feat: add laptop/display brightness modulation

kernel/src/Api/BuildNo.hpp

@@ -12,4 +12,4 @@
 
 #pragma once
 
-#define MONTAUK_BUILD_NUMBER 283
+#define MONTAUK_BUILD_NUMBER 287

kernel/src/Api/Graphics.hpp

@@ -11,6 +11,7 @@
 #include <Memory/HHDM.hpp>
 #include <Graphics/Framebuffer.hpp>
 #include <Drivers/Graphics/IntelGPU.hpp>
+#include <ACPI/AcpiVideo.hpp>
 #include <Terminal/Terminal.hpp>
 
 #include "Syscall.hpp"
@@ -128,8 +129,30 @@ namespace montauk::abi {
         return GPU::Flip((int)index, (flags & 1) != 0, proc->pid);
     }
 
+    // The firmware's ACPI brightness methods drive the built-in panel, so
+    // they stand in only when that panel is what is on screen (or no native
+    // driver knows what is): never for an external monitor without DDC/CI.
+    static bool AcpiBrightnessApplies() {
+        if (!Drivers::Graphics::IntelGPU::IsInitialized()) return true;
+        uint8_t type = Drivers::Graphics::IntelGPU::ActiveConnectorType();
+        return type == DISPLAY_CONNECTOR_EDP || type == DISPLAY_CONNECTOR_LVDS;
+    }
+
+    static int DisplayBrightness(int percent) {
+        int rc = Drivers::Graphics::IntelGPU::Brightness(percent);
+        if (rc == -1 && AcpiBrightnessApplies())
+            rc = Hal::AcpiVideo::Brightness(percent);
+        return rc;
+    }
+
     static int64_t Sys_DisplayInfo(DisplayInfo* out) {
-        return Drivers::Graphics::IntelGPU::GetDisplayInfo(out) ? Success : NoDevice;
+        if (!Drivers::Graphics::IntelGPU::GetDisplayInfo(out)) return NoDevice;
+        if (out->brightness < 0 && AcpiBrightnessApplies()) {
+            out->brightness = Hal::AcpiVideo::Brightness(-1);
+            if (out->brightness >= 0) out->capabilities |= DISPLAY_CAP_BRIGHTNESS;
+            else out->brightness = -1;
+        }
+        return Success;
     }
 
     static int64_t Sys_DisplayModes(DisplayModeInfo* out, int maxCount) {
@@ -145,8 +168,12 @@ namespace montauk::abi {
         return rc;
     }
 
+    // percent < 0 queries. Returns 0..100, NoDevice when the active output
+    // has no controllable backlight, IoFailure when the hardware refused.
     static int64_t Sys_DisplayBrightness(int percent) {
-        return Drivers::Graphics::IntelGPU::Brightness(percent);
+        int rc = DisplayBrightness(percent);
+        if (rc >= 0) return rc;
+        return rc == -1 ? NoDevice : IoFailure;
     }
 
     static uint64_t Sys_TermSize() {

kernel/src/Api/Syscall.cpp

@@ -207,7 +207,7 @@ namespace montauk::abi {
                 return Sys_DisplaySetMode((int)frame->arg1);
             case SYS_DISPLAYBRIGHTNESS:
                 if ((int64_t)frame->arg1 >= 0 &&
-                    !Sched::Consent::Require(CAP_DISPLAY_ADMIN)) return CapabilityDenied;
+                    !Sched::Consent::Require(CAP_BRIGHTNESS)) return CapabilityDenied;
                 return Sys_DisplayBrightness((int)frame->arg1);
             case SYS_GETEXECPATH:
                 if (!UserMemory::Range(frame->arg2 ? frame->arg1 : frame->arg1, frame->arg2, true)) return BadPointer;

kernel/src/Api/Syscall.hpp

@@ -385,8 +385,14 @@ namespace montauk::abi {
        never part of a session mask, never part of "all", and never
        delegated by the compositor to what it launches. */
     static constexpr uint64_t CAP_COMPOSITOR     = 1ULL << 13;
-    static constexpr uint64_t CAP_ALL            = (1ULL << 14) - 1;
-    static constexpr uint64_t CAP_STANDARD_SESSION = CAP_POWER_REQUEST | CAP_SUSPEND;
+    /* Set the backlight of the active display (SYS_DISPLAYBRIGHTNESS).
+       A seat preference rather than display administration, so every
+       session holds it; CAP_DISPLAY_ADMIN keeps mode setting. Reading the
+       level needs nothing. */
+    static constexpr uint64_t CAP_BRIGHTNESS     = 1ULL << 14;
+    static constexpr uint64_t CAP_ALL            = (1ULL << 15) - 1;
+    static constexpr uint64_t CAP_STANDARD_SESSION =
+        CAP_POWER_REQUEST | CAP_SUSPEND | CAP_BRIGHTNESS;
     static constexpr uint64_t CAP_ADMIN_SESSION =
         CAP_STANDARD_SESSION | CAP_PROCESS_ADMIN | CAP_STORAGE_ADMIN |
         CAP_RAW_STORAGE | CAP_NETWORK_ADMIN | CAP_SET_TIME | CAP_USER_ADMIN |
@@ -438,7 +444,7 @@ namespace montauk::abi {
        deferred delegation, not amplification. */
     static constexpr uint64_t CAP_CONSENT_ELIGIBLE =
         CAP_STORAGE_ADMIN | CAP_NETWORK_ADMIN | CAP_SET_TIME |
-        CAP_DISPLAY_ADMIN | CAP_LOG_READ;
+        CAP_DISPLAY_ADMIN | CAP_LOG_READ | CAP_BRIGHTNESS;
     static_assert((CAP_CONSENT_ELIGIBLE & ~CAP_ADMIN_SESSION) == 0,
                   "consent can only hand out authority an admin session holds");