patacongo opened a new pull request #248: Network Loopbacka Driver:  A 
configuration option to control packet s…
URL: https://github.com/apache/incubator-nuttx/pull/248
 
 
   …ize.
   
   Historically, the loopback driver used the largest packet size of all 
enabled link layer protocols.  This permitted packets to be forward via the 
loopbak device with no major loss of performance.  However, in experimenting 
with configurations where no other link layer protocols were enabled, this 
means the loopback packet size was set to the smallest possible size, to the 
SLIP minimum of 296 bytes.  This resulted in terrible loopback performance.
   
   This commit adds an option to increase the loopback packet size with the 
option CONFIG_NET_LOOPBACK_PACKETSIZE.
   
   The loopback driver packet buffer should be quite large.  The larger the 
loopback packet buffer, the better will be TCP performance of the loopback 
transfers.  The Linux loopback device historically used packet buffers of size 
16Kb, but that was increased in recent Linux versions to 64Kb.  Those sizes may 
be excessive for resource constrained MCUs, however.
   
   The network still enforces the lower limit that is the maximum packet size 
of all enabled link layer protocols.  But this new option permits the loopback 
packet size to be increased from that.
   
   * net/Kconfig:  Adds CONFIG_NET_LOOPBACK_PKTSIZE option
   * include/nuttx/net/netconfig.h:  Assures that the packet size that is used 
is at least as large as the largest packet size of other link layer protocols.
   * drivers/net/loopback.c:  Use that larger packet size.
   * boards/sim/sim/sim/configs/tcploop/defconfig:  Set the loopback packet 
size to 1500

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