fix(drm): clear inherited CRTC colour state on modeset and close

Another DRM master can leave a CRTC GAMMA_LUT (or DEGAMMA_LUT/CTM) set.
The kernel keeps it across masters, and it applied on top of the PQ
output, distorting SDR content scanned out through the plane colour
pipeline on HDR outputs. forceBypass now writes 0 to the three CRTC
colour properties with the planes' Bypass pipeline.

commit: e4fec9bc3dc2227e3c0a3da3bd2820fb1e66391f
author: brice · 10d ago · 2026-10-12 12:39 UTC
parent: 0e0d396
signature: GPG VerifiedCD88F9E564E0102646FB6077A1A07C7C7C3E349D · arch-gpg Verified for the committer email

3 files · +56 -0

M docs/config.md +1 -0


@@ -199,6 +199,7 @@ Hardware-decoded video can use NV12 (8-bit) or P010 (10-bit) DMA-BUF subsurfaces
199	199		- Fullscreen HDR buffers (10-bit, PQ) go straight to the display unchanged.
200	200		- Opaque SDR (sRGB) buffers also skip composition on HDR outputs when the KMS plane has a colour pipeline (`DRM_CLIENT_CAP_PLANE_COLOR_PIPELINE`): sRGB curve, `sdr-brightness`, BT.709 to BT.2020, PQ, the same maths as composition. This applies to a fullscreen window (direct scanout) and to a lone window on an overlay plane. The compositor looks for that chain on each plane, whatever the driver; amdgpu on recent kernels has it. Without it, with alpha, with YUV or extended-linear content, SDR windows are composed, and the log gives the reason (`hdr_sdr_content`, `yuv`). When the driver refuses a pipeline (tested once per plane, buffer format and cursor state), the window is composed with reason `color_refused`.
201	201		- Composed frames and every modeset leave the planes' pipeline on Bypass.
	202	+	- Every modeset and exit clear the CRTC's `DEGAMMA_LUT`, `CTM` and `GAMMA_LUT`, which this compositor never uses: a gamma ramp left by another DRM master would otherwise distort every frame.
202	203		- A VT switch resets the pipeline first: before the seat is disabled, each output that applies a pipeline commits Bypass (at most 100 ms are spent waiting for it, then the switch goes on and a warning is logged). The pipeline survives a master change, and a client without `DRM_CLIENT_CAP_PLANE_COLOR_PIPELINE` cannot clear it. A crash or `kill -9` cannot do this reset: the next DRM master inherits the pipeline, with its colours distorted, until a client that sets the cap commits a modeset (or this compositor starts again).
203	204		- Windowed and otherwise composed HDR content keeps its full range in a linear fp16 image before PQ output; the display tone-maps it according to its EDID metadata. SDR surfaces are blended in linear light on HDR outputs, so antialiased edges may look slightly different than sRGB-space blending. Dim veils (`floating.dim`, `stash.dim`) darken SDR white as much as on an SDR output; brighter HDR highlights stay a little brighter.
204	205		- Screenshots and capture use opaque 8-bit sRGB. HDR capture applies GPU tone mapping and reduces out-of-sRGB gamut: values below 90% of SDR reference white stay unchanged, while brighter values are compressed to preserve highlight differences (SDR reference white maps to about 245/255 when capturing an HDR output, versus 255/255 on an SDR output). This capture-only transform does not change HDR display output.


@@ -199,6 +199,7 @@ Hardware-decoded video can use NV12 (8-bit) or P010 (10-bit) DMA-BUF subsurfaces
199	- Fullscreen HDR buffers (10-bit, PQ) go straight to the display unchanged.	199		- Fullscreen HDR buffers (10-bit, PQ) go straight to the display unchanged.
200	- Opaque SDR (sRGB) buffers also skip composition on HDR outputs when the KMS plane has a colour pipeline (`DRM_CLIENT_CAP_PLANE_COLOR_PIPELINE`): sRGB curve, `sdr-brightness`, BT.709 to BT.2020, PQ, the same maths as composition. This applies to a fullscreen window (direct scanout) and to a lone window on an overlay plane. The compositor looks for that chain on each plane, whatever the driver; amdgpu on recent kernels has it. Without it, with alpha, with YUV or extended-linear content, SDR windows are composed, and the log gives the reason (`hdr_sdr_content`, `yuv`). When the driver refuses a pipeline (tested once per plane, buffer format and cursor state), the window is composed with reason `color_refused`.	200		- Opaque SDR (sRGB) buffers also skip composition on HDR outputs when the KMS plane has a colour pipeline (`DRM_CLIENT_CAP_PLANE_COLOR_PIPELINE`): sRGB curve, `sdr-brightness`, BT.709 to BT.2020, PQ, the same maths as composition. This applies to a fullscreen window (direct scanout) and to a lone window on an overlay plane. The compositor looks for that chain on each plane, whatever the driver; amdgpu on recent kernels has it. Without it, with alpha, with YUV or extended-linear content, SDR windows are composed, and the log gives the reason (`hdr_sdr_content`, `yuv`). When the driver refuses a pipeline (tested once per plane, buffer format and cursor state), the window is composed with reason `color_refused`.
201	- Composed frames and every modeset leave the planes' pipeline on Bypass.	201		- Composed frames and every modeset leave the planes' pipeline on Bypass.
		202	+	- Every modeset and exit clear the CRTC's `DEGAMMA_LUT`, `CTM` and `GAMMA_LUT`, which this compositor never uses: a gamma ramp left by another DRM master would otherwise distort every frame.
202	- A VT switch resets the pipeline first: before the seat is disabled, each output that applies a pipeline commits Bypass (at most 100 ms are spent waiting for it, then the switch goes on and a warning is logged). The pipeline survives a master change, and a client without `DRM_CLIENT_CAP_PLANE_COLOR_PIPELINE` cannot clear it. A crash or `kill -9` cannot do this reset: the next DRM master inherits the pipeline, with its colours distorted, until a client that sets the cap commits a modeset (or this compositor starts again).	203		- A VT switch resets the pipeline first: before the seat is disabled, each output that applies a pipeline commits Bypass (at most 100 ms are spent waiting for it, then the switch goes on and a warning is logged). The pipeline survives a master change, and a client without `DRM_CLIENT_CAP_PLANE_COLOR_PIPELINE` cannot clear it. A crash or `kill -9` cannot do this reset: the next DRM master inherits the pipeline, with its colours distorted, until a client that sets the cap commits a modeset (or this compositor starts again).
203	- Windowed and otherwise composed HDR content keeps its full range in a linear fp16 image before PQ output; the display tone-maps it according to its EDID metadata. SDR surfaces are blended in linear light on HDR outputs, so antialiased edges may look slightly different than sRGB-space blending. Dim veils (`floating.dim`, `stash.dim`) darken SDR white as much as on an SDR output; brighter HDR highlights stay a little brighter.	204		- Windowed and otherwise composed HDR content keeps its full range in a linear fp16 image before PQ output; the display tone-maps it according to its EDID metadata. SDR surfaces are blended in linear light on HDR outputs, so antialiased edges may look slightly different than sRGB-space blending. Dim veils (`floating.dim`, `stash.dim`) darken SDR white as much as on an SDR output; brighter HDR highlights stay a little brighter.
204	- Screenshots and capture use opaque 8-bit sRGB. HDR capture applies GPU tone mapping and reduces out-of-sRGB gamut: values below 90% of SDR reference white stay unchanged, while brighter values are compressed to preserve highlight differences (SDR reference white maps to about 245/255 when capturing an HDR output, versus 255/255 on an SDR output). This capture-only transform does not change HDR display output.	205		- Screenshots and capture use opaque 8-bit sRGB. HDR capture applies GPU tone mapping and reduces out-of-sRGB gamut: values below 90% of SDR reference white stay unchanged, while brighter values are compressed to preserve highlight differences (SDR reference white maps to about 245/255 when capturing an HDR output, versus 255/255 on an SDR output). This capture-only transform does not change HDR display output.

M internal/adapters/drm/colorop.go +6 -0


@@ -90,9 +90,15 @@ func (o Output) primaryColorProps(req atomicReq, mode colorMode) {
90	90		// forceBypass sets Bypass on the primary and overlay planes whether or not
91	91		// the applied state is known to differ: for requests that must leave a
92	92		// known state (modeset, close).
	93	+	// The CRTC's DEGAMMA_LUT, CTM and GAMMA_LUT are cleared with them: this
	94	+	// compositor never sets them, and a ramp left by another DRM master would
	95	+	// apply on top of the PQ output.
93	96		func (o Output) forceBypass(req atomicReq) {
94	97		o.primary.colorProps(req, colorBypass, 0, 0)
95	98		o.overlay.colorProps(req, colorBypass, 0, 0)
	99	+	for _, name := range [...]string{"DEGAMMA_LUT", "CTM", "GAMMA_LUT"} {
	100	+	req.set(o.crtc, o.crtcProps[name], 0)
	101	+	}
96	102		}
97	103
98	104		// overlayColorProps puts mode on the overlay plane if it applies another one.


@@ -90,9 +90,15 @@ func (o Output) primaryColorProps(req atomicReq, mode colorMode) {
90	// forceBypass sets Bypass on the primary and overlay planes whether or not	90		// forceBypass sets Bypass on the primary and overlay planes whether or not
91	// the applied state is known to differ: for requests that must leave a	91		// the applied state is known to differ: for requests that must leave a
92	// known state (modeset, close).	92		// known state (modeset, close).
		93	+	// The CRTC's DEGAMMA_LUT, CTM and GAMMA_LUT are cleared with them: this
		94	+	// compositor never sets them, and a ramp left by another DRM master would
		95	+	// apply on top of the PQ output.
93	func (o Output) forceBypass(req atomicReq) {	96		func (o Output) forceBypass(req atomicReq) {
94	o.primary.colorProps(req, colorBypass, 0, 0)	97		o.primary.colorProps(req, colorBypass, 0, 0)
95	o.overlay.colorProps(req, colorBypass, 0, 0)	98		o.overlay.colorProps(req, colorBypass, 0, 0)
		99	+	for _, name := range [...]string{"DEGAMMA_LUT", "CTM", "GAMMA_LUT"} {
		100	+	req.set(o.crtc, o.crtcProps[name], 0)
		101	+	}
96	}	102		}
97		103
98	// overlayColorProps puts mode on the overlay plane if it applies another one.	104		// overlayColorProps puts mode on the overlay plane if it applies another one.

M internal/adapters/drm/colorop_test.go +49 -0


@@ -2,6 +2,7 @@ package drm
2	2
3	3		import (
4	4		"encoding/binary"
	5	+	"maps"
5	6		"math"
6	7		"os"
7	8		"regexp"

@@ -526,6 +527,54 @@ func TestColorBypassOnComposedProtectedOffCloseAndModeset(t *testing.T) {
526	527		})
527	528		}
528	529
	530	+	// A modeset and Close clear the CRTC's DEGAMMA_LUT, CTM and GAMMA_LUT,
	531	+	// which another DRM master may have left: a ramp kept there distorts every
	532	+	// frame, the PQ ones included.
	533	+	func TestCRTCColorClearedOnModesetAndClose(t *testing.T) {
	534	+	const pCTM, pGamma = 63, 64
	535	+	cleared := func(t *testing.T, c commitRec) {
	536	+	t.Helper()
	537	+	for _, p := range []uint32{pDegamma, pCTM, pGamma} {
	538	+	if v, ok := c.req.value(tCrtc, p); !ok \|\| v != 0 {
	539	+	t.Errorf("CRTC property %d = %d %v, want 0", p, v, ok)
	540	+	}
	541	+	}
	542	+	}
	543	+	setup := func(t testing.T) (Output, mockkms, []commitRec) {
	544	+	o, k, commits := colorOutput(t, false, nil)
	545	+	o.crtcProps = maps.Clone(o.crtcProps)
	546	+	o.crtcProps["DEGAMMA_LUT"], o.crtcProps["CTM"], o.crtcProps["GAMMA_LUT"] = pDegamma, pCTM, pGamma
	547	+	return o, k, commits
	548	+	}
	549	+	t.Run("modeset", func(t *testing.T) {
	550	+	o, k, commits := setup(t)
	551	+	k.EXPECT().createBlob(mock.Anything).Return(99, nil).Once()
	552	+	if err := o.modeset(); err != nil {
	553	+	t.Fatal(err)
	554	+	}
	555	+	cleared(t, (*commits)[0])
	556	+	})
	557	+	t.Run("close", func(t *testing.T) {
	558	+	o, k, commits := setup(t)
	559	+	k.EXPECT().rmFB(mock.Anything).Return(nil)
	560	+	k.EXPECT().destroyBlob(uint32(61)).Return(nil).Once()
	561	+	o.Close()
	562	+	cleared(t, (*commits)[0])
	563	+	})
	564	+	t.Run("frame", func(t *testing.T) {
	565	+	// A frame commit leaves them alone: they stay as the modeset left them.
	566	+	o, _, commits := setup(t)
	567	+	if ok, err := o.commitScanoutRect(80, sdrContent(o), pendingFrame{}, fullPlaneRect(200, 100), colorSDRToPQ); !ok \|\| err != nil {
	568	+	t.Fatalf("ok=%v err=%v", ok, err)
	569	+	}
	570	+	for _, p := range []uint32{pDegamma, pCTM, pGamma} {
	571	+	if _, ok := (*commits)[0].req.value(tCrtc, p); ok {
	572	+	t.Errorf("frame commit writes CRTC property %d", p)
	573	+	}
	574	+	}
	575	+	})
	576	+	}
	577	+
529	578		// The overlay's pipeline is driven with the primary on Bypass; a frame
530	579		// without the overlay turns it back to Bypass once.
531	580		func TestColorOverlayTransitions(t *testing.T) {


@@ -2,6 +2,7 @@ package drm
2		2
3	import (	3		import (
4	"encoding/binary"	4		"encoding/binary"
		5	+	"maps"
5	"math"	6		"math"
6	"os"	7		"os"
7	"regexp"	8		"regexp"

@@ -526,6 +527,54 @@ func TestColorBypassOnComposedProtectedOffCloseAndModeset(t *testing.T) {
526	})	527		})
527	}	528		}
528		529
		530	+	// A modeset and Close clear the CRTC's DEGAMMA_LUT, CTM and GAMMA_LUT,
		531	+	// which another DRM master may have left: a ramp kept there distorts every
		532	+	// frame, the PQ ones included.
		533	+	func TestCRTCColorClearedOnModesetAndClose(t *testing.T) {
		534	+	const pCTM, pGamma = 63, 64
		535	+	cleared := func(t *testing.T, c commitRec) {
		536	+	t.Helper()
		537	+	for _, p := range []uint32{pDegamma, pCTM, pGamma} {
		538	+	if v, ok := c.req.value(tCrtc, p); !ok \|\| v != 0 {
		539	+	t.Errorf("CRTC property %d = %d %v, want 0", p, v, ok)
		540	+	}
		541	+	}
		542	+	}
		543	+	setup := func(t testing.T) (Output, mockkms, []commitRec) {
		544	+	o, k, commits := colorOutput(t, false, nil)
		545	+	o.crtcProps = maps.Clone(o.crtcProps)
		546	+	o.crtcProps["DEGAMMA_LUT"], o.crtcProps["CTM"], o.crtcProps["GAMMA_LUT"] = pDegamma, pCTM, pGamma
		547	+	return o, k, commits
		548	+	}
		549	+	t.Run("modeset", func(t *testing.T) {
		550	+	o, k, commits := setup(t)
		551	+	k.EXPECT().createBlob(mock.Anything).Return(99, nil).Once()
		552	+	if err := o.modeset(); err != nil {
		553	+	t.Fatal(err)
		554	+	}
		555	+	cleared(t, (*commits)[0])
		556	+	})
		557	+	t.Run("close", func(t *testing.T) {
		558	+	o, k, commits := setup(t)
		559	+	k.EXPECT().rmFB(mock.Anything).Return(nil)
		560	+	k.EXPECT().destroyBlob(uint32(61)).Return(nil).Once()
		561	+	o.Close()
		562	+	cleared(t, (*commits)[0])
		563	+	})
		564	+	t.Run("frame", func(t *testing.T) {
		565	+	// A frame commit leaves them alone: they stay as the modeset left them.
		566	+	o, _, commits := setup(t)
		567	+	if ok, err := o.commitScanoutRect(80, sdrContent(o), pendingFrame{}, fullPlaneRect(200, 100), colorSDRToPQ); !ok \|\| err != nil {
		568	+	t.Fatalf("ok=%v err=%v", ok, err)
		569	+	}
		570	+	for _, p := range []uint32{pDegamma, pCTM, pGamma} {
		571	+	if _, ok := (*commits)[0].req.value(tCrtc, p); ok {
		572	+	t.Errorf("frame commit writes CRTC property %d", p)
		573	+	}
		574	+	}
		575	+	})
		576	+	}
		577	+
529	// The overlay's pipeline is driven with the primary on Bypass; a frame	578		// The overlay's pipeline is driven with the primary on Bypass; a frame
530	// without the overlay turns it back to Bypass once.	579		// without the overlay turns it back to Bypass once.
531	func TestColorOverlayTransitions(t *testing.T) {	580		func TestColorOverlayTransitions(t *testing.T) {