drm/amd/display: Add a trace event for brightness programming

[Why]
Brightness programming may involve a conversion of a user requested
brightness against what was in a custom brightness curve. The values
might not match what a user programmed.

[How]
Add a new trace event to show specific converted brightness values.

Reviewed-by: Alex Hung <alex.hung@amd.com>
Link: https://lore.kernel.org/r/20250623171114.1156451-2-mario.limonciello@amd.com
Signed-off-by: Mario Limonciello <mario.limonciello@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>

3 files changed, 52 insertions, 0 deletions

diff --git a/Documentation/gpu/amdgpu/debugging.rst b/Documentation/gpu/amdgpu/debugging.rst
index 7cbfea0606e1..ac914d524741 100644
--- a/Documentation/gpu/amdgpu/debugging.rst
+++ b/Documentation/gpu/amdgpu/debugging.rst
@@ -85,3 +85,21 @@ UMR
 GPU debugging and diagnostics tool.  Please see the umr
 `documentation <https://umr.readthedocs.io/en/main/>`_ for more information
 about its capabilities.
+
+Debugging backlight brightness
+==============================
+Default backlight brightness is intended to be set via the policy advertised
+by the firmware.  Firmware will often provide different defaults for AC or DC.
+Furthermore, some userspace software will save backlight brightness during
+the previous boot and attempt to restore it.
+
+Some firmware also has support for a feature called "Custom Backlight Curves"
+where an input value for brightness is mapped along a linearly interpolated
+curve of brightness values that better match display characteristics.
+
+In the event of problems happening with backlight, there is a trace event
+that can be enabled at bootup to log every brightness change request.
+This can help isolate where the problem is. To enable the trace event add
+the following to the kernel command line:
+
+  tp_printk trace_event=amdgpu_dm:amdgpu_dm_brightness:mod:amdgpu trace_buf_size=1M
diff --git a/drivers/gpu/drm/amd/display/amdgpu_dm/amdgpu_dm.c b/drivers/gpu/drm/amd/display/amdgpu_dm/amdgpu_dm.c
index 14e7c2aea206..9310cb7629a2 100644
--- a/drivers/gpu/drm/amd/display/amdgpu_dm/amdgpu_dm.c
+++ b/drivers/gpu/drm/amd/display/amdgpu_dm/amdgpu_dm.c
@@ -4861,6 +4861,14 @@ static void amdgpu_dm_backlight_set_level(struct amdgpu_display_manager *dm,
 		reallow_idle = true;
 	}
 
+	if (trace_amdgpu_dm_brightness_enabled()) {
+		trace_amdgpu_dm_brightness(__builtin_return_address(0),
+					   user_brightness,
+					   brightness,
+					   caps->aux_support,
+					   power_supply_is_system_supplied() > 0);
+	}
+
 	if (caps->aux_support) {
 		rc = dc_link_set_backlight_level_nits(link, true, brightness,
 						      AUX_BL_DEFAULT_TRANSITION_TIME_MS);
diff --git a/drivers/gpu/drm/amd/display/amdgpu_dm/amdgpu_dm_trace.h b/drivers/gpu/drm/amd/display/amdgpu_dm/amdgpu_dm_trace.h
index 4686d4b0cbad..95f890fda8aa 100644
--- a/drivers/gpu/drm/amd/display/amdgpu_dm/amdgpu_dm_trace.h
+++ b/drivers/gpu/drm/amd/display/amdgpu_dm/amdgpu_dm_trace.h
@@ -726,6 +726,32 @@ TRACE_EVENT(dcn_optc_lock_unlock_state,
 	    )
 );
 
+TRACE_EVENT(amdgpu_dm_brightness,
+	TP_PROTO(void *function, u32 user_brightness, u32 converted_brightness, bool aux, bool ac),
+	TP_ARGS(function, user_brightness, converted_brightness, aux, ac),
+	TP_STRUCT__entry(
+		__field(void *, function)
+		__field(u32, user_brightness)
+		__field(u32, converted_brightness)
+		__field(bool, aux)
+		__field(bool, ac)
+	),
+	TP_fast_assign(
+		__entry->function = function;
+		__entry->user_brightness = user_brightness;
+		__entry->converted_brightness = converted_brightness;
+		__entry->aux = aux;
+		__entry->ac = ac;
+	),
+	TP_printk("%ps: brightness requested=%u converted=%u aux=%s power=%s",
+		  (void *)__entry->function,
+		  (u32)__entry->user_brightness,
+		  (u32)__entry->converted_brightness,
+		  (__entry->aux) ? "true" : "false",
+		  (__entry->ac) ? "AC" : "DC"
+	)
+);
+
 #endif /* _AMDGPU_DM_TRACE_H_ */
 
 #undef TRACE_INCLUDE_PATH