From: Christoph Hellwig <hch@infradead.org>
Date: Tue, 12 Jul 2005 21:27:54 +0100

quoted

+/* Enable 2 buffer mode by default for SGI system */
+#ifdef CONFIG_IA64_SGI_SN2
+#define CONFIG_2BUFF_MODE
+#endif

this enabled it only on kernel that are built to only run on SN2
hardware, which is completely useless in practice.  Besides that defining
a CONFIG_ symbol from source files is a big no-go.

Yes, do this in the Kconfig file instead.