--- /dev/null
+/*
+ * Copyright © 2020 Valve Corporation
+ *
+ * Permission is hereby granted, free of charge, to any person obtaining a
+ * copy of this software and associated documentation files (the "Software"),
+ * to deal in the Software without restriction, including without limitation
+ * the rights to use, copy, modify, merge, publish, distribute, sublicense,
+ * and/or sell copies of the Software, and to permit persons to whom the
+ * Software is furnished to do so, subject to the following conditions:
+ *
+ * The above copyright notice and this permission notice (including the next
+ * paragraph) shall be included in all copies or substantial portions of the
+ * Software.
+ *
+ * THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
+ * IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
+ * FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL
+ * THE AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER
+ * LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM,
+ * OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN THE
+ * SOFTWARE.
+ */
+
+#ifndef __FREEDRENO_GUARDBAND_H__
+#define __FREEDRENO_GUARDBAND_H__
+
+#include <math.h>
+#include <stdbool.h>
+#include <assert.h>
+
+static inline unsigned
+fd_calc_guardband(float offset, float scale, bool is_a3xx)
+{
+ /* On a3xx, the viewport max is 4k and the docs say the max guardband
+ * width is 8k. That is, GRAS cannot handle triangle coordinates more than
+ * 8k, positive or negative. On a4xx+ the viewport width was bumped to
+ * 16k, and so the guardband width was necessarily also bumped. Note that
+ * the numbers here should correspond to
+ * VkPhysicalDeviceLimits::viewportBoundsRange in Vulkan.
+ */
+ const float gb_min = is_a3xx ? -8192. : -32768.;
+ const float gb_max = is_a3xx ? 8191. : 32767.;
+
+ /* Clipping happens in normalized device coordinates, so we have to
+ * transform gb_min and gb_max to ndc using the inverse of the viewport
+ * transform. Avoid flipping min and max by using the absolute value of
+ * the scale.
+ */
+ const float gb_min_ndc = (gb_min - offset) / fabsf(scale);
+ const float gb_max_ndc = (gb_max - offset) / fabsf(scale);
+
+ /* There's only one GB_ADJ field, so presumably the guardband is
+ * [-GB_ADJ, GB_ADJ] like on Radeon. It's always safe to make the
+ * guardband smaller, so we have to take the min to get the largest range
+ * contained in [gb_min_ndc, gb_max_ndc].
+ */
+ const float gb_adj = fminf(-gb_min_ndc, gb_max_ndc);
+
+ /* The viewport should always be contained in the guardband. */
+ assert(gb_adj >= 1.0);
+
+ /* frexp returns an unspecified value if given an infinite value, which
+ * can happen if scale == 0.
+ */
+ if (isinf(gb_adj))
+ return 0x1ff;
+
+ /* Convert gb_adj to 3.6 floating point, rounding down since it's always
+ * safe to make the guard band smaller (but not the other way around!).
+ *
+ * Note: After converting back to a float, the value the blob returns here
+ * is sometimes a little smaller than the value we return. This seems to
+ * happen around the boundary between two different rounded values. For
+ * example, using the a6xx blob:
+ *
+ * min | width | unrounded gb_adj | blob result | mesa result
+ * ------------------------------------------------------------
+ * 0 | 510 | 127.498 | 127. | 127.
+ * 0 | 511 | 127.247 | 126. | 127.
+ * 0 | 512 | 126.996 | 126. | 126.
+ *
+ * The guardband must be 32767 wide, since that's what the blob reports
+ * for viewportBoundsRange, so I'm guessing that they're rounding slightly
+ * more conservatively somehow.
+ */
+ int gb_adj_exp;
+ float gb_adj_mantissa = frexpf(gb_adj, &gb_adj_exp);
+ assert(gb_adj_exp > 0);
+
+ /* Round non-representable numbers down to the largest possible number. */
+ if (gb_adj_exp > 8)
+ return 0x1ff;
+
+ return ((gb_adj_exp - 1) << 6) |
+ ((unsigned) truncf(gb_adj_mantissa * (1 << 7)) - (1 << 6));
+}
+
+#endif /* __FREEDRENO_GUARDBAND_H__ */
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