-ab2a-4bbb-a880-fe41995c9f82
> Name: "backlight-level"
> Attributes:
> 	Non-Volatile
> 	Boot Service Access
> 	Runtime Service Access
> Value:
> 00000000  7a ff                                             |z.              |
> 
> aditya@MacBook:~$ cat /sys/class/backlight/gmux_backlight/brightness
> 65402
> 
> 
> 4. Finally, any attempt to reduce the brightness seems worthless as unless booted back to macOS, Linux always boots at max brightness, but the nvram variable is changed successfully:
> 
> aditya@MacBook:~$ efivar -p -n '7c436110-ab2a-4bbb-a880-fe41995c9f82-backlight-level'
> GUID: 7c436110-ab2a-4bbb-a880-fe41995c9f82
> Name: "backlight-level"
> Attributes:
> 	Non-Volatile
> 	Boot Service Access
> 	Runtime Service Access
> Value:
> 00000000  eb 41                                             |.A              |
> 
> aditya@MacBook:~$ cat /sys/class/backlight/gmux_backlight/brightness
> 65402
> 
> 
> Overall, it’s very inconsistent. Also, I’ve noticed Windows doesn't mess around with efivars, rather stores the brightness locally and implements it on every boot, something which systemd should do on Linux.
> 
> 

I've done another test as well:

1. I booted into macOS, and set a normal brightness, not too low, not 
too high.
2. I booted back to Linux. The brightness was consistent. These are the 
values:

aditya@MacBook:~$ efivar -p -n 
'7c436110-ab2a-4bbb-a880-fe41995c9f82-backlight-level'
GUID: 7c436110-ab2a-4bbb-a880-fe41995c9f82
Name: "backlight-level"
Attributes:
	Non-Volatile
	Boot Service Access
	Runtime Service Access
Value:
00000000  92 00                                             |.. 
     |

aditya@MacBook:~$ cat /sys/class/backlight/gmux_backlight/brightness
7792

This is the journalctl of that boot: https://termbin.com/p0vf

3. I slightly modified the brightness and restarted. Upon a restart, the 
brightness was max again:

aditya@MacBook:~$ efivar -p -n 
'7c436110-ab2a-4bbb-a880-fe41995c9f82-backlight-level'
GUID: 7c436110-ab2a-4bbb-a880-fe41995c9f82
Name: "backlight-level"
Attributes:
	Non-Volatile
	Boot Service Access
	Runtime Service Access
Value:
00000000  51 3a                                             |Q: 
     |

aditya@MacBook:~$ cat /sys/class/backlight/gmux_backlight/brightness
63589

This is the journalctl of that boot: https://termbin.com/1z9r