Truncation of usage_o port in fifo_v3 when parameter DEPTH is a power of 2

[WRoenninger]

The usage_o port of fifo_v3 has currently a width of $clog(DEPTH).
This causes the pointer to wrap around to all zeros, if the FIFO has a power of 2 DEPTH and is full. This is an issue for the following exapmle:

localparam int unsigned Depth     = 32'd16;
localparam logic [3:0]  Threshold = 4'hF;
logic [3:0] usage;
if (usage > threshold) begin
  /* do something */
end

This if statement would not trigger in this case.
This could be resolved by assigning the whole status_cnt_q to the usage_o port instead of the truncated one as is currently the case.

The workaround I am using currently is prepending the full_o signal to the ussage_o, however this causes jumps in this new usage pointer value when the FIFO is not configured to a power of 2 DEPTH.

[zarubaf]

Good catch and thanks for reporting. We should probably fix this in a fifo_v4 or a breaking change (major version bump) in the common cells repo (which is unfortunately anyway required near term).

[Juan-Gg]

Came across this today. Was thinking about the same workaround.