2 * Copyright (C) 2019 Collabora, Ltd.
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5 * copy of this software and associated documentation files (the "Software"),
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11 * The above copyright notice and this permission notice (including the next
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16 * IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
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24 * Alyssa Rosenzweig <alyssa.rosenzweig@collabora.com>
27 #include "util/u_math.h"
28 #include "pan_encoder.h"
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
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 1024 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]
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 256 bytes. Stack is allocated
51 * with the framebuffer 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):
56 * allocated = npot(max(size, 256)) * (# of threads/core) * (# of cores)
58 * The size of Thread Local Storage is signaled to the GPU in a dedicated
59 * log_stack_size field. Since stack sizes are powers of two, it follows that
60 * stack_size is logarithmic. Consider some sample values:
62 * stack size | log_stack_size
63 * ---------------------------
68 * Noting that log2(256) = 8, we have the relation:
70 * stack_size <= 2^(log_stack_size + 4)
72 * Given the constraints about powers-of-two and the minimum of 256, we thus
73 * derive a formula for log_stack_size in terms of stack size (s):
75 * log_stack_size = ceil(log2(max(s, 256))) - 4
77 * There are other valid characterisations of this formula, of course, but this
78 * is computationally simple, so good enough for our purposes.
81 /* Computes log_stack_size = ceil(log2(max(s, 256))) - 4 */
84 panfrost_get_stack_shift(unsigned stack_size
)
86 return util_logbase2_ceil(MAX2(stack_size
, 256)) - 4;
89 /* Computes the aligned stack size given the shift and thread count */
92 panfrost_get_total_stack_size(
94 unsigned threads_per_core
,
97 unsigned stack_size
= 1 << (stack_shift
+ 4);
98 return stack_size
* threads_per_core
* core_count
;