scsi.v

/* verilator lint_off UNUSED */

// scsi.v
// implements a target only scsi device
  
module scsi
(
	input      clk,

	// scsi interface
	input 	  rst, // bus reset from initiator
	input 	  sel,
	input 	  atn, // initiator requests to send a message
	output 	  bsy, // target holds bus

	output 	  msg,
	output 	  cd,
	output 	  io,

	output 	  req,
	input 	  ack, // initiator acknowledges a request

	input   [7:0] din, // data from initiator to target
	output  [7:0] dout, // data from target to initiator

	// interface to io controller
	input         img_mounted,
	input  [31:0] img_blocks,
	output [31:0] io_lba,
	output reg 	  io_rd,
	output reg 	  io_wr,
	input         io_ack,

	input      [8:0] sd_buff_addr,
	input      [7:0] sd_buff_dout,
	output reg [7:0] sd_buff_din,
	input            sd_buff_wr
);

// SCSI device id
parameter [2:0] ID = 0;

localparam PHASE_IDLE        = 3'd0;
localparam PHASE_CMD_IN      = 3'd1;
localparam PHASE_DATA_OUT    = 3'd2;
localparam PHASE_DATA_IN     = 3'd3;
localparam PHASE_STATUS_OUT  = 3'd4;
localparam PHASE_MESSAGE_OUT = 3'd5;
reg [2:0]  phase;

// ------------ sector buffer IO controller read/write -----------------------
// the buffer itself. Can hold two sectors
reg sd_buff_sel;
reg [7:0] buffer [1024];
always @(posedge clk) begin
	sd_buff_din <= buffer[{sd_buff_sel, sd_buff_addr}];
	if(sd_buff_wr) buffer[{sd_buff_sel, sd_buff_addr}] <= sd_buff_dout;
end

reg old_io_ack;
always @(posedge clk) begin
	old_io_ack <= io_ack;
	if (phase == PHASE_IDLE)
		sd_buff_sel <= 0;
	else
		if (old_io_ack & ~io_ack) sd_buff_sel <= !sd_buff_sel;
end

// -----------------------------------------------------------

// status replies
reg [7:0]  status;
`define STATUS_OK 8'h00
`define STATUS_CHECK_CONDITION 8'h02

// message codes
`define MSG_CMD_COMPLETE 8'h00
	
// drive scsi signals according to phase
assign msg = (phase == PHASE_MESSAGE_OUT);
assign cd = (phase == PHASE_CMD_IN) || (phase == PHASE_STATUS_OUT) || (phase == PHASE_MESSAGE_OUT);
assign io = (phase == PHASE_DATA_OUT) || (phase == PHASE_STATUS_OUT) || (phase == PHASE_MESSAGE_OUT);

wire   io_busy = (phase == PHASE_DATA_OUT && (io_rd | io_ack) && data_cnt[9] == sd_buff_sel) ||
                 (phase == PHASE_DATA_IN  && (io_wr | io_ack) && data_cnt[9] == sd_buff_sel) ||
                 (phase != PHASE_DATA_OUT && phase != PHASE_DATA_IN && (io_rd | io_wr | io_ack));
assign req = (phase != PHASE_IDLE) && !ack && !io_busy;

assign bsy = (phase != PHASE_IDLE);

assign dout = (phase == PHASE_STATUS_OUT)?status:
	 (phase == PHASE_MESSAGE_OUT)?`MSG_CMD_COMPLETE:
	 (phase == PHASE_DATA_OUT)?cmd_dout:
	 8'h00;

// de-multiplex different data sources
wire [7:0] cmd_dout =
		cmd_read?buffer_dout:
		cmd_inquiry?inquiry_dout:
		cmd_read_capacity?read_capacity_dout:
		cmd_mode_sense?mode_sense_dout:
		8'h00;

// output of inquiry command, identify as "SEAGATE ST225N"
wire [7:0] inquiry_dout =
		(data_cnt == 32'd4 )?8'd32:  // length

		(data_cnt == 32'd8 )?" ":(data_cnt == 32'd9 )?"S":
		(data_cnt == 32'd10)?"E":(data_cnt == 32'd11)?"A":
		(data_cnt == 32'd12)?"G":(data_cnt == 32'd13)?"A":
		(data_cnt == 32'd14)?"T":(data_cnt == 32'd15)?"E":
		(data_cnt == 32'd16)?" ":(data_cnt == 32'd17)?" ":
		(data_cnt == 32'd18)?" ":(data_cnt == 32'd19)?" ":
		(data_cnt == 32'd20)?" ":(data_cnt == 32'd21)?" ":
		(data_cnt == 32'd22)?" ":(data_cnt == 32'd23)?" ":
		(data_cnt == 32'd24)?" ":(data_cnt == 32'd25)?" ":

		(data_cnt == 32'd26)?"S":(data_cnt == 32'd27)?"T":
		(data_cnt == 32'd28)?"2":(data_cnt == 32'd29)?"2":
		(data_cnt == 32'd30)?"5":(data_cnt == 32'd31)?"N" + {5'd0, ID}: // TESTING. ElectronAsh.
		8'h00;

// output of read capacity command
//wire [31:0] capacity = 32'd41056;   // 40960 + 96 blocks = 20MB
//wire [31:0] capacity = 32'd1024096;   // 1024000 + 96 blocks = 500MB
reg [31:0] capacity;
reg        mounted = 0;
always @(posedge clk) begin
	if (img_mounted) begin
		if (|img_blocks) begin
			capacity <= img_blocks;
			$display("Image mounted on target %d, size: %d", ID, img_blocks);
			mounted <= 1;
		end else
			mounted <= 0;
	end
end

wire [7:0] read_capacity_dout =
		(data_cnt == 32'd0 )?capacity[31:24]:
		(data_cnt == 32'd1 )?capacity[23:16]:
		(data_cnt == 32'd2 )?capacity[15:8]:
		(data_cnt == 32'd3 )?capacity[7:0]:
		(data_cnt == 32'd6 )?8'd2:             // 512 bytes per sector
		8'h00;

wire [7:0] mode_sense_dout =
		(data_cnt == 32'd3 )?8'd8:
		(data_cnt == 32'd5 )?capacity[23:16]:
		(data_cnt == 32'd6 )?capacity[15:8]:
		(data_cnt == 32'd7 )?capacity[7:0]:
		(data_cnt == 32'd10 )?8'd2:
		8'h00;

// clock data out of buffer to allow for embedded ram
reg [7:0] buffer_dout;
always @(posedge clk) buffer_dout <= buffer[data_cnt];

// buffer to store incoming commands
reg [3:0]  cmd_cnt;
reg [7:0]  cmd [9:0];

/* ----------------------- request data from/to io controller ----------------------- */

assign io_lba = lba;

// generate an io_rd signal whenever the first byte of a 512 byte block is required
// start fetching the next sector when the 20th byte is read, and it's not the last sector
wire req_rd = ((phase == PHASE_DATA_OUT) && cmd_read && (data_cnt == 0 || (data_cnt[8:0] == 9'd20 && data_cnt[31:9] != ({7'd0, tlen} - 1'd1))) && !data_complete);

// generate an io_wr signal whenever a 512 byte block has been received or when the status
// phase of a write command has been reached
wire req_wr = ((((phase == PHASE_DATA_IN) && (data_cnt[8:0] == 0) && (data_cnt != 0)) || (phase == PHASE_STATUS_OUT)) && cmd_write);

always @(posedge clk) begin
	reg old_rd, old_wr;
	reg wr_pending, rd_pending;

	old_rd <= req_rd;
	old_wr <= req_wr;
	if(~old_rd & req_rd) rd_pending <= 1;
	if(~old_wr & req_wr) wr_pending <= 1;

	if(io_ack) begin
		io_rd <= 1'b0;
		io_wr <= 1'b0;
	end else begin
		if (rd_pending && !io_rd) begin
			io_rd <= 1;
			rd_pending <= 0;
		end

		if (wr_pending && !io_wr) begin
			io_wr <= 1;
			wr_pending <= 0;
		end
	end
end

reg  stb_ack;
reg  stb_adv;
always @(posedge clk) begin
	reg old_ack;
	
	old_ack <= ack;
	stb_ack <= (~old_ack & ack); // on rising edge
	stb_adv <= (old_ack & ~ack); // on falling edge
end

// store data on rising edge of ack, ...
always @(posedge clk) begin
	if(stb_ack) begin
		if(phase == PHASE_CMD_IN)  cmd[cmd_cnt] <= din;
		if(phase == PHASE_DATA_IN) buffer[data_cnt] <= din;
	end
end

// ... advance counter on falling edge
always @(posedge clk) begin
	if(phase == PHASE_IDLE) cmd_cnt <= 4'd0;
	else if(stb_adv && (phase == PHASE_CMD_IN) && (cmd_cnt != 15)) cmd_cnt <= cmd_cnt + 4'd1;
end

// count data bytes. don't increase counter while we are waiting for data from
// the io controller
reg [31:0] data_cnt;
reg        data_complete;

// For block transfers tlen contains the number of 512 bytes blocks to transfer.
// Most other commands have the bytes length stored in the transfer length field.
// And some have a fixed length idependent from any header field.
// The data transfer has finished once the data counter reaches this
// number.
wire [31:0] data_len =
		 cmd_read_capacity?32'd8:
		 cmd_read?{ 7'd0, tlen, 9'd0 }:   // read command length is in 512 bytes blocks
		 cmd_write?{ 7'd0, tlen, 9'd0 }:  // write command length is in 512 bytes blocks
		 { 16'd0, tlen };                 // inquiry etc have length in bytes

always @(posedge clk) begin
	if((phase != PHASE_DATA_OUT) && (phase != PHASE_DATA_IN) && (phase != PHASE_STATUS_OUT) && (phase != PHASE_MESSAGE_OUT)) begin
		data_cnt <= 0;
		data_complete <= 0;
	end else begin	
		if(stb_adv)begin	
			if(!data_complete) data_cnt <= data_cnt + 1'd1;
			data_complete <= (data_len - 1'd1) == data_cnt;
		end
	end
end

// check whether status byte has been sent
reg status_sent;
always @(posedge clk) begin
	if(phase != PHASE_STATUS_OUT) status_sent <= 0;
	else if(stb_adv) status_sent <= 1;
end

// check whether message byte has been sent
reg message_sent;
always @(posedge clk) begin
	if(phase != PHASE_MESSAGE_OUT) message_sent <= 0;
	else if(stb_adv) message_sent <= 1;
end

/* ----------------------- command decoding ------------------------------- */


// parse commands
wire [7:0] op_code = cmd[0];
wire [2:0] cmd_group = op_code[7:5];

// check if a complete command has been received
wire       cmd_cpl = cmd6_cpl || cmd10_cpl;
wire       cmd6_cpl = (cmd_group == 3'b000) && (cmd_cnt == 6);
wire       cmd10_cpl = ((cmd_group == 3'b010) || (cmd_group == 3'b001)) && (cmd_cnt == 10);

// https://en.wikipedia.org/wiki/SCSI_command
wire       cmd_read = cmd_read6 || cmd_read10;
wire       cmd_read6 = (op_code == 8'h08);
wire       cmd_read10 = (op_code == 8'h28);
wire       cmd_write = cmd_write6 || cmd_write10;
wire       cmd_write6 = (op_code == 8'h0a);
wire       cmd_write10 = (op_code == 8'h2a);
wire       cmd_inquiry = (op_code == 8'h12);
wire       cmd_format = (op_code == 8'h04);
wire       cmd_mode_select = (op_code == 8'h15);
wire       cmd_mode_sense = (op_code == 8'h1a);
wire       cmd_test_unit_ready = (op_code == 8'h00);
wire       cmd_read_capacity = (op_code == 8'h25);
wire       cmd_read_buffer = (op_code == 8'h3b);  // fake
wire       cmd_write_buffer = (op_code == 8'h3c); // fake
wire       cmd_verify6 = (op_code == 8'h13); // fake
wire       cmd_verify10 = (op_code == 8'h2f); // fake

// valid command in buffer? TODO: check for valid command parameters
wire  cmd_ok = cmd_read || cmd_write || cmd_inquiry || cmd_test_unit_ready || 
		  cmd_read_capacity || cmd_mode_select || cmd_format || cmd_mode_sense ||
		  cmd_read_buffer || cmd_write_buffer || cmd_verify6 || cmd_verify10;

// latch parameters once command is complete
reg [31:0] lba;
reg [15:0] tlen;

always @(posedge clk) begin
	if (old_io_ack & ~io_ack) lba <= lba + 1'd1;
	if(cmd_cpl && (phase == PHASE_CMD_IN)) begin
		lba <= cmd6_cpl?{11'd0, lba6}:lba10;
		tlen <= cmd6_cpl?{7'd0, tlen6}:tlen10;
	end
end
   
// logical block address
wire [7:0] cmd1 = cmd[1];
wire [20:0] lba6 = { cmd1[4:0], cmd[2], cmd[3] };
wire [31:0] lba10 = { cmd[2], cmd[3], cmd[4], cmd[5] };

// transfer length
wire [8:0]  tlen6 = (cmd[4] == 0)?9'd256:{1'b0,cmd[4]};
wire [15:0] tlen10 = { cmd[7], cmd[8] };


// the 5380 changes phase in the falling edge, thus we monitor it
// on the rising edge
always @(posedge clk) begin
	if(rst) begin
		phase <= PHASE_IDLE;
	end else begin
		if(phase == PHASE_IDLE) begin
			if(sel && din[ID] && mounted)  // own id on bus during selection?
				phase <= PHASE_CMD_IN;
		end

		else if(phase == PHASE_CMD_IN) begin
			// check if a full command is in the buffer
			if(cmd_cpl) begin
				$display("New command on target %d: %02x %02x %02x %02x %02x %02x %02x %02x %02x %02x", ID, cmd[0], cmd[1], cmd[2], cmd[3], cmd[4], cmd[5], cmd[6], cmd[7], cmd[8], cmd[9]);
				// is this a supported and valid command?
				if(cmd_ok) begin
					// yes, continue
					status <= `STATUS_OK;

					// continue according to command

					// these commands return data
					if(cmd_read || cmd_inquiry || cmd_read_capacity || cmd_mode_sense || cmd_read_buffer) phase <= PHASE_DATA_OUT;
					// these commands receive dataa
					else if(cmd_write || cmd_mode_select || cmd_write_buffer) phase <= PHASE_DATA_IN;
					// and all other valid commands are just "ok"
					else phase <= PHASE_STATUS_OUT;
				end else begin
					// no, report failure
					status <= `STATUS_CHECK_CONDITION;
					phase <= PHASE_STATUS_OUT;
				end
			end
		end

		else if(phase == PHASE_DATA_OUT) begin
			if(data_complete) phase <= PHASE_STATUS_OUT;
		end

		else if(phase == PHASE_DATA_IN) begin
			if(data_complete) phase <= PHASE_STATUS_OUT;
		end

		else if(phase == PHASE_STATUS_OUT) begin
			if(status_sent) phase <= PHASE_MESSAGE_OUT;
		end

		else if(phase == PHASE_MESSAGE_OUT) begin
			if(message_sent) phase <= PHASE_IDLE;
		end
		
		else
			phase <= PHASE_IDLE;  // should never happen
	end
end
   
   
endmodule