Expose IC_SIZE_BYTES as a module parameter in ibex_top

[jamesrbailey]

I would like to expose CacheSizeBytes as a module parameter in ibex_top so that it can be set at the instantiation of Ibex. Currently it is defined in ibex_pkg.sv and modifying it requires updating the package - this also makes it a bit tricky to have multiple instances of Ibex with different cache sizes since they share the same package.

I understand that the default cache size has been the focus of testing to date, so exposing the parameter does allow users to pick potentially undertested settings. For what it's worth I have extensive experience using Ibex on a 1KiB cache size.

Looking forward to hearing people's feedback on this.

[marnovandermaas]

This is an interesting question, it needs a bit of rearchitecting in the Ibex package because there are multiple values that are derived from the size of the instruction cache. My main concern is as you say that our DV has mainly been on the 4 KiB instruction cache. Have you replicated any of our instruction cache DV results for your 1 KiB configuration.

The other question is whether this can lead to many more parameters being requested, for example number of ways and line size. We would probably have to have some kind of instruction cache configuration with multiple parameters. My current feeling is that it is not worth the maintenance burden.

[marnovandermaas]

For future reference here are the parameters in Ibex package:

ibex/rtl/ibex_pkg.sv

Lines 380 to 401 in 9742d89

	// ICache constants
	parameter int unsigned ADDR_W = 32;
	parameter int unsigned BUS_SIZE = 32;
	parameter int unsigned BUS_BYTES = BUS_SIZE/8;
	parameter int unsigned BUS_W = $clog2(BUS_BYTES);
	parameter int unsigned IC_SIZE_BYTES = 4096;
	parameter int unsigned IC_NUM_WAYS = 2;
	parameter int unsigned IC_LINE_SIZE = 64;
	parameter int unsigned IC_LINE_BYTES = IC_LINE_SIZE/8;
	parameter int unsigned IC_LINE_W = $clog2(IC_LINE_BYTES);
	parameter int unsigned IC_NUM_LINES = IC_SIZE_BYTES / IC_NUM_WAYS / IC_LINE_BYTES;
	parameter int unsigned IC_LINE_BEATS = IC_LINE_BYTES / BUS_BYTES;
	parameter int unsigned IC_LINE_BEATS_W = $clog2(IC_LINE_BEATS);
	parameter int unsigned IC_INDEX_W = $clog2(IC_NUM_LINES);
	parameter int unsigned IC_INDEX_HI = IC_INDEX_W + IC_LINE_W - 1;
	parameter int unsigned IC_TAG_SIZE = ADDR_W - IC_INDEX_W - IC_LINE_W + 1; // 1 valid bit
	parameter int unsigned IC_OUTPUT_BEATS = (BUS_BYTES / 2); // number of halfwords
	parameter int unsigned IC_DATA_ECC_SIZE = 7;
	parameter int unsigned IC_TAG_ECC_SIZE = 6;
	// ICache Scrambling Parameters
	parameter int unsigned SCRAMBLE_KEY_W = 128;
	parameter int unsigned SCRAMBLE_NONCE_W = 64;