478a788b116d54a6cf77b961c62c4bcff9615ba5
[mesa.git] / src / panfrost / lib / pan_scratch.c
1 /*
2 * Copyright (C) 2019 Collabora, Ltd.
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"),
6 * to deal in the Software without restriction, including without limitation
7 * the rights to use, copy, modify, merge, publish, distribute, sublicense,
8 * and/or sell copies of the Software, and to permit persons to whom the
9 * Software is furnished to do so, subject to the following conditions:
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
18 * THE AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER
19 * LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM,
20 * OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN THE
21 * SOFTWARE.
22 *
23 * Authors:
24 * Alyssa Rosenzweig <alyssa.rosenzweig@collabora.com>
25 */
26
27 #include "util/u_math.h"
28 #include "pan_encoder.h"
29
30 /* Midgard has a small register file, so shaders with high register pressure
31 * need to spill from the register file onto the stack. In addition to
32 * spilling, it is desireable to allocate temporary arrays on the stack (for
33 * instance because the register file does not support indirect access but the
34 * stack does).
35 *
36 * The stack is located in "Thread Local Storage", sometimes abbreviated TLS in
37 * the kernel source code. Thread local storage is allocated per-thread,
38 * per-core, so threads executing concurrently do not interfere with each
39 * other's stacks. On modern kernels, we may query
40 * DRM_PANFROST_PARAM_THREAD_TLS_ALLOC for the number of threads per core we
41 * must allocate for, and DRM_PANFROST_PARAM_SHADER_PRESENT for a bitmask of
42 * shader cores (so take a popcount of that mask for the number of shader
43 * cores). On older kernels that do not support querying these values,
44 * following kbase, we may use the worst-case value of 256 threads for
45 * THREAD_TLS_ALLOC, and the worst-case value of 16 cores for Midgard per the
46 * "shader core count" column of the implementations table in
47 * https://en.wikipedia.org/wiki/Mali_%28GPU% [citation needed]
48 *
49 * Within a particular thread, there is stack allocated. If it is present, its
50 * size is a power-of-two, and it is at least 16 bytes. Stack is allocated
51 * with the shared memory descriptor used for all shaders within a frame (note
52 * that they don't execute concurrently so it's fine). So, consider the maximum
53 * stack size used by any shader within a job, and then compute (where npot
54 * denotes the next power of two):
55 *
56 * bytes/thread = npot(max(size, 16))
57 * allocated = (# of bytes/thread) * (# of threads/core) * (# of cores)
58 *
59 * The size of Thread Local Storage is signaled to the GPU in a dedicated
60 * log_stack_size field. Since stack sizes are powers of two, it follows that
61 * stack_size is logarithmic. Consider some sample values:
62 *
63 * stack size | log_stack_size
64 * ---------------------------
65 * 256 | 4
66 * 512 | 5
67 * 1024 | 6
68 *
69 * Noting that log2(256) = 8, we have the relation:
70 *
71 * stack_size <= 2^(log_stack_size + 4)
72 *
73 * Given the constraints about powers-of-two and the minimum of 256, we thus
74 * derive a formula for log_stack_size in terms of stack size (s), where s is
75 * positive:
76 *
77 * log_stack_size = ceil(log2(max(s, 16))) - 4
78 *
79 * There are other valid characterisations of this formula, of course, but this
80 * is computationally simple, so good enough for our purposes. If s=0, since
81 * there is no spilling used whatsoever, we may set log_stack_size to 0 to
82 * disable the stack.
83 */
84
85 /* Computes log_stack_size = ceil(log2(max(s, 16))) - 4 */
86
87 unsigned
88 panfrost_get_stack_shift(unsigned stack_size)
89 {
90 if (stack_size)
91 return util_logbase2_ceil(MAX2(stack_size, 16)) - 4;
92 else
93 return 0;
94 }
95
96 /* Computes the aligned stack size given the shift and thread count. The blob
97 * reserves an extra page, and since this is hardware-internal, we do too. */
98
99 unsigned
100 panfrost_get_total_stack_size(
101 unsigned stack_shift,
102 unsigned threads_per_core,
103 unsigned core_count)
104 {
105 unsigned size_per_thread = MAX2(1 << (stack_shift + 4), 32);
106 unsigned size = size_per_thread * threads_per_core * core_count;
107
108 return size + 4096;
109 }