mesa,gallium: Make point coord origin a CAP

When lower_wpos_pntc is used, the state tracker inserts code to
transform gl_PointCoord.y according to a uniform, to account for
API-requested point coordinate origin and framebuffer orientation. With
the transformation, driver-supplied point coordinates are expected to
have an upper left origin.

If the hardware point coordinate supports (only) a lower left origin,
the backend has to use lower_wpos_pntc and then lower *again* to flip
back. This ends up transforming twice, which is wasteful:

   a = load point coord Y with lower left origin
   a' = 1.0 - a
   a'' = uniform_transform(a')

However, lower_wpos_pntc is quite capable of transforming for a lower
left origin too, it just needs to flip the transformation. Add a CAP
specifying the point coordinate origin convention, rather than assuming
upper-left. This simplifies the Asahi code greatly.

Signed-off-by: Alyssa Rosenzweig <alyssa@rosenzweig.io>
Reviewed-by: Erik Faye-Lund <erik.faye-lund@collabora.com>
Reviewed-by: Marek Olšák <marek.olsak@amd.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/16829>

docs/gallium/screen.rst

@@ -72,6 +72,8 @@ The integer capabilities:
   pixel-center fragment convention is supported.
 * ``PIPE_CAP_FS_COORD_PIXEL_CENTER_INTEGER``: Whether the integer
   pixel-center fragment convention is supported.
+* ``PIPE_CAP_POINT_COORD_ORIGIN_UPPER_LEFT``: Whether point coordinates use the
+  upper-left origin convention. Otherwise the lower-left convention is used.
 * ``PIPE_CAP_DEPTH_CLIP_DISABLE``: Whether the driver is capable of disabling
   depth clipping (through pipe_rasterizer_state).
 * ``PIPE_CAP_DEPTH_CLIP_DISABLE_SEPARATE``: Whether the driver is capable of

src/gallium/auxiliary/util/u_screen.c

@@ -74,6 +74,11 @@ u_pipe_screen_get_param_defaults(struct pipe_screen *pscreen,
    case PIPE_CAP_FS_COORD_ORIGIN_LOWER_LEFT:
    case PIPE_CAP_FS_COORD_PIXEL_CENTER_HALF_INTEGER:
    case PIPE_CAP_FS_COORD_PIXEL_CENTER_INTEGER:
+      return 0;
+
+   case PIPE_CAP_POINT_COORD_ORIGIN_UPPER_LEFT:
+      return 1;
+
    case PIPE_CAP_DEPTH_CLIP_DISABLE:
    case PIPE_CAP_DEPTH_CLIP_DISABLE_SEPARATE:
    case PIPE_CAP_DEPTH_CLAMP_ENABLE:

src/gallium/drivers/nouveau/nv50/nv50_screen.c

@@ -219,6 +219,7 @@ nv50_screen_get_param(struct pipe_screen *pscreen, enum pipe_cap param)
    case PIPE_CAP_INDEP_BLEND_ENABLE:
    case PIPE_CAP_FS_COORD_ORIGIN_UPPER_LEFT:
    case PIPE_CAP_FS_COORD_PIXEL_CENTER_HALF_INTEGER:
+   case PIPE_CAP_POINT_COORD_ORIGIN_UPPER_LEFT:
    case PIPE_CAP_PRIMITIVE_RESTART:
    case PIPE_CAP_PRIMITIVE_RESTART_FIXED_INDEX:
    case PIPE_CAP_VS_INSTANCEID:

src/gallium/drivers/nouveau/nvc0/nvc0_screen.c

@@ -244,6 +244,7 @@ nvc0_screen_get_param(struct pipe_screen *pscreen, enum pipe_cap param)
    case PIPE_CAP_INDEP_BLEND_FUNC:
    case PIPE_CAP_FS_COORD_ORIGIN_UPPER_LEFT:
    case PIPE_CAP_FS_COORD_PIXEL_CENTER_HALF_INTEGER:
+   case PIPE_CAP_POINT_COORD_ORIGIN_UPPER_LEFT:
    case PIPE_CAP_PRIMITIVE_RESTART:
    case PIPE_CAP_PRIMITIVE_RESTART_FIXED_INDEX:
    case PIPE_CAP_VS_INSTANCEID:

src/gallium/include/pipe/p_defines.h

@@ -766,6 +766,7 @@ enum pipe_cap
    PIPE_CAP_FS_COORD_ORIGIN_LOWER_LEFT,
    PIPE_CAP_FS_COORD_PIXEL_CENTER_HALF_INTEGER,
    PIPE_CAP_FS_COORD_PIXEL_CENTER_INTEGER,
+   PIPE_CAP_POINT_COORD_ORIGIN_UPPER_LEFT,
    PIPE_CAP_DEPTH_CLIP_DISABLE,
    PIPE_CAP_DEPTH_CLIP_DISABLE_SEPARATE,
    PIPE_CAP_DEPTH_CLAMP_ENABLE,

src/mesa/main/consts_exts.h

@@ -996,5 +996,8 @@ struct gl_constants
 
    /** Use hardware accelerated GL_SELECT */
    bool HardwareAcceleratedSelect;
+
+   /** Origin of point coordinates. True if upper left, false if lower left. */
+   bool PointCoordOriginUpperLeft;
 };
 #endif

src/mesa/program/prog_statevars.c

@@ -743,7 +743,8 @@ fetch_state(struct gl_context *ctx, const gl_state_index16 state[],
 
    case STATE_FB_PNTC_Y_TRANSFORM:
       {
-         bool flip_y = (ctx->Point.SpriteOrigin == GL_LOWER_LEFT) ^
+         bool flip_y = (ctx->Point.SpriteOrigin == GL_UPPER_LEFT) ^
+            (ctx->Const.PointCoordOriginUpperLeft) ^
             (ctx->DrawBuffer->FlipY);
 
          value[0] = flip_y ? -1.0F : 1.0F;

src/mesa/state_tracker/st_context.c

@@ -682,6 +682,9 @@ st_create_context_priv(struct gl_context *ctx, struct pipe_context *pipe,
    ctx->Point.MaxSize = MAX2(ctx->Const.MaxPointSize,
                              ctx->Const.MaxPointSizeAA);
 
+   ctx->Const.PointCoordOriginUpperLeft =
+      screen->get_param(screen, PIPE_CAP_POINT_COORD_ORIGIN_UPPER_LEFT);
+
    ctx->Const.NoClippingOnCopyTex = screen->get_param(screen,
                                                       PIPE_CAP_NO_CLIP_ON_COPY_TEX);