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nexus: Add support for deriving timing constraints in packer

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Signed-off-by: David Shah <dave@ds0.me>
This commit is contained in:
David Shah 2020-12-02 09:44:17 +00:00
parent 5a291e4622
commit b666c85824
1 changed files with 96 additions and 0 deletions
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96
nexus/pack.cc
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@ -1709,6 +1709,101 @@ struct NexusPacker
}
}
void generate_constraints()
{
log_info("Generating derived timing constraints...\n");
auto MHz = [&](delay_t a) { return 1000.0 / ctx->getDelayNS(a); };
auto equals_epsilon = [](delay_t a, delay_t b) { return (std::abs(a - b) / std::max(double(b), 1.0)) < 1e-3; };
std::unordered_set<IdString> user_constrained, changed_nets;
for (auto &net : ctx->nets) {
if (net.second->clkconstr != nullptr)
user_constrained.insert(net.first);
changed_nets.insert(net.first);
}
auto set_period = [&](CellInfo *ci, IdString port, delay_t period) {
if (!ci->ports.count(port))
return;
NetInfo *to = ci->ports.at(port).net;
if (to == nullptr)
return;
if (to->clkconstr != nullptr) {
if (!equals_epsilon(to->clkconstr->period.min_delay, period) && user_constrained.count(to->name))
log_warning(
" Overriding derived constraint of %.1f MHz on net %s with user-specified constraint of "
"%.1f MHz.\n",
MHz(to->clkconstr->period.min_delay), to->name.c_str(ctx), MHz(period));
return;
}
to->clkconstr = std::unique_ptr<ClockConstraint>(new ClockConstraint());
to->clkconstr->low.min_delay = period / 2;
to->clkconstr->low.max_delay = period / 2;
to->clkconstr->high.min_delay = period / 2;
to->clkconstr->high.max_delay = period / 2;
to->clkconstr->period.min_delay = period;
to->clkconstr->period.max_delay = period;
log_info(" Derived frequency constraint of %.1f MHz for net %s\n", MHz(to->clkconstr->period.min_delay),
to->name.c_str(ctx));
changed_nets.insert(to->name);
};
auto copy_constraint = [&](CellInfo *ci, IdString fromPort, IdString toPort, double ratio = 1.0) {
if (!ci->ports.count(fromPort) || !ci->ports.count(toPort))
return;
NetInfo *from = ci->ports.at(fromPort).net, *to = ci->ports.at(toPort).net;
if (from == nullptr || from->clkconstr == nullptr || to == nullptr)
return;
if (to->clkconstr != nullptr) {
if (!equals_epsilon(to->clkconstr->period.min_delay,
delay_t(from->clkconstr->period.min_delay / ratio)) &&
user_constrained.count(to->name))
log_warning(
" Overriding derived constraint of %.1f MHz on net %s with user-specified constraint of "
"%.1f MHz.\n",
MHz(to->clkconstr->period.min_delay), to->name.c_str(ctx),
MHz(delay_t(from->clkconstr->period.min_delay / ratio)));
return;
}
to->clkconstr = std::unique_ptr<ClockConstraint>(new ClockConstraint());
to->clkconstr->low = ctx->getDelayFromNS(ctx->getDelayNS(from->clkconstr->low.min_delay) / ratio);
to->clkconstr->high = ctx->getDelayFromNS(ctx->getDelayNS(from->clkconstr->high.min_delay) / ratio);
to->clkconstr->period = ctx->getDelayFromNS(ctx->getDelayNS(from->clkconstr->period.min_delay) / ratio);
log_info(" Derived frequency constraint of %.1f MHz for net %s\n", MHz(to->clkconstr->period.min_delay),
to->name.c_str(ctx));
changed_nets.insert(to->name);
};
// Run in a loop while constraints are changing to deal with dependencies
// Iteration limit avoids hanging in crazy loopback situation (self-fed PLLs or dividers, etc)
int iter = 0;
const int itermax = 5000;
while (!changed_nets.empty() && iter < itermax) {
++iter;
std::unordered_set<IdString> changed_cells;
for (auto net : changed_nets) {
for (auto &user : ctx->nets.at(net)->users)
if (user.port == id_CLKI)
changed_cells.insert(user.cell->name);
auto &drv = ctx->nets.at(net)->driver;
if (iter == 1 && drv.cell != nullptr && (drv.port == id_HFCLKOUT || drv.port == id_LFCLKOUT))
changed_cells.insert(drv.cell->name);
}
changed_nets.clear();
for (auto cell : sorted(changed_cells)) {
CellInfo *ci = ctx->cells.at(cell).get();
if (ci->type == id_DCC) {
copy_constraint(ci, id_CLKI, id_CLKO, 1);
} else if (ci->type == id_OSC_CORE) {
int div = int_or_default(ci->params, ctx->id("HF_CLK_DIV"), 128);
set_period(ci, id_HFCLKOUT, delay_t((1.0e6 / 450) * (div + 1)));
set_period(ci, id_LFCLKOUT, delay_t((1.0e3 / 10)));
}
}
}
}
explicit NexusPacker(Context *ctx) : ctx(ctx) {}
void operator()()
@ -1725,6 +1820,7 @@ struct NexusPacker
pack_luts();
promote_globals();
place_globals();
generate_constraints();
}
};