virgl: Add support for glGetMultisample

Use caps to obtain the multisample sample positions for up to 16
positions and implement the according Gallium interface.

This implemenation (plus its counterpart in virglrenderer) assume that
the fixed sample position are always the same for a given number of samples
over the whole live time of a qemu session. It also assumes that sample
series are only given for 2, 4, 8, and 16 samples, and for intermediate
numbers N of samples the next higher supported set from above list is picked
and the sample positions for the first N samples are returned accordingly.

Fixes (when run on GL host):
    dEQP-GLES31.functional.texture.multisample.samples_1.sample_position
    dEQP-GLES31.functional.texture.multisample.samples_2.sample_position
    dEQP-GLES31.functional.texture.multisample.samples_3.sample_position
    dEQP-GLES31.functional.texture.multisample.samples_4.sample_position
    dEQP-GLES31.functional.texture.multisample.samples_8.sample_position
    dEQP-GLES31.functional.texture.multisample.samples_10.sample_position
    dEQP-GLES31.functional.texture.multisample.samples_12.sample_position
    dEQP-GLES31.functional.texture.multisample.samples_13.sample_position
    dEQP-GLES31.functional.texture.multisample.samples_16.sample_position

v2: remove unrelated chunk (thanks Ilia Mirkin)
v3: - also return positions for intermediate sample counts
    - fix unused varible warning
    - update description
v4: explain better what this patch assumes and how it handles sample numbers
    that are not directly advertised (thanks go to Erik Faye-Lund for making
    me aware that this should be documented)

Signed-off-by: Gert Wollny <[email protected]>
Reviewed-by: Erik Faye-Lund <[email protected]>

2 files changed, 39 insertions, 0 deletions

diff --git a/src/gallium/drivers/virgl/virgl_context.c b/src/gallium/drivers/virgl/virgl_context.c
index e6f8dc85256..d985bf906fb 100644
--- a/src/gallium/drivers/virgl/virgl_context.c
+++ b/src/gallium/drivers/virgl/virgl_context.c
@@ -920,6 +920,42 @@ virgl_context_destroy( struct pipe_context *ctx )
    FREE(vctx);
 }
 
+static void virgl_get_sample_position(struct pipe_context *ctx,
+				      unsigned sample_count,
+				      unsigned index,
+				      float *out_value)
+{
+   struct virgl_context *vctx = virgl_context(ctx);
+   struct virgl_screen *vs = virgl_screen(vctx->base.screen);
+
+   if (sample_count > vs->caps.caps.v1.max_samples) {
+      debug_printf("VIRGL: requested %d MSAA samples, but only %d supported\n",
+		   sample_count, vs->caps.caps.v1.max_samples);
+      return;
+   }
+
+   /* The following is basically copied from dri/i965gen6_get_sample_position
+    * The only addition is that we hold the msaa positions for all sample
+    * counts in a flat array. */
+   uint32_t bits = 0;
+   if (sample_count == 1) {
+      out_value[0] = out_value[1] = 0.5f;
+      return;
+   } else if (sample_count == 2) {
+      bits = vs->caps.caps.v2.msaa_sample_positions[0] >> (8 * index);
+   } else if (sample_count < 4) {
+      bits = vs->caps.caps.v2.msaa_sample_positions[1] >> (8 * index);
+   } else if (sample_count < 8) {
+      bits = vs->caps.caps.v2.msaa_sample_positions[2 + (index >> 2)] >> (8 * (index & 3));
+   } else if (sample_count < 8) {
+      bits = vs->caps.caps.v2.msaa_sample_positions[4 + (index >> 2)] >> (8 * (index & 3));
+   }
+   out_value[0] = ((bits >> 4) & 0xf) / 16.0f;
+   out_value[1] = (bits & 0xf) / 16.0f;
+   debug_printf("VIRGL: sample postion [%2d/%2d] = (%f, %f)\n",
+		index, sample_count, out_value[0], out_value[1]);
+}
+
 struct pipe_context *virgl_context_create(struct pipe_screen *pscreen,
                                           void *priv,
                                           unsigned flags)
@@ -994,6 +1030,8 @@ struct pipe_context *virgl_context_create(struct pipe_screen *pscreen,
 
    vctx->base.set_blend_color = virgl_set_blend_color;
 
+   vctx->base.get_sample_position = virgl_get_sample_position;
+
    vctx->base.resource_copy_region = virgl_resource_copy_region;
    vctx->base.flush_resource = virgl_flush_resource;
    vctx->base.blit =  virgl_blit;
diff --git a/src/gallium/drivers/virgl/virgl_hw.h b/src/gallium/drivers/virgl/virgl_hw.h
index d338051d9b3..157267558ac 100644
--- a/src/gallium/drivers/virgl/virgl_hw.h
+++ b/src/gallium/drivers/virgl/virgl_hw.h
@@ -299,6 +299,7 @@ struct virgl_caps_v2 {
         uint32_t uniform_buffer_offset_alignment;
         uint32_t shader_buffer_offset_alignment;
         uint32_t capability_bits;
+	uint32_t msaa_sample_positions[8];
 };
 
 union virgl_caps {