From 7b227b3ec1e297148ac39b2e722d22ba5ccae1c7 Mon Sep 17 00:00:00 2001
From: ts-korhonen <teemu.s.korhonen@gmail.com>
Date: Sat, 1 May 2021 10:17:47 +0300
Subject: [PATCH] win_opengl: Add fallback feature for gpus without
 ARB__buffer_storage extension.

---
 src/win/win_opengl.c | 41 +++++++++++++++++++++++++++++++----------
 1 file changed, 31 insertions(+), 10 deletions(-)

diff --git a/src/win/win_opengl.c b/src/win/win_opengl.c
index 9c7753151..3aac702b7 100644
--- a/src/win/win_opengl.c
+++ b/src/win/win_opengl.c
@@ -361,14 +361,26 @@ static int initialize_glcontext(gl_identifiers* gl)
 	glGenBuffers(1, &gl->unpackBufferID);
 	glBindBuffer(GL_PIXEL_UNPACK_BUFFER, gl->unpackBufferID);
 
-	/* Create persistent buffer for pixel transfer. */
-	glBufferStorage(GL_PIXEL_UNPACK_BUFFER, BUFFERBYTES * BUFFERCOUNT, NULL, GL_MAP_WRITE_BIT | GL_MAP_PERSISTENT_BIT | GL_MAP_COHERENT_BIT);
-	
-	void* buf_ptr = glMapBufferRange(GL_PIXEL_UNPACK_BUFFER, 0, BUFFERBYTES * BUFFERCOUNT, GL_MAP_WRITE_BIT | GL_MAP_PERSISTENT_BIT | GL_MAP_COHERENT_BIT);
+	void* buf_ptr = NULL;
+
+	if (GLAD_GL_ARB_buffer_storage)
+	{
+		/* Create persistent buffer for pixel transfer. */
+		glBufferStorage(GL_PIXEL_UNPACK_BUFFER, BUFFERBYTES * BUFFERCOUNT, NULL, GL_MAP_WRITE_BIT | GL_MAP_PERSISTENT_BIT | GL_MAP_COHERENT_BIT);
+
+		buf_ptr = glMapBufferRange(GL_PIXEL_UNPACK_BUFFER, 0, BUFFERBYTES * BUFFERCOUNT, GL_MAP_WRITE_BIT | GL_MAP_PERSISTENT_BIT | GL_MAP_COHERENT_BIT);		
+	}
+	else
+	{
+		/* Fallback; create our own buffer. */
+		buf_ptr = malloc(BUFFERBYTES * BUFFERCOUNT);
+
+		glBufferData(GL_PIXEL_UNPACK_BUFFER, BUFFERBYTES * BUFFERCOUNT, NULL, GL_STREAM_DRAW);
+	}
 
 	if (buf_ptr == NULL)
-		return 0;
-	
+		return 0; /* Most likely out of memory. */
+
 	/* Split the buffer area for each blit_info and set them available for use. */
 	for (int i = 0; i < BUFFERCOUNT; i++)
 	{
@@ -389,8 +401,12 @@ static int initialize_glcontext(gl_identifiers* gl)
 */
 static void finalize_glcontext(gl_identifiers* gl)
 {
+	if (GLAD_GL_ARB_buffer_storage)
+		glUnmapBuffer(GL_PIXEL_UNPACK_BUFFER);
+	else
+		free(blit_info[0].buffer);
+
 	glDeleteProgram(gl->shader_progID);
-	glUnmapBuffer(GL_PIXEL_UNPACK_BUFFER);
 	glDeleteBuffers(1, &gl->unpackBufferID);
 	glDeleteTextures(1, &gl->textureID);
 	glDeleteBuffers(1, &gl->vertexBufferID);
@@ -440,7 +456,6 @@ static void __stdcall opengl_debugmsg_callback(GLenum source, GLenum type, GLuin
 	OutputDebugStringA(message);
 	OutputDebugStringA("\n");
 }
-#endif
 */
 
 /**
@@ -491,7 +506,7 @@ static void opengl_main(void* param)
 
 	SDL_GL_SetSwapInterval(options.vsync);
 
-	if (!gladLoadGLLoader(SDL_GL_GetProcAddress) || !GLAD_GL_ARB_buffer_storage)
+	if (!gladLoadGLLoader(SDL_GL_GetProcAddress))
 	{
 		SDL_GL_DeleteContext(context);
 		opengl_fail();
@@ -608,6 +623,12 @@ static void opengl_main(void* param)
 					SetEvent(sync_objects.resize);
 			}
 
+			if (!GLAD_GL_ARB_buffer_storage)
+			{
+				/* Fallback method, copy data to pixel buffer. */
+				glBufferSubData(GL_PIXEL_UNPACK_BUFFER, BUFFERBYTES * read_pos, info->w * (info->y2 - info->y1) * sizeof(uint32_t), info->buffer);
+			}
+
 			/* Update texture from pixel buffer. */
 			glPixelStorei(GL_UNPACK_SKIP_PIXELS, BUFFERPIXELS * read_pos);
 			glTexSubImage2D(GL_TEXTURE_2D, 0, 0, info->y1, info->w, info->y2 - info->y1, GL_BGRA, GL_UNSIGNED_INT_8_8_8_8_REV, NULL);
@@ -765,7 +786,7 @@ static void opengl_blit(int x, int y, int y1, int y2, int w, int h)
 		return;
 	}
 
-	memcpy(blit_info[write_pos].buffer, &(render_buffer->dat)[y1 * w], w * (y2 - y1) * 4);
+	memcpy(blit_info[write_pos].buffer, &(render_buffer->dat)[y1 * w], w * (y2 - y1) * sizeof(uint32_t));
 
 	video_blit_complete();