Panels using a PWM-controlled backlight source do not have a standard
way to communicate their valid PWM ranges.
On x86 the ranges are read from ACPI through driver-specific tables.
The built-in ranges are not necessarily correct, or may grow stale if an
older device can be retrofitted with newer panels.

Add a quirk infrastructure with which the minimum valid backlight value
can be maintained as part of the kernel.

Signed-off-by: Thomas Weißschuh <linux@weissschuh.net>
Tested-by: Dustin L. Howett <dustin@howett.net>
Reviewed-by: Mario Limonciello <mario.limonciello@amd.com>
Reviewed-by: Harry Wentland <harry.wentland@amd.com>
---
 Documentation/gpu/drm-kms-helpers.rst        |  3 ++
 drivers/gpu/drm/Kconfig                      |  4 ++
 drivers/gpu/drm/Makefile                     |  1 +
 drivers/gpu/drm/drm_panel_backlight_quirks.c | 70 ++++++++++++++++++++++++++++
 include/drm/drm_utils.h                      |  4 ++
 5 files changed, 82 insertions(+)

diff --git a/Documentation/gpu/drm-kms-helpers.rst b/Documentation/gpu/drm-kms-helpers.rst
index 8cf2f041af4704875910ce8228ae04615d0f21bd..b4ee25af1702b0019e0de5f9ee66d2dbdac2c664 100644
--- a/Documentation/gpu/drm-kms-helpers.rst
+++ b/Documentation/gpu/drm-kms-helpers.rst
@@ -221,6 +221,9 @@ Panel Helper Reference
 .. kernel-doc:: drivers/gpu/drm/drm_panel_orientation_quirks.c
    :export:
 
+.. kernel-doc:: drivers/gpu/drm/drm_panel_backlight_quirks.c
+   :export:
+
 Panel Self Refresh Helper Reference
 ===================================
 
diff --git a/drivers/gpu/drm/Kconfig b/drivers/gpu/drm/Kconfig
index 5504721007cc190e7d768d42aa9633baa0115f5e..7cf9aa476b4745419f1002a7866fe974d852a2be 100644
--- a/drivers/gpu/drm/Kconfig
+++ b/drivers/gpu/drm/Kconfig
@@ -526,6 +526,10 @@ config DRM_HYPERV
 config DRM_EXPORT_FOR_TESTS
 	bool
 
+# Separate option as not all DRM drivers use it
+config DRM_PANEL_BACKLIGHT_QUIRKS
+	tristate
+
 config DRM_LIB_RANDOM
 	bool
 	default n
diff --git a/drivers/gpu/drm/Makefile b/drivers/gpu/drm/Makefile
index 463afad1b5ca6275e61223adc8ca036c3d4d6b03..06b73068d2483045f819b35095c7dc83d6fcbd53 100644
--- a/drivers/gpu/drm/Makefile
+++ b/drivers/gpu/drm/Makefile
@@ -95,6 +95,7 @@ drm-$(CONFIG_DRM_PANIC_SCREEN_QR_CODE) += drm_panic_qr.o
 obj-$(CONFIG_DRM)	+= drm.o
 
 obj-$(CONFIG_DRM_PANEL_ORIENTATION_QUIRKS) += drm_panel_orientation_quirks.o
+obj-$(CONFIG_DRM_PANEL_BACKLIGHT_QUIRKS) += drm_panel_backlight_quirks.o
 
 #
 # Memory-management helpers
diff --git a/drivers/gpu/drm/drm_panel_backlight_quirks.c b/drivers/gpu/drm/drm_panel_backlight_quirks.c
new file mode 100644
index 0000000000000000000000000000000000000000..6b8bbed77c7f1f3570a6e68c276244cfe4386f6c
--- /dev/null
+++ b/drivers/gpu/drm/drm_panel_backlight_quirks.c
@@ -0,0 +1,70 @@
+// SPDX-License-Identifier: GPL-2.0
+
+#include <linux/array_size.h>
+#include <linux/dmi.h>
+#include <linux/mod_devicetable.h>
+#include <linux/module.h>
+#include <drm/drm_edid.h>
+#include <drm/drm_utils.h>
+
+struct drm_panel_min_backlight_quirk {
+	struct {
+		enum dmi_field field;
+		const char * const value;
+	} dmi_match;
+	struct drm_edid_ident ident;
+	u8 min_brightness;
+};
+
+static const struct drm_panel_min_backlight_quirk drm_panel_min_backlight_quirks[] = {
+};
+
+static bool drm_panel_min_backlight_quirk_matches(const struct drm_panel_min_backlight_quirk *quirk,
+						  const struct drm_edid *edid)
+{
+	if (!dmi_match(quirk->dmi_match.field, quirk->dmi_match.value))
+		return false;
+
+	if (!drm_edid_match(edid, &quirk->ident))
+		return false;
+
+	return true;
+}
+
+/**
+ * drm_get_panel_min_brightness_quirk - Get minimum supported brightness level for a panel.
+ * @edid: EDID of the panel to check
+ *
+ * This function checks for platform specific (e.g. DMI based) quirks
+ * providing info on the minimum backlight brightness for systems where this
+ * cannot be probed correctly from the hard-/firm-ware.
+ *
+ * Returns:
+ * A negative error value or
+ * an override value in the range [0, 255] representing 0-100% to be scaled to
+ * the drivers target range.
+ */
+int drm_get_panel_min_brightness_quirk(const struct drm_edid *edid)
+{
+	const struct drm_panel_min_backlight_quirk *quirk;
+	size_t i;
+
+	if (!IS_ENABLED(CONFIG_DMI))
+		return -ENODATA;
+
+	if (!edid)
+		return -EINVAL;
+
+	for (i = 0; i < ARRAY_SIZE(drm_panel_min_backlight_quirks); i++) {
+		quirk = &drm_panel_min_backlight_quirks[i];
+
+		if (drm_panel_min_backlight_quirk_matches(quirk, edid))
+			return quirk->min_brightness;
+	}
+
+	return -ENODATA;
+}
+EXPORT_SYMBOL(drm_get_panel_min_brightness_quirk);
+
+MODULE_DESCRIPTION("Quirks for panel backlight overrides");
+MODULE_LICENSE("GPL");
diff --git a/include/drm/drm_utils.h b/include/drm/drm_utils.h
index 70775748d243b0fd6e13a088d05c45f31cf34a4a..15fa9b6865f448cc8338c935874015fd74cb7bb2 100644
--- a/include/drm/drm_utils.h
+++ b/include/drm/drm_utils.h
@@ -12,8 +12,12 @@
 
 #include <linux/types.h>
 
+struct drm_edid;
+
 int drm_get_panel_orientation_quirk(int width, int height);
 
+int drm_get_panel_min_brightness_quirk(const struct drm_edid *edid);
+
 signed long drm_timeout_abs_to_jiffies(int64_t timeout_nsec);
 
 #endif

-- 
2.47.0