[Dynamic buffer] [Mellanox] Calculate the peer response time according to the speed (sonic-net#1930)

- What I did
Peer response time was deduced from a fixed value for all the ports speed. According to IEEE802.3 31B.3.7 at different operating speeds, the peer response time is also different, to have a more accurate headroom allocation, the way to calculate the peer response time shall be changed accordingly. See HLD update PR sonic-net/SONiC#870 for more info

- Why I did it
To have the peer response time more accurate, calculate according to the definition in IEEE802.3 31B.3.7

- How I verified it
Run buffer regression tests on various Mellanox platforms at various port speeds, cable lengths, with or w/o gearbox.

Author: keboliu

cfgmgr/buffer_headroom_mellanox.lua

@@ -35,6 +35,23 @@ local state_db = "6"
 
 local ret = {}
 
+-- pause quanta should be taken for each operating speed is defined in IEEE 802.3 31B.3.7
+-- the key of table pause_quanta_per_speed is operating speed at Mb/s
+-- the value of table pause_quanta_per_speed is the number of pause_quanta
+local pause_quanta_per_speed = {}
+pause_quanta_per_speed[400000] = 905
+pause_quanta_per_speed[200000] = 453
+pause_quanta_per_speed[100000] = 394
+pause_quanta_per_speed[50000] = 147
+pause_quanta_per_speed[40000] = 118
+pause_quanta_per_speed[25000] = 80
+pause_quanta_per_speed[10000] = 67
+pause_quanta_per_speed[1000] = 2
+pause_quanta_per_speed[100] = 1
+
+-- Get pause_quanta from the pause_quanta_per_speed table
+local pause_quanta = pause_quanta_per_speed[port_speed]
+
 if gearbox_delay == nil then
     gearbox_delay = 0
 end
@@ -55,7 +72,8 @@ for i = 1, #asic_table_content, 2 do
     if asic_table_content[i] == "mac_phy_delay" then
         mac_phy_delay = tonumber(asic_table_content[i+1]) * 1024
     end
-    if asic_table_content[i] == "peer_response_time" then
+    -- If failed to get pause_quanta from the table, then use the default peer_response_time stored in state_db
+    if asic_table_content[i] == "peer_response_time" and  pause_quanta == nil then
         peer_response_time = tonumber(asic_table_content[i+1]) * 1024
     end
 end
@@ -124,6 +142,11 @@ else
     bytes_on_gearbox = port_speed * gearbox_delay / (8 * 1024)
 end
 
+-- If successfully get pause_quanta from the table, then calculate peer_response_time from it
+if pause_quanta ~= nil then
+    peer_response_time = (pause_quanta) * 512 / 8
+end
+
 bytes_on_cable = 2 * cable_length * port_speed * 1000000000 / speed_of_light / (8 * 1024)
 propagation_delay = port_mtu + bytes_on_cable + 2 * bytes_on_gearbox + mac_phy_delay + peer_response_time