linux-kernel - Re: DMA-API attr - DMA_ATTR_NO_KERNEL

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Message-ID: <CACDBo56fNVxVyNEGtKM+2R0X7DyZrrHMQr6Yw4NwJ6USjD5Png@mail.gmail.com>
Date:   Fri, 28 Jun 2019 21:59:25 +0530
From:   Pankaj Suryawanshi <pankajssuryawanshi@...il.com>
To:     Christoph Hellwig <hch@...radead.org>
Cc:     linux-mm@...ck.org, Michal Hocko <mhocko@...nel.org>,
        linux-kernel@...r.kernel.org, Vlastimil Babka <vbabka@...e.cz>,
        iommu@...ts.linux-foundation.org
Subject: Re: DMA-API attr - DMA_ATTR_NO_KERNEL_MAPPING

On Wed, Jun 26, 2019 at 11:21 PM Christoph Hellwig <hch@...radead.org> 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@...ts.linux-foundation.org.  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.