egl: Use the Mesa surfaceless platform instead of GBM

Mesa surfaceless platform appears to be a better fit for the use-case at hand:
1. Like GBM it is Mesa specific, so no change in supported setups is
   expected. If ever required, a fallback to the generic device platform
   could be added on top.
2. It leaves the complexity of selecting a renderer device to the
   driver, reducing code and dependencies.
3. It allows to use llvmpipe / software drivers without dri device,
   which can be useful on CI or debugging (with LIBGL_ALWAYS_SOFTWARE=1).

Signed-off-by: Robert Mader <robert.mader@collabora.com>
Reviewed-by: Bryan O'Donoghue <bryan.odonoghue@linaro.org>
Tested-by: Bryan O'Donoghue <bryan.odonoghue@linaro.org> # sm8250/rb5, x1e/Dell Insprion14p
Reviewed-by: Milan Zamazal <mzamazal@redhat.com>
Tested-by: Milan Zamazal <mzamazal@redhat.com> # TI AM69
Tested-by: Barnabás Pőcze <barnabas.pocze@ideasonboard.com> # ThinkPad X1 Yoga Gen 7 + ov2740
Reviewed-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Signed-off-by: Kieran Bingham <kieran.bingham@ideasonboard.com>

include/libcamera/internal/egl.h

@@ -16,8 +16,6 @@
 #include <libcamera/base/span.h>
 #include <libcamera/base/utils.h>
 
-#include "libcamera/internal/gbm.h"
-
 #define EGL_EGLEXT_PROTOTYPES
 #include <EGL/egl.h>
 #include <EGL/eglext.h>
@@ -96,7 +94,7 @@ public:
 	eGL();
 	~eGL();
 
-	int initEGLContext(GBM *gbmContext);
+	int initEGLContext();
 
 	int createInputDMABufTexture2D(eGLImage &eglImage, int fd);
 	int createOutputDMABufTexture2D(eGLImage &eglImage, int fd);

src/libcamera/egl.cpp

@@ -256,24 +256,22 @@ void eGL::createTexture2D(eGLImage &eglImage, GLint format, uint32_t width, uint
 
 /**
  * \brief Initialise the EGL context
- * \param[in] gbmContext Pointer to initialised GBM context
  *
- * Sets up the EGL display from the GBM device, creates an OpenGL ES 2.0
- * context, and retrieves function pointers for required extensions
- * including:
+ * Sets up the EGL display, creates an OpenGL ES 2.0 context, and retrieves
+ * function pointers for required extensions including:
  * - eglCreateImageKHR / eglDestroyImageKHR
  * - glEGLImageTargetTexture2DOES
  *
  * \return 0 on success, or -ENODEV on failure
  */
-int eGL::initEGLContext(GBM *gbmContext)
+int eGL::initEGLContext()
 {
 	EGLint configAttribs[] = {
 		EGL_RED_SIZE, 8,
 		EGL_GREEN_SIZE, 8,
 		EGL_BLUE_SIZE, 8,
 		EGL_ALPHA_SIZE, 8,
-		EGL_SURFACE_TYPE, EGL_WINDOW_BIT,
+		EGL_SURFACE_TYPE, EGL_PBUFFER_BIT,
 		EGL_RENDERABLE_TYPE, EGL_OPENGL_ES2_BIT,
 		EGL_NONE
 	};
@@ -291,7 +289,9 @@ int eGL::initEGLContext(GBM *gbmContext)
 		goto fail;
 	}
 
-	display_ = eglGetDisplay(gbmContext->device());
+	display_ = eglGetPlatformDisplay(EGL_PLATFORM_SURFACELESS_MESA,
+					 EGL_DEFAULT_DISPLAY,
+					 nullptr);
 	if (display_ == EGL_NO_DISPLAY) {
 		LOG(eGL, Error) << "Unable to get EGL display";
 		goto fail;

src/libcamera/software_isp/debayer_egl.cpp

@@ -141,8 +141,8 @@ int DebayerEGL::initBayerShaders(PixelFormat inputFormat, PixelFormat outputForm
 	egl_.pushEnv(shaderEnv, "#extension GL_OES_EGL_image_external: enable");
 
 	/*
-	 * Tell shaders how to re-order output taking account of how the
-	 * pixels are actually stored by GBM
+	 * Tell shaders how to re-order output taking account of how the pixels
+	 * are actually stored by EGL.
 	 */
 	switch (outputFormat) {
 	case formats::ARGB8888:
@@ -586,10 +586,7 @@ int DebayerEGL::start()
 {
 	GLint maxTextureImageUnits;
 
-	if (gbmSurface_.createDevice())
-		return -ENODEV;
-
-	if (egl_.initEGLContext(&gbmSurface_))
+	if (egl_.initEGLContext())
 		return -ENODEV;
 
 	glGetIntegerv(GL_MAX_TEXTURE_IMAGE_UNITS, &maxTextureImageUnits);

src/libcamera/software_isp/debayer_egl.h

@@ -113,7 +113,6 @@ private:
 	Rectangle window_;
 	std::unique_ptr<SwStatsCpu> stats_;
 	eGL egl_;
-	GBM gbmSurface_;
 	uint32_t width_;
 	uint32_t height_;
 	GLint glFormat_;