when O=dir is used in make:

  CC      drivers/accessibility/braille/braille_console.o
Assembler messages:
Fatal error: can't create drivers/accessibility/braille/braille_console.o: No such file or directory
  CC      crypto/aes_generic.o
make[3]: *** [drivers/accessibility/braille/braille_console.o] Error 2
make[2]: *** [drivers/accessibility] Error 2
make[1]: *** [drivers] Error 2

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~Randy