i965: Add driconf option clamp_max_samples

The new option clamps GL_MAX_SAMPLES to a hardware-supported MSAA mode.
If negative, then no clamping occurs.

v2: (for Paul)
  - Add option to i965 only, not to all DRI drivers.
  - Do not realy on int->uint cast to convert negative
    values to large positive values. Explicitly check for
    clamp_max_samples < 0.
v3: (for Ken)
   - Don't allow clamp_max_samples to alter context version.
   - Use clearer for-loop and correct comment.
   - Rename variables.
v4: (for Ken)
   - Merge identical if-branches.

Reviewed-and-tested-by: Kenneth Graunke <[email protected]>
Signed-off-by: Chad Versace <[email protected]>

2 files changed, 67 insertions, 12 deletions

diff --git a/src/mesa/drivers/dri/i965/brw_context.c b/src/mesa/drivers/dri/i965/brw_context.c
index 38147e9fe48..13569add74d 100644
--- a/src/mesa/drivers/dri/i965/brw_context.c
+++ b/src/mesa/drivers/dri/i965/brw_context.c
@@ -268,6 +268,53 @@ brw_init_driver_functions(struct brw_context *brw,
       functions->GetSamplePosition = gen6_get_sample_position;
 }
 
+/**
+ * Return array of MSAA modes supported by the hardware. The array is
+ * zero-terminated and sorted in decreasing order.
+ */
+static const int*
+brw_supported_msaa_modes(const struct brw_context *brw)
+{
+   if (brw->gen >= 7) {
+      return (int[]){8, 4, 0};
+   } else if (brw->gen == 6) {
+      return (int[]){4, 0};
+   } else {
+      return (int[]){0};
+   }
+}
+
+/**
+ * Override GL_MAX_SAMPLES and related constants according to value of driconf
+ * option 'clamp_max_samples'.
+ */
+static void
+brw_override_max_samples(struct brw_context *brw)
+{
+   const int clamp_max_samples = driQueryOptioni(&brw->optionCache,
+                                                 "clamp_max_samples");
+   if (clamp_max_samples < 0)
+      return;
+
+   const int *supported_msaa_modes = brw_supported_msaa_modes(brw);
+   int max_samples = 0;
+
+   /* Select the largest supported MSAA mode that does not exceed
+    * clamp_max_samples.
+    */
+   for (int i = 0; supported_msaa_modes[i] != 0; ++i) {
+      if (supported_msaa_modes[i] <= clamp_max_samples) {
+         max_samples = supported_msaa_modes[i];
+         break;
+      }
+   }
+
+   brw->ctx.Const.MaxSamples = max_samples;
+   brw->ctx.Const.MaxColorTextureSamples = max_samples;
+   brw->ctx.Const.MaxDepthTextureSamples = max_samples;
+   brw->ctx.Const.MaxIntegerSamples = max_samples;
+}
+
 static void
 brw_initialize_context_constants(struct brw_context *brw)
 {
@@ -333,18 +380,14 @@ brw_initialize_context_constants(struct brw_context *brw)
 
    ctx->Const.AlwaysUseGetTransformFeedbackVertexCount = true;
 
-   if (brw->gen == 6) {
-      ctx->Const.MaxSamples = 4;
-      ctx->Const.MaxColorTextureSamples = 4;
-      ctx->Const.MaxDepthTextureSamples = 4;
-      ctx->Const.MaxIntegerSamples = 4;
-   } else if (brw->gen >= 7) {
-      ctx->Const.MaxSamples = 8;
-      ctx->Const.MaxColorTextureSamples = 8;
-      ctx->Const.MaxDepthTextureSamples = 8;
-      ctx->Const.MaxIntegerSamples = 8;
+   const int max_samples = brw_supported_msaa_modes(brw)[0];
+   ctx->Const.MaxSamples = max_samples;
+   ctx->Const.MaxColorTextureSamples = max_samples;
+   ctx->Const.MaxDepthTextureSamples = max_samples;
+   ctx->Const.MaxIntegerSamples = max_samples;
+
+   if (brw->gen >= 7)
       ctx->Const.MaxProgramTextureGatherComponents = 4;
-   }
 
    ctx->Const.MinLineWidth = 1.0;
    ctx->Const.MinLineWidthAA = 1.0;
@@ -696,6 +739,12 @@ brwCreateContext(gl_api api,
 
    _mesa_compute_version(ctx);
 
+   /* Here we override context constants. We apply the overrides after
+    * calculation of the context version because we do not want the overridden
+    * constants to change the version.
+    */
+   brw_override_max_samples(brw);
+
    _mesa_initialize_dispatch_tables(ctx);
    _mesa_initialize_vbo_vtxfmt(ctx);
 
diff --git a/src/mesa/drivers/dri/i965/intel_screen.c b/src/mesa/drivers/dri/i965/intel_screen.c
index eafafa2ccdf..ce8124bf5ef 100644
--- a/src/mesa/drivers/dri/i965/intel_screen.c
+++ b/src/mesa/drivers/dri/i965/intel_screen.c
@@ -63,11 +63,17 @@ DRI_CONF_BEGIN
       DRI_CONF_OPT_BEGIN_B(disable_derivative_optimization, "false")
 	 DRI_CONF_DESC(en, "Derivatives with finer granularity by default")
       DRI_CONF_OPT_END
-
    DRI_CONF_SECTION_END
+
    DRI_CONF_SECTION_QUALITY
       DRI_CONF_FORCE_S3TC_ENABLE("false")
+
+      DRI_CONF_OPT_BEGIN(clamp_max_samples, int, -1)
+              DRI_CONF_DESC(en, "Clamp the value of GL_MAX_SAMPLES to the "
+                            "given integer. If negative, then do not clamp.")
+      DRI_CONF_OPT_END
    DRI_CONF_SECTION_END
+
    DRI_CONF_SECTION_DEBUG
       DRI_CONF_NO_RAST("false")
       DRI_CONF_ALWAYS_FLUSH_BATCH("false")