fwb_slave.v

////////////////////////////////////////////////////////////////////////////////
//
// Filename:	fwb_slave.v
//
// Project:	WB2AXIPSP: bus bridges and other odds and ends
//
// Purpose:	This file describes the rules of a wishbone interaction from the
//		perspective of a wishbone slave.  These formal rules may be used
//	with yosys-smtbmc to *prove* that the slave properly handles outgoing
//	responses to (assumed correct) incoming requests.
//
//	This module contains no functional logic.  It is intended for formal
//	verification only.  The outputs returned, the number of requests that
//	have been made, the number of acknowledgements received, and the number
//	of outstanding requests, are designed for further formal verification
//	purposes *only*.
//
//	This file is different from a companion formal_master.v file in that
//	assumptions are made about the inputs to the slave: i_wb_cyc,
//	i_wb_stb, i_wb_we, i_wb_addr, i_wb_data, and i_wb_sel, while full
//	assertions are made about the outputs: o_wb_stall, o_wb_ack, o_wb_data,
//	o_wb_err.  In the formal_master.v, assertions are made about the
//	master outputs (slave inputs)), and assumptions are made about the
//	master inputs (the slave outputs).
//
//	In order to make it easier to compare the slave against the master,
//	assumptions with respect to the slave have been marked with the
//	`SLAVE_ASSUME macro.  Similarly, assertions the slave would make have
//	been marked with `SLAVE_ASSERT.  This allows the master to redefine
//	these two macros to be from his perspective, and therefore the
//	diffs between the two files actually show true differences, rather
//	than just these differences in perspective.
//
//
// Creator:	Dan Gisselquist, Ph.D.
//		Gisselquist Technology, LLC
//
////////////////////////////////////////////////////////////////////////////////
//
// Copyright (C) 2017-2020, Gisselquist Technology, LLC
//
// This file is part of the WB2AXIP project.
//
// The WB2AXIP project contains free software and gateware, licensed under the
// Apache License, Version 2.0 (the "License").  You may not use this project,
// or this file, except in compliance with the License.  You may obtain a copy
// of the License at
//
//	http://www.apache.org/licenses/LICENSE-2.0
//
// Unless required by applicable law or agreed to in writing, software
// distributed under the License is distributed on an "AS IS" BASIS, WITHOUT
// WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.  See the
// License for the specific language governing permissions and limitations
// under the License.
//
////////////////////////////////////////////////////////////////////////////////
//
//
`default_nettype none
//
module	fwb_slave(i_clk, i_reset,
		// The Wishbone bus
		i_wb_cyc, i_wb_stb, i_wb_we, i_wb_addr, i_wb_data, i_wb_sel,
			i_wb_ack, i_wb_stall, i_wb_idata, i_wb_err,
		// Some convenience output parameters
		f_nreqs, f_nacks, f_outstanding);
	parameter		AW=32, DW=32;
	parameter		F_MAX_STALL = 0,
				F_MAX_ACK_DELAY = 0;
	parameter		F_LGDEPTH = 4;
	parameter [(F_LGDEPTH-1):0] F_MAX_REQUESTS = 0;
	//
	// If true, allow the bus to be kept open when there are no outstanding
	// requests.  This is useful for any master that might execute a
	// read modify write cycle, such as an atomic add.
	parameter [0:0]		F_OPT_RMW_BUS_OPTION = 1;
	//
	// 
	// If true, allow the bus to issue multiple discontinuous requests.
	// Unlike F_OPT_RMW_BUS_OPTION, these requests may be issued while other
	// requests are outstanding
	parameter	[0:0]	F_OPT_DISCONTINUOUS = 1;
	//
	//
	// If true, insist that there be a minimum of a single clock delay
	// between request and response.  This defaults to off since the
	// wishbone specification specifically doesn't require this.  However,
	// some interfaces do, so we allow it as an option here.
	parameter	[0:0]	F_OPT_MINCLOCK_DELAY = 0;
	//
	//
	//
	localparam [(F_LGDEPTH-1):0] MAX_OUTSTANDING = {(F_LGDEPTH){1'b1}};
	localparam	MAX_DELAY = (F_MAX_STALL > F_MAX_ACK_DELAY)
				? F_MAX_STALL : F_MAX_ACK_DELAY;
	localparam	DLYBITS= (MAX_DELAY < 4) ? 2
				: ((MAX_DELAY <    16) ? 4
				: ((MAX_DELAY <    64) ? 6
				: ((MAX_DELAY <   256) ? 8
				: ((MAX_DELAY <  1024) ? 10
				: ((MAX_DELAY <  4096) ? 12
				: ((MAX_DELAY < 16384) ? 14
				: ((MAX_DELAY < 65536) ? 16
				: 32)))))));
	//
	input	wire			i_clk, i_reset;
	// Input/master bus
	input	wire			i_wb_cyc, i_wb_stb, i_wb_we;
	input	wire	[(AW-1):0]	i_wb_addr;
	input	wire	[(DW-1):0]	i_wb_data;
	input	wire	[(DW/8-1):0]	i_wb_sel;
	//
	input	wire			i_wb_ack;
	input	wire			i_wb_stall;
	input	wire	[(DW-1):0]	i_wb_idata;
	input	wire			i_wb_err;
	//
	output	reg	[(F_LGDEPTH-1):0]	f_nreqs, f_nacks;
	output	wire	[(F_LGDEPTH-1):0]	f_outstanding;

`define	SLAVE_ASSUME	assume
`define	SLAVE_ASSERT	assert
	//
	// Let's just make sure our parameters are set up right
	//
	initial	assert(F_MAX_REQUESTS < {(F_LGDEPTH){1'b1}});

	//
	// Wrap the request line in a bundle.  The top bit, named STB_BIT,
	// is the bit indicating whether the request described by this vector
	// is a valid request or not.
	//
	localparam	STB_BIT = 2+AW+DW+DW/8-1;
	wire	[STB_BIT:0]	f_request;
	assign	f_request = { i_wb_stb, i_wb_we, i_wb_addr, i_wb_data, i_wb_sel };

	//
	// A quick register to be used later to know if the $past() operator
	// will yield valid result
	reg	f_past_valid;
	initial	f_past_valid = 1'b0;
	always @(posedge i_clk)
		f_past_valid <= 1'b1;
	always @(*)
	if (!f_past_valid)
		`SLAVE_ASSUME(i_reset);
	//
	//
	// Assertions regarding the initial (and reset) state
	//
	//

	//
	// Assume we start from a reset condition
	initial assert(i_reset);
	initial `SLAVE_ASSUME(!i_wb_cyc);
	initial `SLAVE_ASSUME(!i_wb_stb);
	//
	initial	`SLAVE_ASSERT(!i_wb_ack);
	initial	`SLAVE_ASSERT(!i_wb_err);

`ifdef	VERIFIC
	always @(*)
	if (!f_past_valid)
	begin
		`SLAVE_ASSUME(!i_wb_cyc);
		`SLAVE_ASSUME(!i_wb_stb);
		//
		`SLAVE_ASSERT(!i_wb_ack);
		`SLAVE_ASSERT(!i_wb_err);
	end
`endif
	always @(posedge i_clk)
	if ((!f_past_valid)||($past(i_reset)))
	begin
		`SLAVE_ASSUME(!i_wb_cyc);
		`SLAVE_ASSUME(!i_wb_stb);
		//
		`SLAVE_ASSERT(!i_wb_ack);
		`SLAVE_ASSERT(!i_wb_err);
	end

	always @(*)
	if (!f_past_valid)
		`SLAVE_ASSUME(!i_wb_cyc);

	//
	//
	// Bus requests
	//
	//

	// Following any bus error, the CYC line should be dropped to abort
	// the transaction
	always @(posedge i_clk)
	if ((f_past_valid)&&($past(i_wb_err))&&($past(i_wb_cyc)))
		`SLAVE_ASSUME(!i_wb_cyc);

	// STB can only be true if CYC is also true
	always @(*)
	if (i_wb_stb)
		`SLAVE_ASSUME(i_wb_cyc);

	// If a request was both outstanding and stalled on the last clock,
	// then nothing should change on this clock regarding it.
	always @(posedge i_clk)
	if ((f_past_valid)&&(!$past(i_reset))&&($past(i_wb_stb))
			&&($past(i_wb_stall))&&(i_wb_cyc))
	begin
		`SLAVE_ASSUME(i_wb_stb);
		`SLAVE_ASSUME(i_wb_we   == $past(i_wb_we));
		`SLAVE_ASSUME(i_wb_addr == $past(i_wb_addr));
		`SLAVE_ASSUME(i_wb_sel  == $past(i_wb_sel));
		if (i_wb_we)
			`SLAVE_ASSUME(i_wb_data == $past(i_wb_data));
	end

	// Within any series of STB/requests, the direction of the request
	// may not change.
	always @(posedge i_clk)
	if ((f_past_valid)&&($past(i_wb_stb))&&(i_wb_stb))
		`SLAVE_ASSUME(i_wb_we == $past(i_wb_we));


	// Within any given bus cycle, the direction may *only* change when
	// there are no further outstanding requests.
	always @(posedge i_clk)
	if ((f_past_valid)&&(f_outstanding > 0))
		`SLAVE_ASSUME(i_wb_we == $past(i_wb_we));

	// Write requests must also set one (or more) of i_wb_sel
	//
	// This test has been removed since down-sizers (taking bus from width
	// DW to width dw < DW) might actually create empty requests that this
	// would prevent.  Re-enabling it would also complicate AXI to WB
	// transfers, since AXI explicitly allows WSTRB == 0.  Finally, this
	// criteria isn't found in the WB spec--so while it might be a good
	// idea to check, in hind sight there are too many exceptions to be
	// dogmatic about it.
	//
	// always @(*)
	// if ((i_wb_stb)&&(i_wb_we))
	//	`SLAVE_ASSUME(|i_wb_sel);


	//
	//
	// Bus responses
	//
	//

	// If CYC was low on the last clock, then both ACK and ERR should be
	// low on this clock.
	always @(posedge i_clk)
	if ((f_past_valid)&&(!$past(i_wb_cyc))&&(!i_wb_cyc))
	begin
		`SLAVE_ASSERT(!i_wb_ack);
		`SLAVE_ASSERT(!i_wb_err);
		// Stall may still be true--such as when we are not
		// selected at some arbiter between us and the slave
	end

	//
	// Any time the CYC line drops, it is possible that there may be a
	// remaining (registered) ACK or ERR that hasn't yet been returned.
	// Restrict such out of band returns so that they are *only* returned
	// if there is an outstanding operation.
	//
	// Update: As per spec, WB-classic to WB-pipeline conversions require
	// that the ACK|ERR might come back on the same cycle that STB
	// is low, yet also be registered.  Hence, if STB & STALL are true on
	// one cycle, then CYC is dropped, ACK|ERR might still be true on the
	// cycle when CYC is dropped
	always @(posedge i_clk)
	if ((f_past_valid)&&(!$past(i_reset))&&($past(i_wb_cyc))&&(!i_wb_cyc))
	begin
		// Note that, unlike f_outstanding, f_nreqs and f_nacks are both
		// registered.  Hence, we can check here if a response is still
		// pending.  If not, no response should be returned.
		if (f_nreqs == f_nacks)
		begin
			`SLAVE_ASSERT(!i_wb_ack);
			`SLAVE_ASSERT(!i_wb_err);
		end
	end

	// ACK and ERR may never both be true at the same time
	always @(*)
		`SLAVE_ASSERT((!i_wb_ack)||(!i_wb_err));

	generate if (F_MAX_STALL > 0)
	begin : MXSTALL
		//
		// Assume the slave cannnot stall for more than F_MAX_STALL
		// counts.  We'll count this forward any time STB and STALL
		// are both true.
		//
		reg	[(DLYBITS-1):0]		f_stall_count;

		initial	f_stall_count = 0;
		always @(posedge i_clk)
		if ((!i_reset)&&(i_wb_stb)&&(i_wb_stall))
			f_stall_count <= f_stall_count + 1'b1;
		else
			f_stall_count <= 0;

		always @(*)
		if (i_wb_cyc)
			`SLAVE_ASSERT(f_stall_count < F_MAX_STALL);
	end endgenerate

	generate if (F_MAX_ACK_DELAY > 0)
	begin : MXWAIT
		//
		// Assume the slave will respond within F_MAX_ACK_DELAY cycles,
		// counted either from the end of the last request, or from the
		// last ACK received
		//
		reg	[(DLYBITS-1):0]		f_ackwait_count;

		initial	f_ackwait_count = 0;
		always @(posedge i_clk)
		if ((!i_reset)&&(i_wb_cyc)&&(!i_wb_stb)
				&&(!i_wb_ack)&&(!i_wb_err)
				&&(f_outstanding > 0))
			f_ackwait_count <= f_ackwait_count + 1'b1;
		else
			f_ackwait_count <= 0;

		always @(*)
		if ((!i_reset)&&(i_wb_cyc)&&(!i_wb_stb)
					&&(!i_wb_ack)&&(!i_wb_err)
					&&(f_outstanding > 0))
			`SLAVE_ASSERT(f_ackwait_count < F_MAX_ACK_DELAY);
	end endgenerate

	//
	// Count the number of requests that have been received
	//
	initial	f_nreqs = 0;
	always @(posedge i_clk)
	if ((i_reset)||(!i_wb_cyc))
		f_nreqs <= 0;
	else if ((i_wb_stb)&&(!i_wb_stall))
		f_nreqs <= f_nreqs + 1'b1;


	//
	// Count the number of acknowledgements that have been returned
	//
	initial	f_nacks = 0;
	always @(posedge i_clk)
	if (i_reset)
		f_nacks <= 0;
	else if (!i_wb_cyc)
		f_nacks <= 0;
	else if ((i_wb_ack)||(i_wb_err))
		f_nacks <= f_nacks + 1'b1;

	//
	// The number of outstanding requests is the difference between
	// the number of requests and the number of acknowledgements
	//
	assign	f_outstanding = (i_wb_cyc) ? (f_nreqs - f_nacks):0;

	always @(*)
	if ((i_wb_cyc)&&(F_MAX_REQUESTS > 0))
	begin
		if (i_wb_stb)
			`SLAVE_ASSUME(f_nreqs < F_MAX_REQUESTS);
		else
			`SLAVE_ASSUME(f_nreqs <= F_MAX_REQUESTS);
		`SLAVE_ASSERT(f_nacks <= f_nreqs);
		assert(f_outstanding < (1<<F_LGDEPTH)-1);
	end else
		assume(f_outstanding < (1<<F_LGDEPTH)-1);

	always @(*)
	if ((i_wb_cyc)&&(f_outstanding == 0))
	begin
		// If nothing is outstanding, then there should be
		// no acknowledgements ... however, an acknowledgement
		// *can* come back on the same clock as the stb is
		// going out.
		if (F_OPT_MINCLOCK_DELAY)
		begin
			`SLAVE_ASSERT(!i_wb_ack);
			`SLAVE_ASSERT(!i_wb_err);
		end else begin
			`SLAVE_ASSERT((!i_wb_ack)||((i_wb_stb)&&(!i_wb_stall)));
			// The same is true of errors.  They may not be
			// created before the request gets through
			`SLAVE_ASSERT((!i_wb_err)||((i_wb_stb)&&(!i_wb_stall)));
		end
	end else if (!i_wb_cyc && f_nacks == f_nreqs)
	begin
		`SLAVE_ASSERT(!i_wb_ack);
		`SLAVE_ASSERT(!i_wb_err);
	end

	generate if (!F_OPT_RMW_BUS_OPTION)
	begin
		// If we aren't waiting for anything, and we aren't issuing
		// any requests, then then our transaction is over and we
		// should be dropping the CYC line.
		always @(*)
		if (f_outstanding == 0)
			`SLAVE_ASSUME((i_wb_stb)||(!i_wb_cyc));
		// Not all masters will abide by this restriction.  Some
		// masters may wish to implement read-modify-write bus
		// interactions.  These masters need to keep CYC high between
		// transactions, even though nothing is outstanding.  For
		// these busses, turn F_OPT_RMW_BUS_OPTION on.
	end endgenerate

	generate if ((!F_OPT_DISCONTINUOUS)&&(!F_OPT_RMW_BUS_OPTION))
	begin : INSIST_ON_NO_DISCONTINUOUS_STBS
		// Within my own code, once a request begins it goes to
		// completion and the CYC line is dropped.  The master
		// is not allowed to raise STB again after dropping it.
		// Doing so would be a *discontinuous* request.
		//
		// However, in any RMW scheme, discontinuous requests are
		// necessary, and the spec doesn't disallow them.  Hence we
		// make this check optional.
		always @(posedge i_clk)
		if ((f_past_valid)&&($past(i_wb_cyc))&&(!$past(i_wb_stb)))
			`SLAVE_ASSUME(!i_wb_stb);
	end endgenerate

endmodule
`undef	SLAVE_ASSUME
`undef	SLAVE_ASSERT