Re: DMA-API attr - DMA_ATTR_NO_KERNEL_MAPPING

[Pankaj Suryawanshi]

On Wed, Jun 26, 2019 at 11:21 PM Christoph Hellwig <hch@xxxxxxxxxxxxx> wrote:
>
> On Wed, Jun 26, 2019 at 10:12:45PM +0530, Pankaj Suryawanshi wrote:
> > [CC: linux kernel and Vlastimil Babka]
>
> The right list is the list for the DMA mapping subsystem, which is
> iommu@xxxxxxxxxxxxxxxxxxxxxxxxxxx  I've also added that.
>
> > > I am writing driver in which I used DMA_ATTR_NO_KERNEL_MAPPING attribute
> > > for cma allocation using dma_alloc_attr(), as per kernel docs
> > > https://www.kernel.org/doc/Documentation/DMA-attributes.txt  buffers
> > > allocated with this attribute can be only passed to user space by calling
> > > dma_mmap_attrs().
> > >
> > > how can I mapped in kernel space (after dma_alloc_attr with
> > > DMA_ATTR_NO_KERNEL_MAPPING ) ?
>
> You can't.  And that is the whole point of that API.

1. We can again mapped in kernel space using dma_remap() api , because
when we are using  DMA_ATTR_NO_KERNEL_MAPPING for dma_alloc_attr it
returns the page as virtual address(in case of CMA) so we can mapped
it again using dma_remap().

2. We can mapped in kernel space using vmap() as used for ion-cma
https://github.com/torvalds/linux/tree/master/drivers/staging/android/ion
 as used in function ion_heap_map_kernel().

Please let me know if i am missing anything.