[PATCH v2] glamor: check max native ALU instructions

[Olivier Fourdan]

When using glamor (either in Xephyr or Xwayland) on hardware with too
low instructions limit, glamor fallbacks to sw due to large shaders.

This makes glamor unbearably slow on such hardware.

Check reported value for GL_MAX_PROGRAM_NATIVE_ALU_INSTRUCTIONS_ARB
and fail in glamor_init() if the limit is lower than 128.

Can be overridden with the environment variable
GLAMOR_MIN_ALU_INSTRUCTIONS for testing and debugging purpose.

Fixes: https://bugs.freedesktop.org/show_bug.cgi?id=88316
Signed-off-by: Olivier Fourdan <ofourdan at redhat.com>
---
v2: Check for GL version < 30 

 glamor/glamor.c      | 47 +++++++++++++++++++++++++++++++++++++++++++++++
 glamor/glamor_priv.h |  2 ++
 2 files changed, 49 insertions(+)

diff --git a/glamor/glamor.c b/glamor/glamor.c
index 017266a..8270914 100644
--- a/glamor/glamor.c
+++ b/glamor/glamor.c
@@ -305,6 +305,50 @@ glamor_create_screen_resources(ScreenPtr screen)
     return ret;
 }
 
+static int
+glamor_get_min_instructions(void)
+{
+    char *min_instructions_string;
+    int min_instructions;
+
+    min_instructions_string = getenv("GLAMOR_MIN_ALU_INSTRUCTIONS");
+    if (min_instructions_string
+        && sscanf(min_instructions_string, "%d", &min_instructions) == 1)
+        return min_instructions;
+
+    return GLAMOR_MIN_ALU_INSTRUCTIONS;
+}
+
+static Bool
+glamor_check_limits(int gl_version)
+{
+    GLint max_native_alu_instructions;
+    int min_instructions;
+
+    /* Avoid using glamor if the reported instructions limit is too low,
+     * as this would cause glamor to fallback on sw due to large shaders
+     * which ends up being unbearably slow.
+     */
+    min_instructions = glamor_get_min_instructions();
+    if (gl_version < 30 && min_instructions > 0) {
+        if (!epoxy_has_gl_extension("GL_ARB_fragment_program")) {
+            ErrorF("GL_ARB_fragment_program required\n");
+            return FALSE;
+        }
+
+        glGetProgramivARB(GL_FRAGMENT_PROGRAM_ARB,
+                          GL_MAX_PROGRAM_NATIVE_ALU_INSTRUCTIONS_ARB,
+                          &max_native_alu_instructions);
+        if (max_native_alu_instructions < min_instructions) {
+            ErrorF("Require at least %d instructions (%d reported)\n",
+                   min_instructions, max_native_alu_instructions);
+            return FALSE;
+        }
+    }
+
+    return TRUE;
+}
+
 /** Set up glamor for an already-configured GL context. */
 Bool
 glamor_init(ScreenPtr screen, unsigned int flags)
@@ -384,6 +428,9 @@ glamor_init(ScreenPtr screen, unsigned int flags)
             ErrorF("Require OpenGL version 2.1 or later.\n");
             goto fail;
         }
+
+        if (!glamor_check_limits(gl_version))
+            goto fail;
     } else {
         if (gl_version < 20) {
             ErrorF("Require Open GLES2.0 or later.\n");
diff --git a/glamor/glamor_priv.h b/glamor/glamor_priv.h
index ed6e2d1..3d13a27 100644
--- a/glamor/glamor_priv.h
+++ b/glamor/glamor_priv.h
@@ -1095,4 +1095,6 @@ void glamor_xv_render(glamor_port_private *port_priv);
 
 #include "glamor_font.h"
 
+#define GLAMOR_MIN_ALU_INSTRUCTIONS 128 /* Minimum required number of native ALU instructions */
+
 #endif                          /* GLAMOR_PRIV_H */
-- 
2.1.0