On 28/06/2019 17:29, Pankaj Suryawanshi wrote:
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@xxxxxxxxxxxxxxxxxxxxxxxxxx. 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().
No, you really can't. A caller of dma_alloc_attrs(...,
DMA_ATTR_NO_KERNEL_MAPPING) cannot make any assumptions about the void*
it returns, other than that it must be handed back to dma_free_attrs()
later. The implementation is free to ignore the flag and give back a
virtual mapping anyway. Any driver which depends on how one particular
implementation on one particular platform happens to behave today is,
essentially, wrong.
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.
If you want a kernel mapping, *don't* explicitly request not to have a
kernel mapping in the first place. It's that simple.
Robin.
