[PATCH v2] platform/x86: dell-uart-backlight: fall back to PCH PWM on UART timeout
From: chitranshmandhaniya
Date: Fri Oct 09 2026 - 03:28:46 EST
From: Chitransh Mandhaniya <Chitransh_Mandhaniya@xxxxxxxx>
Some Dell OptiPlex AIO 7410 (35W, board 01KP8D) ship with DELL0501 in
the ACPI DSDT (status = 0x0F) even though no UART scalar board is
physically present. This causes acpi_video_get_backlight_type() to
return acpi_backlight_dell_uart, which makes i915 skip registering
intel_backlight. The dell-uart-backlight driver then times out talking
to the absent board and the probe fails, leaving the machine with zero
backlight control.
A BIOS update fixes this on some models (e.g. OptiPlex 7420, board
0914RT) by setting DELL0501 _STA to 0, but Dell has not shipped a
corrected BIOS for the 7410 even in the latest release (1.37.0,
2026-05-28).
When the initial get_version command returns -ETIMEDOUT, fall back to
driving the backlight through the Intel PCH PWM registers (BLC_PWM_CTL /
BLC_PWM_FREQ / BLC_PWM_DUTY at MMIO BAR0 + 0xC8250). The fallback
reads the BIOS-programmed PWM frequency as max_brightness and registers
a BACKLIGHT_PLATFORM device so that compositors (Weston, GNOME, KDE)
pick it up normally.
Tested on:
- OptiPlex AIO 7410 35W (01KP8D) -- timeout, fallback activates, works
- OptiPlex AIO 7420 35W (0914RT, old BIOS) -- timeout, fallback works
- OptiPlex AIO 7420 35W (0914RT, new BIOS) -- DELL0501 absent, driver
not loaded, intel_backlight works natively (no regression)
Signed-off-by: Chitransh Mandhaniya <Chitransh_Mandhaniya@xxxxxxxx>
---
v1 -> v2:
- Use __free(pci_dev_put) for automatic PCI device cleanup (Ilpo)
- Replace 0x10 with SZ_16, add linux/sizes.h (Ilpo)
- Name BIT(31) as PCH_BLC_PWM_ENABLE (Ilpo)
- Fix double-space in block comment (Ilpo)
- Reduce devm_backlight_device_register() to fewer lines (Ilpo)
- Extract dell_uart_bl_init() for the normal UART backlight path
so the probe function reads more clearly (Ilpo)
.../platform/x86/dell/dell-uart-backlight.c | 157 +++++++++++++++---
1 file changed, 134 insertions(+), 23 deletions(-)
diff --git a/drivers/platform/x86/dell/dell-uart-backlight.c b/drivers/platform/x86/dell/dell-uart-backlight.c
index f323a667dc2d..edc7cfe38069 100644
--- a/drivers/platform/x86/dell/dell-uart-backlight.c
+++ b/drivers/platform/x86/dell/dell-uart-backlight.c
@@ -13,10 +13,13 @@
#include <linux/delay.h>
#include <linux/device.h>
#include <linux/err.h>
+#include <linux/io.h>
#include <linux/module.h>
#include <linux/mutex.h>
+#include <linux/pci.h>
#include <linux/platform_device.h>
#include <linux/serdev.h>
+#include <linux/sizes.h>
#include <linux/string.h>
#include <linux/types.h>
#include <linux/wait.h>
@@ -50,6 +53,12 @@
#define MIN_RESP_LEN 3
#define MAX_RESP_LEN 80
+/* PCH PWM backlight registers relative to Intel GPU MMIO BAR0 */
+#define PCH_BLC_PWM_CTL 0xC8250
+#define PCH_BLC_PWM_FREQ 0xC8254
+#define PCH_BLC_PWM_DUTY 0xC8258
+#define PCH_BLC_PWM_ENABLE BIT(31)
+
struct dell_uart_backlight {
struct mutex mutex;
wait_queue_head_t wait_queue;
@@ -64,6 +73,11 @@ struct dell_uart_backlight {
int power;
};
+struct dell_pwm_backlight {
+ void __iomem *mmio;
+ u32 max_brightness;
+};
+
/* Checksum: SUM(Length and Cmd and Data) xor 0xFF */
static u8 dell_uart_checksum(u8 *buf, int len)
{
@@ -267,32 +281,97 @@ static const struct serdev_device_ops dell_uart_bl_serdev_ops = {
.write_wakeup = serdev_device_write_wakeup,
};
-static int dell_uart_bl_serdev_probe(struct serdev_device *serdev)
+/*
+ * Some Dell AIO models (e.g. OptiPlex 7410) declare DELL0501 in ACPI but
+ * have no physical UART scalar board. When the UART times out fall back
+ * to driving the Intel PCH PWM backlight registers directly.
+ */
+static int dell_pwm_update_status(struct backlight_device *bd)
{
- u8 get_version[GET_CMD_LEN], resp[MAX_RESP_LEN];
+ struct dell_pwm_backlight *pwm = bl_get_data(bd);
+ u32 duty;
+
+ if (bd->props.power != BACKLIGHT_POWER_ON ||
+ bd->props.state & BL_CORE_SUSPENDED)
+ duty = 0;
+ else
+ duty = bd->props.brightness;
+
+ writel(duty, pwm->mmio + (PCH_BLC_PWM_DUTY - PCH_BLC_PWM_CTL));
+ return 0;
+}
+
+static int dell_pwm_get_brightness(struct backlight_device *bd)
+{
+ struct dell_pwm_backlight *pwm = bl_get_data(bd);
+
+ return readl(pwm->mmio + (PCH_BLC_PWM_DUTY - PCH_BLC_PWM_CTL));
+}
+
+static const struct backlight_ops dell_pwm_backlight_ops = {
+ .update_status = dell_pwm_update_status,
+ .get_brightness = dell_pwm_get_brightness,
+};
+
+static int dell_pwm_fallback_register(struct device *dev)
+{
+ struct pci_dev *gpu __free(pci_dev_put) = NULL;
struct backlight_properties props = {};
- struct dell_uart_backlight *dell_bl;
- struct device *dev = &serdev->dev;
- int ret;
+ struct dell_pwm_backlight *pwm;
+ struct backlight_device *bl;
+ resource_size_t bar0;
+ u32 freq, duty, ctl;
- dell_bl = devm_kzalloc(dev, sizeof(*dell_bl), GFP_KERNEL);
- if (!dell_bl)
+ gpu = pci_get_class(PCI_CLASS_DISPLAY_VGA << 8, NULL);
+ if (!gpu || gpu->vendor != PCI_VENDOR_ID_INTEL) {
+ dev_warn(dev, "No Intel GPU found, cannot fall back to PCH PWM\n");
+ return -ENODEV;
+ }
+
+ bar0 = pci_resource_start(gpu, 0);
+ if (!bar0) {
+ dev_warn(dev, "Intel GPU BAR0 not assigned\n");
+ return -ENODEV;
+ }
+
+ pwm = devm_kzalloc(dev, sizeof(*pwm), GFP_KERNEL);
+ if (!pwm)
return -ENOMEM;
- mutex_init(&dell_bl->mutex);
- init_waitqueue_head(&dell_bl->wait_queue);
- dell_bl->dev = dev;
+ pwm->mmio = devm_ioremap(dev, bar0 + PCH_BLC_PWM_CTL, SZ_16);
+ if (!pwm->mmio) {
+ dev_warn(dev, "Failed to ioremap PCH PWM registers\n");
+ return -ENOMEM;
+ }
- serdev_device_set_drvdata(serdev, dell_bl);
- serdev_device_set_client_ops(serdev, &dell_uart_bl_serdev_ops);
+ ctl = readl(pwm->mmio);
+ freq = readl(pwm->mmio + (PCH_BLC_PWM_FREQ - PCH_BLC_PWM_CTL));
+ duty = readl(pwm->mmio + (PCH_BLC_PWM_DUTY - PCH_BLC_PWM_CTL));
- ret = devm_serdev_device_open(dev, serdev);
- if (ret)
- return dev_err_probe(dev, ret, "opening UART device\n");
+ if (!(ctl & PCH_BLC_PWM_ENABLE) || freq == 0) {
+ dev_warn(dev, "PCH PWM not enabled by BIOS (ctl=0x%08x freq=%u)\n", ctl, freq);
+ return -ENODEV;
+ }
- /* 9600 bps, no flow control, these are the default but set them to be sure */
- serdev_device_set_baudrate(serdev, 9600);
- serdev_device_set_flow_control(serdev, false);
+ pwm->max_brightness = freq;
+
+ dev_info(dev, "Falling back to PCH PWM backlight (max=%u current=%u)\n", freq, duty);
+
+ props.type = BACKLIGHT_PLATFORM;
+ props.brightness = duty;
+ props.max_brightness = freq;
+
+ bl = devm_backlight_device_register(dev, "dell_uart_backlight", dev, pwm,
+ &dell_pwm_backlight_ops, &props);
+ return PTR_ERR_OR_ZERO(bl);
+}
+
+static int dell_uart_bl_init(struct device *dev, struct serdev_device *serdev,
+ struct dell_uart_backlight *dell_bl)
+{
+ u8 get_version[GET_CMD_LEN], resp[MAX_RESP_LEN];
+ struct backlight_properties props = {};
+ int ret;
get_version[0] = DELL_SOF(GET_CMD_LEN);
get_version[1] = CMD_GET_VERSION;
@@ -300,7 +379,7 @@ static int dell_uart_bl_serdev_probe(struct serdev_device *serdev)
ret = dell_uart_bl_command(dell_bl, get_version, GET_CMD_LEN, resp, MAX_RESP_LEN);
if (ret)
- return dev_err_probe(dev, ret, "getting firmware version\n");
+ return ret;
dev_dbg(dev, "Firmware version: %.*s\n", resp[RESP_LEN] - 3, resp + RESP_DATA);
@@ -318,13 +397,45 @@ static int dell_uart_bl_serdev_probe(struct serdev_device *serdev)
props.max_brightness = DELL_BL_MAX_BRIGHTNESS;
props.power = dell_bl->power;
- dell_bl->bl = devm_backlight_device_register(dev, "dell_uart_backlight",
- dev, dell_bl,
- &dell_uart_backlight_ops,
- &props);
+ dell_bl->bl = devm_backlight_device_register(dev, "dell_uart_backlight", dev, dell_bl,
+ &dell_uart_backlight_ops, &props);
return PTR_ERR_OR_ZERO(dell_bl->bl);
}
+static int dell_uart_bl_serdev_probe(struct serdev_device *serdev)
+{
+ struct dell_uart_backlight *dell_bl;
+ struct device *dev = &serdev->dev;
+ int ret;
+
+ dell_bl = devm_kzalloc(dev, sizeof(*dell_bl), GFP_KERNEL);
+ if (!dell_bl)
+ return -ENOMEM;
+
+ mutex_init(&dell_bl->mutex);
+ init_waitqueue_head(&dell_bl->wait_queue);
+ dell_bl->dev = dev;
+
+ serdev_device_set_drvdata(serdev, dell_bl);
+ serdev_device_set_client_ops(serdev, &dell_uart_bl_serdev_ops);
+
+ ret = devm_serdev_device_open(dev, serdev);
+ if (ret)
+ return dev_err_probe(dev, ret, "opening UART device\n");
+
+ /* 9600 bps, no flow control, these are the default but set them to be sure */
+ serdev_device_set_baudrate(serdev, 9600);
+ serdev_device_set_flow_control(serdev, false);
+
+ ret = dell_uart_bl_init(dev, serdev, dell_bl);
+ if (ret == -ETIMEDOUT) {
+ dev_info(dev, "UART backlight controller not responding, trying PCH PWM fallback\n");
+ return dell_pwm_fallback_register(dev);
+ }
+
+ return ret;
+}
+
static struct serdev_device_driver dell_uart_bl_serdev_driver = {
.probe = dell_uart_bl_serdev_probe,
.driver = {
--
2.43.0