From 4206046f12b7f029ff95c6fda4fd5bfc81564131 Mon Sep 17 00:00:00 2001
From: Xaver Hugl <xaver.hugl@gmail.com>
Date: Tue, 28 Sep 2021 22:31:11 +0200
Subject: [PATCH] Re-enable proper gamma ramps with AMS

Instead of downgrading everyone else to legacy gamma, only exclude
the huge and buggy gamma ramp of Intel TIgerLake hardware.
---
 src/colordevice.cpp                           | 3 ++-
 src/plugins/platforms/drm/drm_object_crtc.cpp | 5 +++++
 src/plugins/platforms/drm/drm_object_crtc.h   | 1 +
 3 files changed, 8 insertions(+), 1 deletion(-)

diff --git a/src/colordevice.cpp b/src/colordevice.cpp
index f3e902b915..585b867dec 100644
--- a/src/colordevice.cpp
+++ b/src/colordevice.cpp
@@ -353,7 +353,8 @@ void ColorDevice::update()
     uint16_t *blueChannel = gammaRamp.blue();
 
     for (uint32_t i = 0; i < gammaRamp.size(); ++i) {
-        const uint16_t index = (i * 0xffff) / (gammaRamp.size() - 1);
+        // ensure 64 bit calculation to prevent overflows
+        const uint16_t index = (static_cast<uint64_t>(i) * 0xffff) / (gammaRamp.size() - 1);
 
         const uint16_t in[3] = { index, index, index };
         uint16_t out[3] = { 0 };
diff --git a/src/plugins/platforms/drm/drm_object_crtc.cpp b/src/plugins/platforms/drm/drm_object_crtc.cpp
index c6f3c73f7f..3979b0b4f1 100644
--- a/src/plugins/platforms/drm/drm_object_crtc.cpp
+++ b/src/plugins/platforms/drm/drm_object_crtc.cpp
@@ -24,6 +24,7 @@ DrmCrtc::DrmCrtc(DrmGpu *gpu, uint32_t crtcId, int pipeIndex, DrmPlane *primaryP
         PropertyDefinition(QByteArrayLiteral("ACTIVE"), Requirement::Required),
         PropertyDefinition(QByteArrayLiteral("VRR_ENABLED"), Requirement::Optional),
         PropertyDefinition(QByteArrayLiteral("GAMMA_LUT"), Requirement::Optional),
+        PropertyDefinition(QByteArrayLiteral("GAMMA_LUT_SIZE"), Requirement::Optional)
     }, DRM_MODE_OBJECT_CRTC)
     , m_crtc(drmModeGetCrtc(gpu->fd(), crtcId))
     , m_pipeIndex(pipeIndex)
@@ -81,6 +82,10 @@ void DrmCrtc::setNext(const QSharedPointer<DrmBuffer> &buffer)
 
 int DrmCrtc::gammaRampSize() const
 {
+    // limit atomic gamma ramp to 4096 to work around https://gitlab.freedesktop.org/drm/intel/-/issues/3916
+    if (auto prop = getProp(PropertyIndex::Gamma_LUT_Size); prop && prop->current() <= 4096) {
+        return prop->current();
+    }
     return m_crtc->gamma_size;
 }
 
diff --git a/src/plugins/platforms/drm/drm_object_crtc.h b/src/plugins/platforms/drm/drm_object_crtc.h
index 1b31189fe6..ce285bd314 100644
--- a/src/plugins/platforms/drm/drm_object_crtc.h
+++ b/src/plugins/platforms/drm/drm_object_crtc.h
@@ -34,6 +34,7 @@ public:
         Active,
         VrrEnabled,
         Gamma_LUT,
+        Gamma_LUT_Size,
         Count
     };