Printf Synthesis: Capturing RTL printf Calls when Running on the FPGA¶
Golden Gate can synthesize printfs present in Chisel/FIRRTL (implemented as
printf statements) that would otherwise be lost in the FPGA synthesis flow.
Rocket and BOOM have printfs of their commit logs and other useful transaction
C0: 409  pc=[008000004c] W[r10=0000000000000000] R[r 0=0000000000000000] R[r20=0000000000000003] inst=[f1402573] csrr a0, mhartid C0: 410  pc=[008000004c] W[r 0=0000000000000000] R[r 0=0000000000000000] R[r20=0000000000000003] inst=[f1402573] csrr a0, mhartid C0: 411  pc=[008000004c] W[r 0=0000000000000000] R[r 0=0000000000000000] R[r20=0000000000000003] inst=[f1402573] csrr a0, mhartid C0: 412  pc= W[r 0=0000000000000000] R[r10=0000000000000000] R[r 0=0000000000000000] inst= bnez a0, pc + 0 C0: 413  pc= W[r 5=0000000080000054] R[r 0=0000000000000000] R[r 0=0000000000000000] inst= auipc t0, 0x0 C0: 414  pc= W[r 5=0000000080000064] R[r 5=0000000080000054] R[r16=0000000000000003] inst= addi t0, t0, 16 C0: 415  pc=[008000005c] W[r 0=0000000000010000] R[r 5=0000000080000064] R[r 5=0000000080000064] inst= csrw mtvec, t0
Synthesizing these printfs lets you capture the same logs on a running FireSim instance.
Enabling Printf Synthesis¶
To synthesize a printf, you need to annotate the specific printfs you’d like to capture in your Chisel source code. Presently, due to a limitation in Chisel and FIRRTL’s annotation system, you need to annotate the arguments to the printf, not the printf itself, like so:
printf(midas.targetutils.SynthesizePrintf("x%d p%d 0x%x\n", rf_waddr, rf_waddr, rf_wdata))
Be judicious, as synthesizing many, frequently active printfs will slow down your simulator.
Once your printfs have been annotated, enable printf synthesis by prepending
WithPrintfSynthesis configuration mixin to your
For example, if your previous
PLATFORM_CONFIG=BaseF1Config_F120MHz, then change it to
PLATFORM_CONFIG=WithPrintfSynthesis_BaseF1Config_F120MHz. Notice that you
must prepend the mixin (rather than appending). During compilation, Golden
Gate will print the number of printfs it has synthesized. In the target’s
generated header (
FireSim-generated.const.h), you’ll find metadata for each of the
printfs Golden Gate synthesized. This is passed as argument to the constructor
synthesized_prints_t bridge driver, which will be automatically
instantiated in FireSim driver.
- Specifies the file name prefix. Generated files will be of the form <print-file><N>, with one output file generated per clock domain. The associated clock domain’s name and frequency relative to the base clock is included in the header of the output file.
- Specifies the target-cycle in cycles of the base clock at which the printf trace should be captured in the simulator. Since capturing high-bandwidth printf traces will slow down simulation, this allows the user to reach the region-of-interest at full simulation speed.
- Specifies the target-cycle in cycles of the base clock at which to stop pulling the synthesized print trace from the simulator.
- By default, a captured printf trace will be written to file formatted as it would be emitted by a software RTL simulator. Setting this dumps the raw binary coming off the FPGA instead, improving simulation rate.
- (Formatted output only) This removes the cycle prefix from each printf to save bandwidth in cases where the printf already includes a cycle field. In binary-output mode, since the target cycle is implicit in the token stream, this flag has no effect.
You can set some of these options by changing the fields in the “synthprint” section of your config_runtime.ini.
[synthprint] start=0 end=-1 cycleprefix=yes
The “start” field corresponds to “print-start”, “end” to “print-end”, and “cycleprefix” to “print-no-cycle-prefix”.