Re: [PATCH] NOMMU: Fix MAP_PRIVATE mmap() of objects where the data can be mapped directly

[David Howells <dhowells@redhat.com>]

graff yang <graff.yang@gmail.com> wrote:

> > +       if (capabilities & BDI_CAP_MAP_DIRECT) {
> 
> This will breaks many drivers, e.g. some frame-buffer drivers, on NOMMU
> system.  Because they don't have get_unmapped_area().  These drivers depend
> on it's mmap() to return the frame-buffer base address.

Then they won't work on NOMMU.  Read Documentation/nommu-mmap.txt:

	============================================
	PROVIDING SHAREABLE CHARACTER DEVICE SUPPORT
	============================================

	To provide shareable character device support, a driver must provide a
	file->f_op->get_unmapped_area() operation. The mmap() routines will
	call this to get a proposed address for the mapping. This may return an
	error if it doesn't wish to honour the mapping because it's too long,
	at a weird offset, under some unsupported combination of flags or
	whatever.

The chardev driver doesn't provide the virtual address through its mmap() fop
in MMU-mode either - that's provided by do_mmap_pgoff().

David