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1 /*
2  * Copyright 2015 Google Inc.
3  *
4  * Use of this source code is governed by a BSD-style license that can be
5  * found in the LICENSE file.
6  */
7 
8 #include "src/gpu/GrCaps.h"
9 
10 #include "include/gpu/GrBackendSurface.h"
11 #include "include/gpu/GrContextOptions.h"
12 #include "include/private/GrTypesPriv.h"
13 #include "src/gpu/GrBackendUtils.h"
14 #include "src/gpu/GrRenderTargetProxy.h"
15 #include "src/gpu/GrSurface.h"
16 #include "src/gpu/GrSurfaceProxy.h"
17 #include "src/gpu/GrWindowRectangles.h"
18 
GrCaps(const GrContextOptions & options)19 GrCaps::GrCaps(const GrContextOptions& options) {
20     fMipmapSupport = false;
21     fNPOTTextureTileSupport = false;
22     fReuseScratchTextures = true;
23     fReuseScratchBuffers = true;
24     fGpuTracingSupport = false;
25     fOversizedStencilSupport = false;
26     fTextureBarrierSupport = false;
27     fSampleLocationsSupport = false;
28     fDrawInstancedSupport = false;
29     fNativeDrawIndirectSupport = false;
30     fUseClientSideIndirectBuffers = false;
31     fConservativeRasterSupport = false;
32     fWireframeSupport = false;
33     fMSAAResolvesAutomatically = false;
34     fPreferDiscardableMSAAAttachment = false;
35     fUsePrimitiveRestart = false;
36     fPreferClientSideDynamicBuffers = false;
37     fPreferFullscreenClears = false;
38     fTwoSidedStencilRefsAndMasksMustMatch = false;
39     fMustClearUploadedBufferData = false;
40     fShouldInitializeTextures = false;
41     fSupportsAHardwareBufferImages = false;
42     fFenceSyncSupport = false;
43     fSemaphoreSupport = false;
44     fCrossContextTextureSupport = false;
45     fHalfFloatVertexAttributeSupport = false;
46     fDynamicStateArrayGeometryProcessorTextureSupport = false;
47     fPerformPartialClearsAsDraws = false;
48     fPerformColorClearsAsDraws = false;
49     fAvoidLargeIndexBufferDraws = false;
50     fPerformStencilClearsAsDraws = false;
51     fTransferFromBufferToTextureSupport = false;
52     fTransferFromSurfaceToBufferSupport = false;
53     fWritePixelsRowBytesSupport = false;
54     fTransferPixelsToRowBytesSupport = false;
55     fReadPixelsRowBytesSupport = false;
56     fShouldCollapseSrcOverToSrcWhenAble = false;
57     fMustSyncGpuDuringAbandon = true;
58     fDriverDisableMSAAClipAtlas = false;
59     fDisableTessellationPathRenderer = false;
60 
61     fBlendEquationSupport = kBasic_BlendEquationSupport;
62     fAdvBlendEqDisableFlags = 0;
63 
64     fMapBufferFlags = kNone_MapFlags;
65 
66     fMaxVertexAttributes = 0;
67     fMaxRenderTargetSize = 1;
68     fMaxPreferredRenderTargetSize = 1;
69     fMaxTextureSize = 1;
70     fMaxWindowRectangles = 0;
71     fInternalMultisampleCount = 0;
72 
73     fSuppressPrints = options.fSuppressPrints;
74 #if GR_TEST_UTILS
75     fWireframeMode = options.fWireframeMode;
76 #else
77     fWireframeMode = false;
78 #endif
79     fBufferMapThreshold = options.fBufferMapThreshold;
80     fAvoidStencilBuffers = false;
81     fAvoidWritePixelsFastPath = false;
82     fRequiresManualFBBarrierAfterTessellatedStencilDraw = false;
83     fNativeDrawIndexedIndirectIsBroken = false;
84     fAvoidReorderingRenderTasks = false;
85     fAvoidDithering = false;
86 
87     fPreferVRAMUseOverFlushes = true;
88 
89     // Default to true, allow older versions of OpenGL to disable explicitly
90     fClampToBorderSupport = true;
91 
92     fDriverBugWorkarounds = options.fDriverBugWorkarounds;
93 }
94 
finishInitialization(const GrContextOptions & options)95 void GrCaps::finishInitialization(const GrContextOptions& options) {
96     if (!fNativeDrawIndirectSupport) {
97         // We will implement indirect draws with a polyfill, so the commands need to reside in CPU
98         // memory.
99         fUseClientSideIndirectBuffers = true;
100     }
101 
102     this->applyOptionsOverrides(options);
103 
104     // Our render targets are always created with textures as the color attachment, hence this min:
105     fMaxRenderTargetSize = std::min(fMaxRenderTargetSize, fMaxTextureSize);
106     fMaxPreferredRenderTargetSize = std::min(fMaxPreferredRenderTargetSize, fMaxRenderTargetSize);
107 }
108 
applyOptionsOverrides(const GrContextOptions & options)109 void GrCaps::applyOptionsOverrides(const GrContextOptions& options) {
110     fShaderCaps->applyOptionsOverrides(options);
111     this->onApplyOptionsOverrides(options);
112     if (options.fDisableDriverCorrectnessWorkarounds) {
113         SkASSERT(!fDriverDisableMSAAClipAtlas);
114         SkASSERT(!fDisableTessellationPathRenderer);
115         SkASSERT(!fAvoidStencilBuffers);
116         SkASSERT(!fAvoidWritePixelsFastPath);
117         SkASSERT(!fRequiresManualFBBarrierAfterTessellatedStencilDraw);
118         SkASSERT(!fNativeDrawIndexedIndirectIsBroken);
119         SkASSERT(!fAdvBlendEqDisableFlags);
120         SkASSERT(!fPerformColorClearsAsDraws);
121         SkASSERT(!fPerformStencilClearsAsDraws);
122         // Don't check the partial-clear workaround, since that is a backend limitation, not a
123         // driver workaround (it just so happens the fallbacks are the same).
124     }
125     if (GrContextOptions::Enable::kNo == options.fUseDrawInsteadOfClear) {
126         fPerformColorClearsAsDraws = false;
127         fPerformStencilClearsAsDraws = false;
128     } else if (GrContextOptions::Enable::kYes == options.fUseDrawInsteadOfClear) {
129         fPerformColorClearsAsDraws = true;
130         fPerformStencilClearsAsDraws = true;
131     }
132 
133     fMaxTextureSize = std::min(fMaxTextureSize, options.fMaxTextureSizeOverride);
134 #if GR_TEST_UTILS
135     if (options.fSuppressAdvancedBlendEquations) {
136         fBlendEquationSupport = kBasic_BlendEquationSupport;
137     }
138     if (options.fClearAllTextures) {
139         fShouldInitializeTextures = true;
140     }
141     if (options.fDisallowWriteAndTransferPixelRowBytes) {
142         fWritePixelsRowBytesSupport = false;
143         fTransferPixelsToRowBytesSupport = false;
144     }
145     if (options.fAlwaysPreferHardwareTessellation) {
146         fMinPathVerbsForHwTessellation = fMinStrokeVerbsForHwTessellation = 0;
147     }
148 #endif
149     if (options.fSuppressMipmapSupport) {
150         fMipmapSupport = false;
151     }
152 
153     if (fMaxWindowRectangles > GrWindowRectangles::kMaxWindows) {
154         SkDebugf("WARNING: capping window rectangles at %i. HW advertises support for %i.\n",
155                  GrWindowRectangles::kMaxWindows, fMaxWindowRectangles);
156         fMaxWindowRectangles = GrWindowRectangles::kMaxWindows;
157     }
158 
159     fInternalMultisampleCount = options.fInternalMultisampleCount;
160 
161     fAvoidStencilBuffers = options.fAvoidStencilBuffers;
162 
163     fDriverBugWorkarounds.applyOverrides(options.fDriverBugWorkarounds);
164 }
165 
166 
167 #ifdef SK_ENABLE_DUMP_GPU
168 #include "src/gpu/GrTestUtils.h"
169 #include "src/utils/SkJSONWriter.h"
170 
map_flags_to_string(uint32_t flags)171 static SkString map_flags_to_string(uint32_t flags) {
172     SkString str;
173     if (GrCaps::kNone_MapFlags == flags) {
174         str = "none";
175     } else {
176         SkASSERT(GrCaps::kCanMap_MapFlag & flags);
177         SkDEBUGCODE(flags &= ~GrCaps::kCanMap_MapFlag);
178         str = "can_map";
179 
180         if (GrCaps::kSubset_MapFlag & flags) {
181             str.append(" partial");
182         } else {
183             str.append(" full");
184         }
185         SkDEBUGCODE(flags &= ~GrCaps::kSubset_MapFlag);
186         if (GrCaps::kAsyncRead_MapFlag & flags) {
187             str.append(" async_read");
188         } else {
189             str.append(" sync_read");
190         }
191         SkDEBUGCODE(flags &= ~GrCaps::kAsyncRead_MapFlag);
192     }
193     SkASSERT(0 == flags); // Make sure we handled all the flags.
194     return str;
195 }
196 
dumpJSON(SkJSONWriter * writer) const197 void GrCaps::dumpJSON(SkJSONWriter* writer) const {
198     writer->beginObject();
199 
200     writer->appendBool("MIP Map Support", fMipmapSupport);
201     writer->appendBool("NPOT Texture Tile Support", fNPOTTextureTileSupport);
202     writer->appendBool("Reuse Scratch Textures", fReuseScratchTextures);
203     writer->appendBool("Reuse Scratch Buffers", fReuseScratchBuffers);
204     writer->appendBool("Gpu Tracing Support", fGpuTracingSupport);
205     writer->appendBool("Oversized Stencil Support", fOversizedStencilSupport);
206     writer->appendBool("Texture Barrier Support", fTextureBarrierSupport);
207     writer->appendBool("Sample Locations Support", fSampleLocationsSupport);
208     writer->appendBool("Draw Instanced Support", fDrawInstancedSupport);
209     writer->appendBool("Native Draw Indirect Support", fNativeDrawIndirectSupport);
210     writer->appendBool("Use client side indirect buffers", fUseClientSideIndirectBuffers);
211     writer->appendBool("Conservative Raster Support", fConservativeRasterSupport);
212     writer->appendBool("Wireframe Support", fWireframeSupport);
213     writer->appendBool("MSAA Resolves Automatically", fMSAAResolvesAutomatically);
214     writer->appendBool("Use primitive restart", fUsePrimitiveRestart);
215     writer->appendBool("Prefer client-side dynamic buffers", fPreferClientSideDynamicBuffers);
216     writer->appendBool("Prefer fullscreen clears (and stencil discard)", fPreferFullscreenClears);
217     writer->appendBool("Two-sided Stencil Refs And Masks Must Match",
218                        fTwoSidedStencilRefsAndMasksMustMatch);
219     writer->appendBool("Must clear buffer memory", fMustClearUploadedBufferData);
220     writer->appendBool("Should initialize textures", fShouldInitializeTextures);
221     writer->appendBool("Supports importing AHardwareBuffers", fSupportsAHardwareBufferImages);
222     writer->appendBool("Fence sync support", fFenceSyncSupport);
223     writer->appendBool("Semaphore support", fSemaphoreSupport);
224     writer->appendBool("Cross context texture support", fCrossContextTextureSupport);
225     writer->appendBool("Half float vertex attribute support", fHalfFloatVertexAttributeSupport);
226     writer->appendBool("Specify GeometryProcessor textures as a dynamic state array",
227                        fDynamicStateArrayGeometryProcessorTextureSupport);
228     writer->appendBool("Use draws for partial clears", fPerformPartialClearsAsDraws);
229     writer->appendBool("Use draws for color clears", fPerformColorClearsAsDraws);
230     writer->appendBool("Avoid Large IndexBuffer Draws", fAvoidLargeIndexBufferDraws);
231     writer->appendBool("Use draws for stencil clip clears", fPerformStencilClearsAsDraws);
232     writer->appendBool("Supports transfers from buffers to textures",
233                        fTransferFromBufferToTextureSupport);
234     writer->appendBool("Supports transfers from textures to buffers",
235                        fTransferFromSurfaceToBufferSupport);
236     writer->appendBool("Write pixels row bytes support", fWritePixelsRowBytesSupport);
237     writer->appendBool("Transfer pixels to row bytes support", fTransferPixelsToRowBytesSupport);
238     writer->appendBool("Read pixels row bytes support", fReadPixelsRowBytesSupport);
239     writer->appendBool("Disable msaa clip mask atlas on current driver [workaround]",
240                        fDriverDisableMSAAClipAtlas);
241     writer->appendBool("Disable TessellationPathRenderer current driver [workaround]",
242                        fDisableTessellationPathRenderer);
243     writer->appendBool("Clamp-to-border", fClampToBorderSupport);
244 
245     writer->appendBool("Prefer VRAM Use over flushes [workaround]", fPreferVRAMUseOverFlushes);
246     writer->appendBool("Avoid stencil buffers [workaround]", fAvoidStencilBuffers);
247     writer->appendBool("Avoid writePixels fast path [workaround]", fAvoidWritePixelsFastPath);
248     writer->appendBool("Requires manual FB barrier after tessellated stencilDraw [workaround]",
249                        fRequiresManualFBBarrierAfterTessellatedStencilDraw);
250     writer->appendBool("Native draw indexed indirect is broken [workaround]",
251                        fNativeDrawIndexedIndirectIsBroken);
252     writer->appendBool("Avoid DAG reordering [workaround]", fAvoidReorderingRenderTasks);
253     writer->appendBool("Avoid Dithering [workaround]", fAvoidDithering);
254 
255     if (this->advancedBlendEquationSupport()) {
256         writer->appendHexU32("Advanced Blend Equation Disable Flags", fAdvBlendEqDisableFlags);
257     }
258 
259     writer->appendS32("Max Vertex Attributes", fMaxVertexAttributes);
260     writer->appendS32("Max Texture Size", fMaxTextureSize);
261     writer->appendS32("Max Render Target Size", fMaxRenderTargetSize);
262     writer->appendS32("Max Preferred Render Target Size", fMaxPreferredRenderTargetSize);
263     writer->appendS32("Max Window Rectangles", fMaxWindowRectangles);
264     writer->appendS32("Sample Count for Internal MSAA", fInternalMultisampleCount);
265     writer->appendS32("Min Path Verbs for HW Tessellation", fMinPathVerbsForHwTessellation);
266     writer->appendS32("Min Stroke Verbs for HW Tessellation", fMinStrokeVerbsForHwTessellation);
267 
268     static const char* kBlendEquationSupportNames[] = {
269         "Basic",
270         "Advanced",
271         "Advanced Coherent",
272     };
273     static_assert(0 == kBasic_BlendEquationSupport);
274     static_assert(1 == kAdvanced_BlendEquationSupport);
275     static_assert(2 == kAdvancedCoherent_BlendEquationSupport);
276     static_assert(SK_ARRAY_COUNT(kBlendEquationSupportNames) == kLast_BlendEquationSupport + 1);
277 
278     writer->appendString("Blend Equation Support",
279                          kBlendEquationSupportNames[fBlendEquationSupport]);
280     writer->appendString("Map Buffer Support", map_flags_to_string(fMapBufferFlags).c_str());
281 
282     this->onDumpJSON(writer);
283 
284     writer->appendName("shaderCaps");
285     this->shaderCaps()->dumpJSON(writer);
286 
287     writer->endObject();
288 }
289 #else
dumpJSON(SkJSONWriter * writer) const290 void GrCaps::dumpJSON(SkJSONWriter* writer) const { }
291 #endif
292 
surfaceSupportsWritePixels(const GrSurface * surface) const293 bool GrCaps::surfaceSupportsWritePixels(const GrSurface* surface) const {
294     return surface->readOnly() ? false : this->onSurfaceSupportsWritePixels(surface);
295 }
296 
canCopySurface(const GrSurfaceProxy * dst,const GrSurfaceProxy * src,const SkIRect & srcRect,const SkIPoint & dstPoint) const297 bool GrCaps::canCopySurface(const GrSurfaceProxy* dst, const GrSurfaceProxy* src,
298                             const SkIRect& srcRect, const SkIPoint& dstPoint) const {
299     if (dst->readOnly()) {
300         return false;
301     }
302 
303     if (dst->backendFormat() != src->backendFormat()) {
304         return false;
305     }
306     return this->onCanCopySurface(dst, src, srcRect, dstPoint);
307 }
308 
validateSurfaceParams(const SkISize & dimensions,const GrBackendFormat & format,GrRenderable renderable,int renderTargetSampleCnt,GrMipmapped mipped,GrTextureType textureType) const309 bool GrCaps::validateSurfaceParams(const SkISize& dimensions, const GrBackendFormat& format,
310                                    GrRenderable renderable, int renderTargetSampleCnt,
311                                    GrMipmapped mipped, GrTextureType textureType) const {
312     if (textureType != GrTextureType::kNone) {
313         if (!this->isFormatTexturable(format, textureType)) {
314             return false;
315         }
316     }
317 
318     if (GrMipmapped::kYes == mipped && !this->mipmapSupport()) {
319         return false;
320     }
321 
322     if (dimensions.width() < 1 || dimensions.height() < 1) {
323         return false;
324     }
325 
326     if (renderable == GrRenderable::kYes) {
327         if (!this->isFormatRenderable(format, renderTargetSampleCnt)) {
328             return false;
329         }
330         int maxRTSize = this->maxRenderTargetSize();
331         if (dimensions.width() > maxRTSize || dimensions.height() > maxRTSize) {
332             return false;
333         }
334     } else {
335         // We currently do not support multisampled textures
336         if (renderTargetSampleCnt != 1) {
337             return false;
338         }
339         int maxSize = this->maxTextureSize();
340         if (dimensions.width() > maxSize || dimensions.height() > maxSize) {
341             return false;
342         }
343     }
344 
345     return true;
346 }
347 
supportedReadPixelsColorType(GrColorType srcColorType,const GrBackendFormat & srcFormat,GrColorType dstColorType) const348 GrCaps::SupportedRead GrCaps::supportedReadPixelsColorType(GrColorType srcColorType,
349                                                            const GrBackendFormat& srcFormat,
350                                                            GrColorType dstColorType) const {
351     SupportedRead read = this->onSupportedReadPixelsColorType(srcColorType, srcFormat,
352                                                               dstColorType);
353 
354     // There are known problems with 24 vs 32 bit BPP with this color type. Just fail for now if
355     // using a transfer buffer.
356     if (GrColorType::kRGB_888x == read.fColorType) {
357         read.fOffsetAlignmentForTransferBuffer = 0;
358     }
359     // It's very convenient to access 1 byte-per-channel 32 bit color types as uint32_t on the CPU.
360     // Make those aligned reads out of the buffer even if the underlying API doesn't require it.
361     auto channelFlags = GrColorTypeChannelFlags(read.fColorType);
362     if ((channelFlags == kRGBA_SkColorChannelFlags || channelFlags == kRGB_SkColorChannelFlags ||
363          channelFlags == kAlpha_SkColorChannelFlag || channelFlags == kGray_SkColorChannelFlag) &&
364         GrColorTypeBytesPerPixel(read.fColorType) == 4) {
365         switch (read.fOffsetAlignmentForTransferBuffer & 0b11) {
366             // offset alignment already a multiple of 4
367             case 0:
368                 break;
369             // offset alignment is a multiple of 2 but not 4.
370             case 2:
371                 read.fOffsetAlignmentForTransferBuffer *= 2;
372                 break;
373             // offset alignment is not a multiple of 2.
374             default:
375                 read.fOffsetAlignmentForTransferBuffer *= 4;
376                 break;
377         }
378     }
379     return read;
380 }
381 
getDefaultBackendFormat(GrColorType colorType,GrRenderable renderable) const382 GrBackendFormat GrCaps::getDefaultBackendFormat(GrColorType colorType,
383                                                 GrRenderable renderable) const {
384     // Unknown color types are always an invalid format, so early out before calling virtual.
385     if (colorType == GrColorType::kUnknown) {
386         return {};
387     }
388 
389     auto format = this->onGetDefaultBackendFormat(colorType);
390     if (!this->isFormatTexturable(format, GrTextureType::k2D)) {
391         return {};
392     }
393     if (!this->areColorTypeAndFormatCompatible(colorType, format)) {
394         return {};
395     }
396     // Currently we require that it be possible to write pixels into the "default" format. Perhaps,
397     // that could be a separate requirement from the caller. It seems less necessary if
398     // renderability was requested.
399     if (this->supportedWritePixelsColorType(colorType, format, colorType).fColorType ==
400         GrColorType::kUnknown) {
401         return {};
402     }
403     if (renderable == GrRenderable::kYes &&
404         !this->isFormatAsColorTypeRenderable(colorType, format)) {
405         return {};
406     }
407     return format;
408 }
409 
areColorTypeAndFormatCompatible(GrColorType grCT,const GrBackendFormat & format) const410 bool GrCaps::areColorTypeAndFormatCompatible(GrColorType grCT,
411                                              const GrBackendFormat& format) const {
412     if (GrColorType::kUnknown == grCT) {
413         return false;
414     }
415 
416     SkImage::CompressionType compression = GrBackendFormatToCompressionType(format);
417     if (compression != SkImage::CompressionType::kNone) {
418         return grCT == (SkCompressionTypeIsOpaque(compression) ? GrColorType::kRGB_888x
419                                                                : GrColorType::kRGBA_8888);
420     }
421 
422     return this->onAreColorTypeAndFormatCompatible(grCT, format);
423 }
424 
getReadSwizzle(const GrBackendFormat & format,GrColorType colorType) const425 skgpu::Swizzle GrCaps::getReadSwizzle(const GrBackendFormat& format, GrColorType colorType) const {
426     SkImage::CompressionType compression = GrBackendFormatToCompressionType(format);
427     if (compression != SkImage::CompressionType::kNone) {
428         if (colorType == GrColorType::kRGB_888x || colorType == GrColorType::kRGBA_8888) {
429             return skgpu::Swizzle::RGBA();
430         }
431         SkDEBUGFAILF("Illegal color type (%d) and compressed format (%d) combination.",
432                      (int)colorType, (int)compression);
433         return {};
434     }
435 
436     return this->onGetReadSwizzle(format, colorType);
437 }
438 
isFormatCompressed(const GrBackendFormat & format) const439 bool GrCaps::isFormatCompressed(const GrBackendFormat& format) const {
440     return GrBackendFormatToCompressionType(format) != SkImage::CompressionType::kNone;
441 }
442 
getDstSampleFlagsForProxy(const GrRenderTargetProxy * rt,bool drawUsesMSAA) const443 GrDstSampleFlags GrCaps::getDstSampleFlagsForProxy(const GrRenderTargetProxy* rt,
444                                                    bool drawUsesMSAA) const {
445     SkASSERT(rt);
446     if (this->textureBarrierSupport() && (!drawUsesMSAA || this->msaaResolvesAutomatically())) {
447         return this->onGetDstSampleFlagsForProxy(rt);
448     }
449     return GrDstSampleFlags::kNone;
450 }
451 
supportsDynamicMSAA(const GrRenderTargetProxy * rtProxy) const452 bool GrCaps::supportsDynamicMSAA(const GrRenderTargetProxy* rtProxy) const {
453     return rtProxy->numSamples() == 1 &&
454            this->internalMultisampleCount(rtProxy->backendFormat()) > 1 &&
455            this->onSupportsDynamicMSAA(rtProxy);
456 }
457 
color_type_fallback(GrColorType ct)458 static inline GrColorType color_type_fallback(GrColorType ct) {
459     switch (ct) {
460         // kRGBA_8888 is our default fallback for many color types that may not have renderable
461         // backend formats.
462         case GrColorType::kAlpha_8:
463         case GrColorType::kBGR_565:
464         case GrColorType::kABGR_4444:
465         case GrColorType::kBGRA_8888:
466         case GrColorType::kRGBA_1010102:
467         case GrColorType::kBGRA_1010102:
468         case GrColorType::kRGBA_F16:
469         case GrColorType::kRGBA_F16_Clamped:
470             return GrColorType::kRGBA_8888;
471         case GrColorType::kAlpha_F16:
472             return GrColorType::kRGBA_F16;
473         case GrColorType::kGray_8:
474             return GrColorType::kRGB_888x;
475         default:
476             return GrColorType::kUnknown;
477     }
478 }
479 
getFallbackColorTypeAndFormat(GrColorType ct,int sampleCnt) const480 std::tuple<GrColorType, GrBackendFormat> GrCaps::getFallbackColorTypeAndFormat(
481                                                                             GrColorType ct,
482                                                                             int sampleCnt) const {
483     do {
484         auto format = this->getDefaultBackendFormat(ct, GrRenderable::kYes);
485         // We continue to the fallback color type if there no default renderable format or we
486         // requested msaa and the format doesn't support msaa.
487         if (format.isValid() && this->isFormatRenderable(format, sampleCnt)) {
488             return {ct, format};
489         }
490         ct = color_type_fallback(ct);
491     } while (ct != GrColorType::kUnknown);
492     return {GrColorType::kUnknown, {}};
493 }
494