From the "Physical Layer Simplified Specification Version 3.01":

  A known data block ("Tuning block") can be used to tune sampling
  point for tuning required hosts. [...]
  This procedure gives the system optimal timing for each specific
  host and card combination and compensates for static delays in
  the timing budget including process, voltage and different PCB
  loads and skews. [...]
  Data block, carried by DAT[3:0], contains a pattern for tuning
  sampling position to receive data on the CMD and DAT[3:0] line.

[based on a patch from Alistair Francis <alistair.francis@xilinx.com>
 from qemu/xilinx tag xilinx-v2015.2]
Signed-off-by: Philippe Mathieu-Daudé <f4bug@amsat.org>
Reviewed-by: Alistair Francis <alistair.francis@xilinx.com>
Message-id: 20180309153654.13518-5-f4bug@amsat.org
Signed-off-by: Peter Maydell <peter.maydell@linaro.org>