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    SubjectRe: DMA using data buffer vmapped in kernel space
    From
    2010/3/7 Russell King - ARM Linux <linux@arm.linux.org.uk>:
    > On Sat, Mar 06, 2010 at 02:07:12PM +0100, Thomas Koeller wrote:
    >> Am Donnerstag, 4. März 2010 22:36:34 schrieb Russell King - ARM Linux:
    >> > Cache maintainence is done using virtual addresses for L1, and
    >> > physical addresses for L2.  There's the need for virtual addresses
    >> > to be translatable to physical addresses, which is only true for
    >> > the kernel direct mapped region (pages between PAGE_OFFSET and
    >> > high_memory).
    >>
    >> Isn't the mapping created by vmap() sufficient for the virt/phys
    >> translation? In which way is this case different from a buffer
    >> passed in from user space, where the constituent pages are not
    >> in the directly mapped kernel region either?
    >
    > No different.
    >
    > The requirement is that dma_map_single() is passed a virtual address
    > in the kernel direct-mapped memory region, which is translatable using
    > virt_to_phys() and friends.
    I had encounter a similiar problem and I simply allocated a new
    buffer, copy the data, then DMA. It seems slow and stupid.

    I'm wondering wether could I translate the vmap virt to phys(don't
    know how to yet), then use phys_to_virt to get the virt in
    direct-mapped memory region?

    Is there other possible ways?

    > Anything which requires a page table lookup to obtain the physical
    > address is just not acceptable - that requires taking locks and other
    > messy things, plus is grossly inefficient.

    Best Regards,
    Mac Lin
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