freedreno: Add a helper for computing guardband sizes
[mesa.git] / src / freedreno / common / freedreno_guardband.h
1 /*
2 * Copyright © 2020 Valve Corporation
3 *
4 * Permission is hereby granted, free of charge, to any person obtaining a
5 * copy of this software and associated documentation files (the "Software"),
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10 *
11 * The above copyright notice and this permission notice (including the next
12 * paragraph) shall be included in all copies or substantial portions of the
13 * Software.
14 *
15 * THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
16 * IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
17 * FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL
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21 * SOFTWARE.
22 */
23
24 #ifndef __FREEDRENO_GUARDBAND_H__
25 #define __FREEDRENO_GUARDBAND_H__
26
27 #include <math.h>
28 #include <stdbool.h>
29 #include <assert.h>
30
31 static inline unsigned
32 fd_calc_guardband(float offset, float scale, bool is_a3xx)
33 {
34 /* On a3xx, the viewport max is 4k and the docs say the max guardband
35 * width is 8k. That is, GRAS cannot handle triangle coordinates more than
36 * 8k, positive or negative. On a4xx+ the viewport width was bumped to
37 * 16k, and so the guardband width was necessarily also bumped. Note that
38 * the numbers here should correspond to
39 * VkPhysicalDeviceLimits::viewportBoundsRange in Vulkan.
40 */
41 const float gb_min = is_a3xx ? -8192. : -32768.;
42 const float gb_max = is_a3xx ? 8191. : 32767.;
43
44 /* Clipping happens in normalized device coordinates, so we have to
45 * transform gb_min and gb_max to ndc using the inverse of the viewport
46 * transform. Avoid flipping min and max by using the absolute value of
47 * the scale.
48 */
49 const float gb_min_ndc = (gb_min - offset) / fabsf(scale);
50 const float gb_max_ndc = (gb_max - offset) / fabsf(scale);
51
52 /* There's only one GB_ADJ field, so presumably the guardband is
53 * [-GB_ADJ, GB_ADJ] like on Radeon. It's always safe to make the
54 * guardband smaller, so we have to take the min to get the largest range
55 * contained in [gb_min_ndc, gb_max_ndc].
56 */
57 const float gb_adj = fminf(-gb_min_ndc, gb_max_ndc);
58
59 /* The viewport should always be contained in the guardband. */
60 assert(gb_adj >= 1.0);
61
62 /* frexp returns an unspecified value if given an infinite value, which
63 * can happen if scale == 0.
64 */
65 if (isinf(gb_adj))
66 return 0x1ff;
67
68 /* Convert gb_adj to 3.6 floating point, rounding down since it's always
69 * safe to make the guard band smaller (but not the other way around!).
70 *
71 * Note: After converting back to a float, the value the blob returns here
72 * is sometimes a little smaller than the value we return. This seems to
73 * happen around the boundary between two different rounded values. For
74 * example, using the a6xx blob:
75 *
76 * min | width | unrounded gb_adj | blob result | mesa result
77 * ------------------------------------------------------------
78 * 0 | 510 | 127.498 | 127. | 127.
79 * 0 | 511 | 127.247 | 126. | 127.
80 * 0 | 512 | 126.996 | 126. | 126.
81 *
82 * The guardband must be 32767 wide, since that's what the blob reports
83 * for viewportBoundsRange, so I'm guessing that they're rounding slightly
84 * more conservatively somehow.
85 */
86 int gb_adj_exp;
87 float gb_adj_mantissa = frexpf(gb_adj, &gb_adj_exp);
88 assert(gb_adj_exp > 0);
89
90 /* Round non-representable numbers down to the largest possible number. */
91 if (gb_adj_exp > 8)
92 return 0x1ff;
93
94 return ((gb_adj_exp - 1) << 6) |
95 ((unsigned) truncf(gb_adj_mantissa * (1 << 7)) - (1 << 6));
96 }
97
98 #endif /* __FREEDRENO_GUARDBAND_H__ */