From cbd034d0b4abce8a713b890fbdbde386e30baeaf Mon Sep 17 00:00:00 2001
From: Eric Kooistra <kooistra@astron.nl>
Date: Thu, 14 Mar 2024 09:11:44 +0100
Subject: [PATCH] Add circular buffer size example with
g_bsn_latency_first_node in description.
---
.../base/dp/src/vhdl/dp_bsn_align_v2.vhd | 36 +++++++++++++------
1 file changed, 25 insertions(+), 11 deletions(-)
diff --git a/libraries/base/dp/src/vhdl/dp_bsn_align_v2.vhd b/libraries/base/dp/src/vhdl/dp_bsn_align_v2.vhd
index 27447448cb..61db3174e6 100644
--- a/libraries/base/dp/src/vhdl/dp_bsn_align_v2.vhd
+++ b/libraries/base/dp/src/vhdl/dp_bsn_align_v2.vhd
@@ -45,6 +45,13 @@
-- the required circular buffer size just enough, such that the next power
-- of two is only a few blocks larger, instead of almost a factor two
-- larger. This then can save a significant amount of block RAM.
+-- For example: The circular buffer size c_buffer_nof_blocks is 1 + the
+-- sum of bsn latencies at each node. Therefor if g_nof_aligners_max = 16
+-- (a power of two) and g_bsn_latency_max = 2, then the circular buffer
+-- becomes true_log_pow2(1 + 16 * 2) = 64 blocks, so almost twice as large
+-- as needed. If the first input stream does not have active remote input,
+-- or is disabled via stream_en_arr, then choose g_bsn_latency_first_node
+-- = 1, to get a buffer size of true_log_pow2(1 + 15 * 2 + 1) = 32 blocks.
-- . In case of a chain of aligners then the circular buffer size depends on
-- the latency of local input. The most remote input will only use a
-- fraction of the buffer. Therefore more block RAM can be saved by using
@@ -65,14 +72,8 @@
-- the local stream. Streams index > 0 is for remote streams. The
-- remote streams arrive later than the local stream, but within
-- g_bsn_latency_max or within an integer multiple of g_bsn_latency_max.
--- . g_bsn_latency_max: maximum travel latency of a remote block in number
--- of block periods T_blk.
--- . g_nof_aligners_max: Number of dp_bsn_align_v2 aligners in a chain.
--- = 1 when only align at last node, or
--- > 1 when align at every intermediate node in a chain of nodes, and then
--- g_nof_aligners_max should equal the number of nodes for
--- chain_node_index range. The g_nof_aligners_max is the number of
--- nodes in the chain including the first node.
+-- . g_bsn_latency_max: >= 1, maximum travel latency of a remote block in
+-- number of block periods T_blk.
-- . g_bsn_latency_first_node: typically <= g_bsn_latency_max of the other
-- nodes in a chain. Use g_bsn_latency_first_node = 0 for immediate
-- output from first node in a chain of nodes. Only used when
@@ -81,6 +82,12 @@
-- size of the circular buffer. If the circular buffer is large enough
-- anyway, then the g_bsn_latency_first_node setting is don't care,
-- assuming that a little extra latency is don't care.
+-- . g_nof_aligners_max: Number of dp_bsn_align_v2 aligners in a chain.
+-- = 1 when only align at last node, or
+-- > 1 when align at every intermediate node in a chain of nodes, and then
+-- g_nof_aligners_max should equal the number of nodes for
+-- chain_node_index range. The g_nof_aligners_max is the number of
+-- nodes in the chain including the first node.
--
-- Inputs:
-- . chain_node_index: Node index in chain of nodes. First node has index 0.
@@ -104,6 +111,13 @@
-- APERTIF. Main differences are that the old component uses FIFO buffers,
-- timeouts and states, and v2 does not, which makes v2 simpler and more
-- robust.
+-- . The g_bsn_latency_first_node = 0 should also be feasible, but does not
+-- work and is not investigated further, because g_bsn_latency_first_node =
+-- 1 in combination with g_bsn_latency_max = 2 is sufficient to reduce the
+-- circular buffer size when g_nof_aligners_max is a power of two.
+-- . Using a circular buffer with optimum size, that does not have to have a
+-- power of two number of blocks, makes the circular buffer control and
+-- access more complicated and is not investigated further.
library IEEE,common_lib;
use IEEE.std_logic_1164.all;
@@ -115,7 +129,7 @@ use work.dp_stream_pkg.all;
entity dp_bsn_align_v2 is
generic (
g_nof_streams : natural := 2; -- >= 2, number of input and output streams
- g_bsn_latency_max : natural := 2;
+ g_bsn_latency_max : natural := 2; -- >= 1
g_bsn_latency_first_node : natural := 2; -- default use same as g_bsn_latency_max
g_nof_aligners_max : positive := 16;
g_block_size : natural := 1024; -- > 1, g_block_size=1 is not supported
@@ -153,7 +167,7 @@ entity dp_bsn_align_v2 is
end dp_bsn_align_v2;
architecture rtl of dp_bsn_align_v2 is
- -- Circular buffer per stream, size is next power of 2 that fits
+ -- Circular buffer per stream, size is next power of two that fits
constant c_buffer_nof_blocks : natural := sel_a_b(g_nof_aligners_max = 1,
true_log_pow2(1 + g_bsn_latency_max),
true_log_pow2(1 + g_bsn_latency_max * (g_nof_aligners_max - 1) + g_bsn_latency_first_node));
@@ -165,7 +179,7 @@ architecture rtl of dp_bsn_align_v2 is
nof_dat => c_ram_size,
init_sl => '0');
- -- Use +1 to ensure that g_block_size that is power of 2 also fits in c_block_size_slv
+ -- Use +1 to ensure that g_block_size that is power of two also fits in c_block_size_slv
constant c_block_size_w : natural := ceil_log2(g_block_size + 1);
constant c_block_size_slv : std_logic_vector(c_block_size_w - 1 downto 0) := TO_UVEC(g_block_size, c_block_size_w);
constant c_blk_pointer_w : natural := ceil_log2(c_buffer_nof_blocks);
--
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