Mesa (main): vc4: Work around a HW bug with 2-vert line loops.

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Module: Mesa
Branch: main
Commit: 2f851f0479959723d9057cde0a44e805d598b7e6
URL:    http://cgit.freedesktop.org/mesa/mesa/commit/?id=2f851f0479959723d9057cde0a44e805d598b7e6

Author: Emma Anholt <emma at anholt.net>
Date:   Sun Jul 10 08:11:15 2022 -0700

vc4: Work around a HW bug with 2-vert line loops.

Reviewed-by: Adam Jackson <ajax at redhat.com>
Reviewed-by: Jose Maria Casanova Crespo <jmcasanova at igalia.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/17449>

---

 src/broadcom/ci/broadcom-rpi3-fails.txt |  5 -----
 src/gallium/drivers/vc4/vc4_draw.c      | 25 +++++++++++++++++++++++++
 2 files changed, 25 insertions(+), 5 deletions(-)

diff --git a/src/broadcom/ci/broadcom-rpi3-fails.txt b/src/broadcom/ci/broadcom-rpi3-fails.txt
index 44783d78f53..3a49166d1c2 100644
--- a/src/broadcom/ci/broadcom-rpi3-fails.txt
+++ b/src/broadcom/ci/broadcom-rpi3-fails.txt
@@ -15,11 +15,6 @@ dEQP-GLES2.functional.clipping.line.wide_line_clip_viewport_corner,Fail
 
 dEQP-GLES2.functional.depth_stencil_clear.depth_stencil_masked,Fail
 
-# 2-count line loop incorrectly draws nothing (HW bug).  Need to demote to being
-# GL_LINES, though that wouldn't be correct with blending/logicop.
-dEQP-GLES2.functional.draw.draw_arrays.line_loop.multiple_attributes,Fail
-dEQP-GLES2.functional.draw.draw_arrays.line_loop.single_attribute,Fail
-
 # A glTexImage, glDraw, glTexSubImage sequence into a texture is missing what looks like the drawing.
 dEQP-GLES2.functional.fbo.render.texsubimage.after_render_tex2d_rgba,Fail
 # A glTexImage, glDraw, glTexSubImage, glDraw sequence into a texture is missing what looks like the first drawing.
diff --git a/src/gallium/drivers/vc4/vc4_draw.c b/src/gallium/drivers/vc4/vc4_draw.c
index ebaddae7624..0143072fe12 100644
--- a/src/gallium/drivers/vc4/vc4_draw.c
+++ b/src/gallium/drivers/vc4/vc4_draw.c
@@ -287,6 +287,28 @@ vc4_hw_2116_workaround(struct pipe_context *pctx, int vert_count)
         }
 }
 
+/* A HW bug fails to draw 2-vert line loops.  Just draw it as two GL_LINES. */
+static bool
+vc4_draw_workaround_line_loop_2(struct pipe_context *pctx, const struct pipe_draw_info *info,
+             unsigned drawid_offset,
+             const struct pipe_draw_indirect_info *indirect,
+             const struct pipe_draw_start_count_bias *draw)
+{
+        if (draw->count != 2 || info->mode != PIPE_PRIM_LINE_LOOP)
+                return false;
+
+        struct pipe_draw_info local_info = *info;
+        local_info.mode = PIPE_PRIM_LINES;
+
+        /* Draw twice.  The vertex order will be wrong on the second prim, but
+         * that's probably not worth rewriting an index buffer over.
+         */
+        for (int i = 0; i < 2; i++)
+                pctx->draw_vbo(pctx, &local_info, drawid_offset, indirect, draw, 1);
+
+        return true;
+}
+
 static void
 vc4_draw_vbo(struct pipe_context *pctx, const struct pipe_draw_info *info,
              unsigned drawid_offset,
@@ -309,6 +331,9 @@ vc4_draw_vbo(struct pipe_context *pctx, const struct pipe_draw_info *info,
 	    !u_trim_pipe_prim(info->mode, (unsigned*)&draws[0].count))
 		return;
 
+        if (vc4_draw_workaround_line_loop_2(pctx, info, drawid_offset, indirect, draws))
+                return;
+
         /* Before setting up the draw, do any fixup blits necessary. */
         vc4_predraw_check_textures(pctx, &vc4->verttex);
         vc4_predraw_check_textures(pctx, &vc4->fragtex);