O_DIRECT can't work on openwrt?

Question

I'm developing a program which need to access a special USB device. This USB device acts as a regular file in filesystem, so I have to open this file with O_DIRECT flag. As follow:

open(pathname, O_CREAT | O_RDWR | O_DIRECT | O_SYNC, S_IRWXU)

The program works well on PC environment. But when I port it to embedded board with openwrt, the "open" function returns EINVAL 22 /* Invalid argument */.

• O_DIRECT support is selected in kernel configuration.
• The filesystem of openwrt is squashfs and jffs2.
• The filesystem of USB device is fat, and mounted on /media/aegis directory.
• The ARCH of board is mips.

It seems that error is returned from following function in kernel:

int open_check_o_direct(struct file *f)
{
         /* NB: we're sure to have correct a_ops only after f_op->open */
         if (f->f_flags & O_DIRECT) {
                 if (!f->f_mapping->a_ops ||
                     ((!f->f_mapping->a_ops->direct_IO) &&
                     (!f->f_mapping->a_ops->get_xip_mem))) {
                         return -EINVAL;
                 }
         }
         return 0;
 }

Is it known that O_DIRECT isn't supported on jffs2 and supported on fat. When operating on file in /media/aegis I guess the a_ops of fat is used, but program doesn't run in my expectation.