[PATCH 1/2] HID: apple: preserve keyboard backlight across T2 resume

[Andre Eikmeyer <dev@deq.rocks>]

The T2 virtual USB host controller re-enumerates the internal keyboard
after system resume. The butterfly keyboard backlight currently uses
LED_CORE_SUSPENDRESUME, so the LED core sends a blocking request to the
old HID device while it is disappearing. That request fails with -ENODEV
and the newly probed device starts with its backlight off.

To fix this, we cache the requested brightness when the old HID device is
removed and restore it when the replacement is probed. We let
re-enumeration handle restoration instead of issuing a request through the
stale device.

Fixes: 1f95a6cd5ad7 ("HID: apple: ensure the keyboard backlight is off if suspending")
Cc: stable@vger.kernel.org
Signed-off-by: Andre Eikmeyer <dev@deq.rocks>
---
 drivers/hid/hid-apple.c | 18 ++++++++++++++++--
 1 file changed, 16 insertions(+), 2 deletions(-)

diff --git a/drivers/hid/hid-apple.c b/drivers/hid/hid-apple.c
index bf7dd0fbf249..a5f232cc2b66 100644
--- a/drivers/hid/hid-apple.c
+++ b/drivers/hid/hid-apple.c
@@ -91,6 +91,9 @@ struct apple_sc_backlight {
 	struct hid_device *hdev;
 };
 
+/* T2 VHCI re-enumerates the internal keyboard across system resume. */
+static int apple_backlight_resume_brightness = -1;
+
 struct apple_backlight_config_report {
 	u8 report_id;
 	u8 version;
@@ -825,6 +828,7 @@ static int apple_backlight_led_set(struct led_classdev *led_cdev,
 static int apple_backlight_init(struct hid_device *hdev)
 {
 	int ret;
+	int brightness;
 	struct apple_sc *asc = hid_get_drvdata(hdev);
 	struct apple_backlight_config_report *rep;
 
@@ -860,13 +864,20 @@ static int apple_backlight_init(struct hid_device *hdev)
 	asc->backlight->cdev.name = "apple::kbd_backlight";
 	asc->backlight->cdev.max_brightness = rep->backlight_on_max;
 	asc->backlight->cdev.brightness_set_blocking = apple_backlight_led_set;
-	asc->backlight->cdev.flags = LED_CORE_SUSPENDRESUME;
+	/* VHCI re-enumeration restores the cached brightness in the next probe. */
+
+	brightness = READ_ONCE(apple_backlight_resume_brightness);
+	if (brightness < 0)
+		brightness = LED_OFF;
+	else
+		brightness = min_t(int, brightness, rep->backlight_on_max);
 
-	ret = apple_backlight_set(hdev, 0, 0);
+	ret = apple_backlight_set(hdev, brightness, 0);
 	if (ret < 0) {
 		hid_err(hdev, "backlight set request failed: %d\n", ret);
 		goto cleanup_and_exit;
 	}
+	asc->backlight->cdev.brightness = brightness;
 
 	ret = devm_led_classdev_register(&hdev->dev, &asc->backlight->cdev);
 
@@ -999,6 +1010,9 @@ static void apple_remove(struct hid_device *hdev)
 
 	if (asc->quirks & APPLE_RDESC_BATTERY)
 		timer_delete_sync(&asc->battery_timer);
+	if (asc->backlight)
+		WRITE_ONCE(apple_backlight_resume_brightness,
+			   asc->backlight->cdev.brightness);
 
 	hid_hw_stop(hdev);
 }
-- 
2.55.0