[spi_device] Watermark and Flip handling
This commit revises the watermark logic to handle the corner case
correctly. The case is when `watermark` and `flip` both events occur at
the same cycle.
If both events happen at the same cycle, it means host system issues a
read request to the next buffer above the threshold.
For example, the threshold is configured to 0x83 (131 Byte) and the
current buffer is {1kB : 2kB}. Then when the host system issues read
request to 0x486 (1,158 byte), it triggers `flip` event and `watermark`
events.
Signed-off-by: Eunchan Kim <eunchan@opentitan.org>
diff --git a/hw/ip/spi_device/rtl/spid_readbuffer.sv b/hw/ip/spi_device/rtl/spid_readbuffer.sv
index c912bab..9d86dc1 100644
--- a/hw/ip/spi_device/rtl/spid_readbuffer.sv
+++ b/hw/ip/spi_device/rtl/spid_readbuffer.sv
@@ -135,13 +135,29 @@
if (!sys_rst_ni) begin
watermark_crossed <= 1'b 0;
end else if (active && watermark_cross) begin
+ // When `watermark_cross` and `flip` both are 1, the watermark_crossed
+ // should remain 1. The event will be reported in this case.
watermark_crossed <= 1'b 1;
end else if (active && flip) begin
watermark_crossed <= 1'b 0;
end
end
- assign event_watermark_o = active && !watermark_crossed && watermark_cross ;
+ always_comb begin : watermark_event_logic
+ event_watermark_o = 1'b 0;
+
+ if (active && watermark_cross) begin
+ if (!watermark_crossed) begin
+ event_watermark_o = 1'b 1;
+ end else if (flip) begin
+ // if flip is set and previous watermark_crossed is set, the event
+ // should be reported also. This means the host issues the next buffer
+ // address pointing above the threshold. This scenario, flip event and
+ // watermark event both should be reported.
+ event_watermark_o = 1'b 1;
+ end
+ end
+ end : watermark_event_logic
///////////////////
// State Machine //
