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1 /*
2  * Copyright (C) 2014 Rob Clark <robclark@freedesktop.org>
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  *    Rob Clark <robclark@freedesktop.org>
25  */
26 
27 #include "pipe/p_screen.h"
28 #include "pipe/p_state.h"
29 #include "tgsi/tgsi_dump.h"
30 #include "tgsi/tgsi_parse.h"
31 #include "util/format/u_format.h"
32 #include "util/u_inlines.h"
33 #include "util/u_memory.h"
34 #include "util/u_string.h"
35 
36 #include "nir/tgsi_to_nir.h"
37 #include "nir_serialize.h"
38 
39 #include "freedreno_context.h"
40 #include "freedreno_util.h"
41 
42 #include "ir3/ir3_cache.h"
43 #include "ir3/ir3_compiler.h"
44 #include "ir3/ir3_gallium.h"
45 #include "ir3/ir3_nir.h"
46 #include "ir3/ir3_shader.h"
47 
48 /**
49  * The hardware cso for shader state
50  *
51  * Initially just a container for the ir3_shader, but this is where we'll
52  * plumb in async compile.
53  */
54 struct ir3_shader_state {
55    struct ir3_shader *shader;
56 
57    /* Fence signalled when async compile is completed: */
58    struct util_queue_fence ready;
59 };
60 
61 /**
62  * Should initial variants be compiled synchronously?
63  *
64  * The only case where util_debug_message() is used in the initial-variants
65  * path is with FD_MESA_DEBUG=shaderdb.  So if either debug is disabled (ie.
66  * debug.debug_message==NULL), or shaderdb stats are not enabled, we can
67  * compile the initial shader variant asynchronously.
68  */
69 static bool
initial_variants_synchronous(struct fd_context * ctx)70 initial_variants_synchronous(struct fd_context *ctx)
71 {
72    return unlikely(ctx->debug.debug_message) || FD_DBG(SHADERDB) ||
73           FD_DBG(SERIALC);
74 }
75 
76 static void
dump_shader_info(struct ir3_shader_variant * v,struct util_debug_callback * debug)77 dump_shader_info(struct ir3_shader_variant *v,
78                  struct util_debug_callback *debug)
79 {
80    if (!FD_DBG(SHADERDB))
81       return;
82 
83    util_debug_message(
84       debug, SHADER_INFO,
85       "%s shader: %u inst, %u nops, %u non-nops, %u mov, %u cov, "
86       "%u dwords, %u last-baryf, %u half, %u full, %u constlen, "
87       "%u cat0, %u cat1, %u cat2, %u cat3, %u cat4, %u cat5, %u cat6, %u cat7, "
88       "%u stp, %u ldp, %u sstall, %u (ss), %u systall, %u (sy), %d waves, "
89       "%d loops\n",
90       ir3_shader_stage(v), v->info.instrs_count, v->info.nops_count,
91       v->info.instrs_count - v->info.nops_count, v->info.mov_count,
92       v->info.cov_count, v->info.sizedwords, v->info.last_baryf,
93       v->info.max_half_reg + 1, v->info.max_reg + 1, v->constlen,
94       v->info.instrs_per_cat[0], v->info.instrs_per_cat[1],
95       v->info.instrs_per_cat[2], v->info.instrs_per_cat[3],
96       v->info.instrs_per_cat[4], v->info.instrs_per_cat[5],
97       v->info.instrs_per_cat[6], v->info.instrs_per_cat[7],
98       v->info.stp_count, v->info.ldp_count, v->info.sstall,
99       v->info.ss, v->info.systall, v->info.sy, v->info.max_waves, v->loops);
100 }
101 
102 static void
upload_shader_variant(struct ir3_shader_variant * v)103 upload_shader_variant(struct ir3_shader_variant *v)
104 {
105    struct ir3_compiler *compiler = v->compiler;
106 
107    assert(!v->bo);
108 
109    v->bo =
110       fd_bo_new(compiler->dev, v->info.size, FD_BO_NOMAP,
111                 "%s:%s", ir3_shader_stage(v), v->name);
112 
113    /* Always include shaders in kernel crash dumps. */
114    fd_bo_mark_for_dump(v->bo);
115 
116    fd_bo_upload(v->bo, v->bin, v->info.size);
117 }
118 
119 struct ir3_shader_variant *
ir3_shader_variant(struct ir3_shader * shader,struct ir3_shader_key key,bool binning_pass,struct util_debug_callback * debug)120 ir3_shader_variant(struct ir3_shader *shader, struct ir3_shader_key key,
121                    bool binning_pass, struct util_debug_callback *debug)
122 {
123    struct ir3_shader_variant *v;
124    bool created = false;
125 
126    /* Some shader key values may not be used by a given ir3_shader (for
127     * example, fragment shader saturates in the vertex shader), so clean out
128     * those flags to avoid recompiling.
129     */
130    ir3_key_clear_unused(&key, shader);
131 
132    v = ir3_shader_get_variant(shader, &key, binning_pass, false, &created);
133 
134    if (created) {
135       if (shader->initial_variants_done) {
136          perf_debug_message(debug, SHADER_INFO,
137                             "%s shader: recompiling at draw time: global "
138                             "0x%08x, vfsamples %x/%x, astc %x/%x\n",
139                             ir3_shader_stage(v), key.global, key.vsamples,
140                             key.fsamples, key.vastc_srgb, key.fastc_srgb);
141       }
142 
143       dump_shader_info(v, debug);
144       upload_shader_variant(v);
145 
146       if (v->binning) {
147          upload_shader_variant(v->binning);
148          dump_shader_info(v->binning, debug);
149       }
150    }
151 
152    return v;
153 }
154 
155 static void
copy_stream_out(struct ir3_stream_output_info * i,const struct pipe_stream_output_info * p)156 copy_stream_out(struct ir3_stream_output_info *i,
157                 const struct pipe_stream_output_info *p)
158 {
159    STATIC_ASSERT(ARRAY_SIZE(i->stride) == ARRAY_SIZE(p->stride));
160    STATIC_ASSERT(ARRAY_SIZE(i->output) == ARRAY_SIZE(p->output));
161 
162    i->num_outputs = p->num_outputs;
163    for (int n = 0; n < ARRAY_SIZE(i->stride); n++)
164       i->stride[n] = p->stride[n];
165 
166    for (int n = 0; n < ARRAY_SIZE(i->output); n++) {
167       i->output[n].register_index = p->output[n].register_index;
168       i->output[n].start_component = p->output[n].start_component;
169       i->output[n].num_components = p->output[n].num_components;
170       i->output[n].output_buffer = p->output[n].output_buffer;
171       i->output[n].dst_offset = p->output[n].dst_offset;
172       i->output[n].stream = p->output[n].stream;
173    }
174 }
175 
176 static void
create_initial_variants(struct ir3_shader_state * hwcso,struct util_debug_callback * debug)177 create_initial_variants(struct ir3_shader_state *hwcso,
178                         struct util_debug_callback *debug)
179 {
180    struct ir3_shader *shader = hwcso->shader;
181    struct ir3_compiler *compiler = shader->compiler;
182    nir_shader *nir = shader->nir;
183 
184    /* Compile standard variants immediately to try to avoid draw-time stalls
185     * to run the compiler.
186     */
187    struct ir3_shader_key key = {
188       .tessellation = IR3_TESS_NONE,
189       .ucp_enables = MASK(nir->info.clip_distance_array_size),
190       .msaa = true,
191    };
192 
193    switch (nir->info.stage) {
194    case MESA_SHADER_TESS_EVAL:
195       key.tessellation = ir3_tess_mode(nir->info.tess._primitive_mode);
196       break;
197 
198    case MESA_SHADER_TESS_CTRL:
199       /* The primitive_mode field, while it exists for TCS, is not
200        * populated (since separable shaders between TCS/TES are legal,
201        * so TCS wouldn't have access to TES's declaration).  Make a
202        * guess so that we shader-db something plausible for TCS.
203        */
204       if (nir->info.outputs_written & VARYING_BIT_TESS_LEVEL_INNER)
205          key.tessellation = IR3_TESS_TRIANGLES;
206       else
207          key.tessellation = IR3_TESS_ISOLINES;
208       break;
209 
210    case MESA_SHADER_GEOMETRY:
211       key.has_gs = true;
212       break;
213 
214    default:
215       break;
216    }
217 
218    key.safe_constlen = false;
219    struct ir3_shader_variant *v = ir3_shader_variant(shader, key, false, debug);
220    if (!v)
221       return;
222 
223    if (v->constlen > compiler->max_const_safe) {
224       key.safe_constlen = true;
225       ir3_shader_variant(shader, key, false, debug);
226    }
227 
228    /* For vertex shaders, also compile initial binning pass shader: */
229    if (nir->info.stage == MESA_SHADER_VERTEX) {
230       key.safe_constlen = false;
231       v = ir3_shader_variant(shader, key, true, debug);
232       if (!v)
233          return;
234 
235       if (v->constlen > compiler->max_const_safe) {
236          key.safe_constlen = true;
237          ir3_shader_variant(shader, key, true, debug);
238       }
239    }
240 
241    shader->initial_variants_done = true;
242 }
243 
244 static void
create_initial_variants_async(void * job,void * gdata,int thread_index)245 create_initial_variants_async(void *job, void *gdata, int thread_index)
246 {
247    struct ir3_shader_state *hwcso = job;
248    struct util_debug_callback debug = {};
249 
250    create_initial_variants(hwcso, &debug);
251 }
252 
253 static void
create_initial_compute_variants_async(void * job,void * gdata,int thread_index)254 create_initial_compute_variants_async(void *job, void *gdata, int thread_index)
255 {
256    struct ir3_shader_state *hwcso = job;
257    struct ir3_shader *shader = hwcso->shader;
258    struct util_debug_callback debug = {};
259    static struct ir3_shader_key key; /* static is implicitly zeroed */
260 
261    ir3_shader_variant(shader, key, false, &debug);
262    shader->initial_variants_done = true;
263 }
264 
265 /* a bit annoying that compute-shader and normal shader state objects
266  * aren't a bit more aligned.
267  */
268 void *
ir3_shader_compute_state_create(struct pipe_context * pctx,const struct pipe_compute_state * cso)269 ir3_shader_compute_state_create(struct pipe_context *pctx,
270                                 const struct pipe_compute_state *cso)
271 {
272    struct fd_context *ctx = fd_context(pctx);
273 
274    /* req_input_mem will only be non-zero for cl kernels (ie. clover).
275     * This isn't a perfect test because I guess it is possible (but
276     * uncommon) for none for the kernel parameters to be a global,
277     * but ctx->set_global_bindings() can't fail, so this is the next
278     * best place to fail if we need a newer version of kernel driver:
279     */
280    if ((cso->req_input_mem > 0) &&
281        fd_device_version(ctx->dev) < FD_VERSION_BO_IOVA) {
282       return NULL;
283    }
284 
285    struct ir3_compiler *compiler = ctx->screen->compiler;
286    nir_shader *nir;
287 
288    if (cso->ir_type == PIPE_SHADER_IR_NIR) {
289       /* we take ownership of the reference: */
290       nir = (nir_shader *)cso->prog;
291    } else if (cso->ir_type == PIPE_SHADER_IR_NIR_SERIALIZED) {
292       const nir_shader_compiler_options *options =
293             ir3_get_compiler_options(compiler);
294       const struct pipe_binary_program_header *hdr = cso->prog;
295       struct blob_reader reader;
296 
297       blob_reader_init(&reader, hdr->blob, hdr->num_bytes);
298       nir = nir_deserialize(NULL, options, &reader);
299 
300       ir3_finalize_nir(compiler, nir);
301    } else {
302       assert(cso->ir_type == PIPE_SHADER_IR_TGSI);
303       if (ir3_shader_debug & IR3_DBG_DISASM) {
304          tgsi_dump(cso->prog, 0);
305       }
306       nir = tgsi_to_nir(cso->prog, pctx->screen, false);
307    }
308 
309    struct ir3_shader *shader =
310       ir3_shader_from_nir(compiler, nir, &(struct ir3_shader_options){
311                               /* TODO: force to single on a6xx with legacy
312                                * ballot extension that uses 64-bit masks
313                                */
314                               .api_wavesize = IR3_SINGLE_OR_DOUBLE,
315                               .real_wavesize = IR3_SINGLE_OR_DOUBLE,
316                           }, NULL);
317    shader->cs.req_input_mem = align(cso->req_input_mem, 4) / 4;     /* byte->dword */
318    shader->cs.req_local_mem = cso->req_local_mem;
319 
320    struct ir3_shader_state *hwcso = calloc(1, sizeof(*hwcso));
321 
322    util_queue_fence_init(&hwcso->ready);
323    hwcso->shader = shader;
324 
325    /* Immediately compile a standard variant.  We have so few variants in our
326     * shaders, that doing so almost eliminates draw-time recompiles.  (This
327     * is also how we get data from shader-db's ./run)
328     */
329 
330    if (initial_variants_synchronous(ctx)) {
331       static struct ir3_shader_key key; /* static is implicitly zeroed */
332       ir3_shader_variant(shader, key, false, &ctx->debug);
333       shader->initial_variants_done = true;
334    } else {
335       struct fd_screen *screen = ctx->screen;
336       util_queue_add_job(&screen->compile_queue, hwcso, &hwcso->ready,
337                          create_initial_compute_variants_async, NULL, 0);
338    }
339 
340    return hwcso;
341 }
342 
343 void *
ir3_shader_state_create(struct pipe_context * pctx,const struct pipe_shader_state * cso)344 ir3_shader_state_create(struct pipe_context *pctx,
345                         const struct pipe_shader_state *cso)
346 {
347    struct fd_context *ctx = fd_context(pctx);
348    struct ir3_compiler *compiler = ctx->screen->compiler;
349    struct ir3_shader_state *hwcso = calloc(1, sizeof(*hwcso));
350 
351    /*
352     * Convert to nir (if necessary):
353     */
354 
355    nir_shader *nir;
356    if (cso->type == PIPE_SHADER_IR_NIR) {
357       /* we take ownership of the reference: */
358       nir = cso->ir.nir;
359    } else {
360       assert(cso->type == PIPE_SHADER_IR_TGSI);
361       if (ir3_shader_debug & IR3_DBG_DISASM) {
362          tgsi_dump(cso->tokens, 0);
363       }
364       nir = tgsi_to_nir(cso->tokens, pctx->screen, false);
365    }
366 
367    /*
368     * Create ir3_shader:
369     *
370     * This part is cheap, it doesn't compile initial variants
371     */
372 
373    struct ir3_stream_output_info stream_output = {};
374    copy_stream_out(&stream_output, &cso->stream_output);
375 
376    hwcso->shader =
377       ir3_shader_from_nir(compiler, nir, &(struct ir3_shader_options){
378                               /* TODO: force to single on a6xx with legacy
379                                * ballot extension that uses 64-bit masks
380                                */
381                               .api_wavesize = IR3_SINGLE_OR_DOUBLE,
382                               .real_wavesize = IR3_SINGLE_OR_DOUBLE,
383                           },
384                           &stream_output);
385 
386    /*
387     * Create initial variants to avoid draw-time stalls.  This is
388     * normally done asynchronously, unless debug is enabled (which
389     * will be the case for shader-db)
390     */
391 
392    util_queue_fence_init(&hwcso->ready);
393 
394    if (initial_variants_synchronous(ctx)) {
395       create_initial_variants(hwcso, &ctx->debug);
396    } else {
397       util_queue_add_job(&ctx->screen->compile_queue, hwcso, &hwcso->ready,
398                          create_initial_variants_async, NULL, 0);
399    }
400 
401    return hwcso;
402 }
403 
404 void
ir3_shader_state_delete(struct pipe_context * pctx,void * _hwcso)405 ir3_shader_state_delete(struct pipe_context *pctx, void *_hwcso)
406 {
407    struct fd_context *ctx = fd_context(pctx);
408    struct fd_screen *screen = ctx->screen;
409    struct ir3_shader_state *hwcso = _hwcso;
410    struct ir3_shader *so = hwcso->shader;
411 
412    ir3_cache_invalidate(ctx->shader_cache, hwcso);
413 
414    /* util_queue_drop_job() guarantees that either:
415     *  1) job did not execute
416     *  2) job completed
417     *
418     * In either case the fence is signaled
419     */
420    util_queue_drop_job(&screen->compile_queue, &hwcso->ready);
421 
422    /* free the uploaded shaders, since this is handled outside of the
423     * shared ir3 code (ie. not used by turnip):
424     */
425    for (struct ir3_shader_variant *v = so->variants; v; v = v->next) {
426       fd_bo_del(v->bo);
427       v->bo = NULL;
428 
429       if (v->binning && v->binning->bo) {
430          fd_bo_del(v->binning->bo);
431          v->binning->bo = NULL;
432       }
433    }
434 
435    ir3_shader_destroy(so);
436    util_queue_fence_destroy(&hwcso->ready);
437    free(hwcso);
438 }
439 
440 struct ir3_shader *
ir3_get_shader(struct ir3_shader_state * hwcso)441 ir3_get_shader(struct ir3_shader_state *hwcso)
442 {
443    if (!hwcso)
444       return NULL;
445 
446    struct ir3_shader *shader = hwcso->shader;
447    perf_time (1000, "waited for %s:%s:%s variants",
448               _mesa_shader_stage_to_abbrev(shader->type),
449               shader->nir->info.name,
450               shader->nir->info.label) {
451       /* wait for initial variants to compile: */
452       util_queue_fence_wait(&hwcso->ready);
453    }
454 
455    return shader;
456 }
457 
458 struct shader_info *
ir3_get_shader_info(struct ir3_shader_state * hwcso)459 ir3_get_shader_info(struct ir3_shader_state *hwcso)
460 {
461    if (!hwcso)
462       return NULL;
463    return &hwcso->shader->nir->info;
464 }
465 
466 /* fixup dirty shader state in case some "unrelated" (from the state-
467  * tracker's perspective) state change causes us to switch to a
468  * different variant.
469  */
470 void
ir3_fixup_shader_state(struct pipe_context * pctx,struct ir3_shader_key * key)471 ir3_fixup_shader_state(struct pipe_context *pctx, struct ir3_shader_key *key)
472 {
473    struct fd_context *ctx = fd_context(pctx);
474 
475    if (!ir3_shader_key_equal(ctx->last.key, key)) {
476       if (ir3_shader_key_changes_fs(ctx->last.key, key)) {
477          fd_context_dirty_shader(ctx, PIPE_SHADER_FRAGMENT,
478                                  FD_DIRTY_SHADER_PROG);
479       }
480 
481       if (ir3_shader_key_changes_vs(ctx->last.key, key)) {
482          fd_context_dirty_shader(ctx, PIPE_SHADER_VERTEX, FD_DIRTY_SHADER_PROG);
483       }
484 
485       /* NOTE: currently only a6xx has gs/tess, but needs no
486        * gs/tess specific lowering.
487        */
488 
489       *ctx->last.key = *key;
490    }
491 }
492 
493 static char *
ir3_screen_finalize_nir(struct pipe_screen * pscreen,void * nir)494 ir3_screen_finalize_nir(struct pipe_screen *pscreen, void *nir)
495 {
496    struct fd_screen *screen = fd_screen(pscreen);
497 
498    ir3_nir_lower_io_to_temporaries(nir);
499    ir3_finalize_nir(screen->compiler, nir);
500 
501    return NULL;
502 }
503 
504 static void
ir3_set_max_shader_compiler_threads(struct pipe_screen * pscreen,unsigned max_threads)505 ir3_set_max_shader_compiler_threads(struct pipe_screen *pscreen,
506                                     unsigned max_threads)
507 {
508    struct fd_screen *screen = fd_screen(pscreen);
509 
510    /* This function doesn't allow a greater number of threads than
511     * the queue had at its creation.
512     */
513    util_queue_adjust_num_threads(&screen->compile_queue, max_threads);
514 }
515 
516 static bool
ir3_is_parallel_shader_compilation_finished(struct pipe_screen * pscreen,void * shader,enum pipe_shader_type shader_type)517 ir3_is_parallel_shader_compilation_finished(struct pipe_screen *pscreen,
518                                             void *shader,
519                                             enum pipe_shader_type shader_type)
520 {
521    struct ir3_shader_state *hwcso = (struct ir3_shader_state *)shader;
522 
523    return util_queue_fence_is_signalled(&hwcso->ready);
524 }
525 
526 void
ir3_prog_init(struct pipe_context * pctx)527 ir3_prog_init(struct pipe_context *pctx)
528 {
529    pctx->create_vs_state = ir3_shader_state_create;
530    pctx->delete_vs_state = ir3_shader_state_delete;
531 
532    pctx->create_tcs_state = ir3_shader_state_create;
533    pctx->delete_tcs_state = ir3_shader_state_delete;
534 
535    pctx->create_tes_state = ir3_shader_state_create;
536    pctx->delete_tes_state = ir3_shader_state_delete;
537 
538    pctx->create_gs_state = ir3_shader_state_create;
539    pctx->delete_gs_state = ir3_shader_state_delete;
540 
541    pctx->create_fs_state = ir3_shader_state_create;
542    pctx->delete_fs_state = ir3_shader_state_delete;
543 }
544 
545 void
ir3_screen_init(struct pipe_screen * pscreen)546 ir3_screen_init(struct pipe_screen *pscreen)
547 {
548    struct fd_screen *screen = fd_screen(pscreen);
549 
550    screen->compiler = ir3_compiler_create(screen->dev, screen->dev_id,
551                                           &(struct ir3_compiler_options) {});
552 
553    /* TODO do we want to limit things to # of fast cores, or just limit
554     * based on total # of both big and little cores.  The little cores
555     * tend to be in-order and probably much slower for compiling than
556     * big cores.  OTOH if they are sitting idle, maybe it is useful to
557     * use them?
558     */
559    unsigned num_threads = sysconf(_SC_NPROCESSORS_ONLN) - 1;
560 
561    /* Create at least one thread - even on single core CPU systems. */
562    num_threads = MAX2(1, num_threads);
563 
564    util_queue_init(&screen->compile_queue, "ir3q", 64, num_threads,
565                    UTIL_QUEUE_INIT_RESIZE_IF_FULL |
566                       UTIL_QUEUE_INIT_SET_FULL_THREAD_AFFINITY, NULL);
567 
568    pscreen->finalize_nir = ir3_screen_finalize_nir;
569    pscreen->set_max_shader_compiler_threads =
570       ir3_set_max_shader_compiler_threads;
571    pscreen->is_parallel_shader_compilation_finished =
572       ir3_is_parallel_shader_compilation_finished;
573 }
574 
575 void
ir3_screen_fini(struct pipe_screen * pscreen)576 ir3_screen_fini(struct pipe_screen *pscreen)
577 {
578    struct fd_screen *screen = fd_screen(pscreen);
579 
580    util_queue_destroy(&screen->compile_queue);
581    ir3_compiler_destroy(screen->compiler);
582    screen->compiler = NULL;
583 }
584 
585 void
ir3_update_max_tf_vtx(struct fd_context * ctx,const struct ir3_shader_variant * v)586 ir3_update_max_tf_vtx(struct fd_context *ctx,
587                       const struct ir3_shader_variant *v)
588 {
589    struct fd_streamout_stateobj *so = &ctx->streamout;
590    const struct ir3_stream_output_info *info = &v->stream_output;
591    uint32_t maxvtxcnt = 0x7fffffff;
592 
593    if (v->stream_output.num_outputs == 0)
594       maxvtxcnt = 0;
595    if (so->num_targets == 0)
596       maxvtxcnt = 0;
597 
598    /* offset to write to is:
599     *
600     *   total_vtxcnt = vtxcnt + offsets[i]
601     *   offset = total_vtxcnt * stride[i]
602     *
603     *   offset =   vtxcnt * stride[i]       ; calculated in shader
604     *            + offsets[i] * stride[i]   ; calculated at emit_tfbos()
605     *
606     * assuming for each vtx, each target buffer will have data written
607     * up to 'offset + stride[i]', that leaves maxvtxcnt as:
608     *
609     *   buffer_size = (maxvtxcnt * stride[i]) + stride[i]
610     *   maxvtxcnt   = (buffer_size - stride[i]) / stride[i]
611     *
612     * but shader is actually doing a less-than (rather than less-than-
613     * equal) check, so we can drop the -stride[i].
614     *
615     * TODO is assumption about `offset + stride[i]` legit?
616     */
617    for (unsigned i = 0; i < so->num_targets; i++) {
618       struct pipe_stream_output_target *target = so->targets[i];
619       unsigned stride = info->stride[i] * 4; /* convert dwords->bytes */
620       if (target) {
621          uint32_t max = target->buffer_size / stride;
622          maxvtxcnt = MIN2(maxvtxcnt, max);
623       }
624    }
625 
626    ctx->streamout.max_tf_vtx = maxvtxcnt;
627 }
628