[Mesa-dev] [PATCH] radeonsi: Fix guardband computation for large render targets

[Oscar Blumberg]

Stop using 12.12 quantization for viewports that are not contained in
the lower 4k corner of the render target as the hardware needs to keep
both absolute and relative coordinates representable.
---
 .../drivers/radeonsi/si_state_viewport.c      | 30 +++++++++++++++++--
 1 file changed, 28 insertions(+), 2 deletions(-)

diff --git a/src/gallium/drivers/radeonsi/si_state_viewport.c b/src/gallium/drivers/radeonsi/si_state_viewport.c
index dac90df1c4f..64bb956b200 100644
--- a/src/gallium/drivers/radeonsi/si_state_viewport.c
+++ b/src/gallium/drivers/radeonsi/si_state_viewport.c
@@ -185,6 +185,16 @@ static void si_emit_guardband(struct si_context *ctx)
 	const unsigned hw_screen_offset_alignment =
 		ctx->chip_class >= VI ? 16 : MAX2(ctx->screen->se_tile_repeat, 16);
 
+	/* Indexed by quantization modes */
+	static unsigned max_viewport_size[] = {65535, 16383, 4095};
+
+	/* Ensure that the whole viewport stays representable in
+	 * absolute coordinates.
+	 * See comment in si_set_viewport_states.
+	 */
+	assert(vp_as_scissor.maxx <= max_viewport_size[vp_as_scissor.quant_mode] &&
+	       vp_as_scissor.maxy <= max_viewport_size[vp_as_scissor.quant_mode]);
+
 	hw_screen_offset_x = CLAMP(hw_screen_offset_x, 0, MAX_PA_SU_HARDWARE_SCREEN_OFFSET);
 	hw_screen_offset_y = CLAMP(hw_screen_offset_y, 0, MAX_PA_SU_HARDWARE_SCREEN_OFFSET);
 
@@ -219,7 +229,6 @@ static void si_emit_guardband(struct si_context *ctx)
 	 *
 	 * The viewport range is [-max_viewport_size/2, max_viewport_size/2].
 	 */
-	static unsigned max_viewport_size[] = {65535, 16383, 4095};
 	assert(vp_as_scissor.quant_mode < ARRAY_SIZE(max_viewport_size));
 	max_range = max_viewport_size[vp_as_scissor.quant_mode] / 2;
 	left   = (-max_range - vp.translate[0]) / vp.scale[0];
@@ -333,6 +342,8 @@ static void si_set_viewport_states(struct pipe_context *pctx,
 		unsigned h = scissor->maxy - scissor->miny;
 		unsigned max_extent = MAX2(w, h);
 
+		int max_corner = MAX2(scissor->maxx, scissor->maxy);
+
 		unsigned center_x = (scissor->maxx + scissor->minx) / 2;
 		unsigned center_y = (scissor->maxy + scissor->miny) / 2;
 		unsigned max_center = MAX2(center_x, center_y);
@@ -358,7 +369,22 @@ static void si_set_viewport_states(struct pipe_context *pctx,
 		if (ctx->family == CHIP_RAVEN)
 			max_extent = 16384; /* Use QUANT_MODE == 16_8. */
 
-		if (max_extent <= 1024) /* 4K scanline area for guardband */
+		/* Another constraint is that all coordinates in the viewport
+		 * are representable in fixed point with respect to the
+		 * surface origin.
+		 *
+		 * It means that PA_SU_HARDWARE_SCREEN_OFFSET can't be given
+		 * an offset that would make the upper corner of the viewport
+		 * greater than the maximum representable number post
+		 * quantization, ie 2^quant_bits.
+		 *
+		 * This does not matter for 14.10 and 16.8 formats since the
+		 * offset is already limited at 8k, but it means we can't use
+		 * 12.12 if we are drawing to some pixels outside the lower
+		 * 4k x 4k of the render target.
+		 */
+
+		if (max_extent <= 1024 && max_corner < 4096) /* 4K scanline area for guardband */
 			scissor->quant_mode = SI_QUANT_MODE_12_12_FIXED_POINT_1_4096TH;
 		else if (max_extent <= 4096) /* 16K scanline area for guardband */
 			scissor->quant_mode = SI_QUANT_MODE_14_10_FIXED_POINT_1_1024TH;
-- 
2.20.1