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Add some FIXME's around VCC assumption in LUT logic.

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Signed-off-by: Keith Rothman <537074+litghost@users.noreply.github.com>
This commit is contained in:
Keith Rothman 2021-03-25 13:36:11 -07:00
parent bbe1881293
commit cc4f2b4516
1 changed files with 17 additions and 0 deletions
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17
fpga_interchange/luts.cc
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@ -45,6 +45,12 @@ bool rotate_and_merge_lut_equation(std::vector<LogicLevel> *result, const LutBel
if ((bel_address & (1 << bel_pin_idx)) == 0) { if ((bel_address & (1 << bel_pin_idx)) == 0) {
// This pin is unused, so the line will be tied high, this // This pin is unused, so the line will be tied high, this
// address is unreachable. // address is unreachable.
//
// FIXME: The assumption is that unused pins are tied VCC.
// This is not generally true.
//
// Use Arch::prefered_constant_net_type to determine what
// constant net should be used for unused pins.
if ((used_pins & (1 << bel_pin_idx)) == 0) { if ((used_pins & (1 << bel_pin_idx)) == 0) {
address_reachable = false; address_reachable = false;
break; break;
@ -134,6 +140,11 @@ uint32_t LutMapper::check_wires(const std::vector<std::vector<int32_t>> &bel_to_
} }
} }
// FIXME: The assumption is that unused pins are tied VCC.
// This is not generally true.
//
// Use Arch::prefered_constant_net_type to determine what
// constant net should be used for unused pins.
uint32_t vcc_mask = 0; uint32_t vcc_mask = 0;
DynamicBitarray<> wire_equation; DynamicBitarray<> wire_equation;
@ -351,6 +362,12 @@ bool LutMapper::remap_luts(const Context *ctx)
} else { } else {
// Look to see if wires can be run from element inputs to unused // Look to see if wires can be run from element inputs to unused
// outputs. If not, block the BEL pin by tying to VCC. // outputs. If not, block the BEL pin by tying to VCC.
//
// FIXME: The assumption is that unused pins are tied VCC.
// This is not generally true.
//
// Use Arch::prefered_constant_net_type to determine what
// constant net should be used for unused pins.
uint32_t vcc_pins = check_wires(bel_to_cell_pin_remaps, lut_bels, used_pins); uint32_t vcc_pins = check_wires(bel_to_cell_pin_remaps, lut_bels, used_pins);
#if defined(DEBUG_LUT_ROTATION) #if defined(DEBUG_LUT_ROTATION)
log_info("vcc_pins = 0x%x", vcc_pins); log_info("vcc_pins = 0x%x", vcc_pins);