tests/avocado: Truncate M2S-FG484 SOM SPI flash to 16MiB

The M2S-FG484 SOM uses a 16 MiB SPI flash (Spansion
S25FL128SDPBHICO).  Since the test asset is bigger,
truncate it to the correct size to avoid when running
the test_arm_emcraft_sf2 test:

  qemu-system-arm: device requires 16777216 bytes, block backend provides 67108864 bytes

Add comment regarding the M2S-FG484 SOM hardware in
hw/arm/msf2-som.c.

Reported-by: Cédric Le Goater <clg@kaod.org>
Signed-off-by: Philippe Mathieu-Daudé <philmd@linaro.org>
Reviewed-by: Cédric Le Goater <clg@kaod.org>
Tested-by: Cédric Le Goater <clg@kaod.org>
Signed-off-by: Cédric Le Goater <clg@kaod.org>

hw/arm/msf2-som.c

@@ -1,6 +1,9 @@
 /*
  * SmartFusion2 SOM starter kit(from Emcraft) emulation.
  *
+ * M2S-FG484 SOM hardware architecture specification:
+ *   https://www.emcraft.com/jdownloads/som/m2s/m2s-som-ha.pdf
+ *
  * Copyright (c) 2017 Subbaraya Sundeep <sundeep.lkml@gmail.com>
  *
  * Permission is hereby granted, free of charge, to any person obtaining a copy
@@ -87,7 +90,7 @@ static void emcraft_sf2_s2s010_init(MachineState *machine)
 
     /* Attach SPI flash to SPI0 controller */
     spi_bus = qdev_get_child_bus(dev, "spi0");
-    spi_flash = qdev_new("s25sl12801");
+    spi_flash = qdev_new("s25sl12801"); /* Spansion S25FL128SDPBHICO */
     qdev_prop_set_uint8(spi_flash, "spansion-cr2nv", 1);
     if (dinfo) {
         qdev_prop_set_drive_err(spi_flash, "drive",

tests/avocado/boot_linux_console.py

@@ -400,6 +400,8 @@ class BootLinuxConsole(LinuxKernelTest):
         spi_hash = '65523a1835949b6f4553be96dec1b6a38fb05501'
         spi_path = self.fetch_asset(spi_url, asset_hash=spi_hash)
 
+        file_truncate(spi_path, 16 << 20) # Spansion S25FL128SDPBHICO is 16 MiB
+
         self.vm.set_console()
         kernel_command_line = self.KERNEL_COMMON_COMMAND_LINE
         self.vm.add_args('-kernel', uboot_path,