Should I use FBINFO_VIRTFB?

I'm cleaning up my fbdev driver, and I just noticed FBINFO_VIRTFB:

#define FBINFO_VIRTFB		0x0004 /* FB is System RAM, not device. */

I am currently not setting this flag, but I am allocating my framebuffer in system ram via dma_alloc_coherent().  I don't see any good documentation for this flag, but I suspect I should be enabling it.  What exactly does this flag do?

I'd also like some explanation for these two macros, which appear to be related:

#define FBINFO_PARTIAL_PAN_OK	0x0040 /* otw use pan only for double-buffering */
#define FBINFO_READS_FAST	0x0080 /* soft-copy faster than rendering */

-- 
Timur Tabi
Linux kernel developer at Freescale