linux-kernel - Re: cma_remap when using dma_alloc_attr :- DMA_ATTR_NO_KERNEL

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Date:   Thu, 25 Jul 2019 23:39:08 +0530
From:   Pankaj Suryawanshi <pankajssuryawanshi@...il.com>
To:     Christoph Hellwig <hch@....de>
Cc:     Robin Murphy <robin.murphy@....com>, linux-kernel@...r.kernel.org,
        linux-mm@...ck.org, iommu@...ts.linux-foundation.org,
        Vlastimil Babka <vbabka@...e.cz>,
        Michal Hocko <mhocko@...nel.org>,
        pankaj.suryawanshi@...fochips.com, minchan@...nel.org,
        minchan.kim@...il.com
Subject: Re: cma_remap when using dma_alloc_attr :- DMA_ATTR_NO_KERNEL_MAPPING

On Tue, Jul 16, 2019 at 5:40 PM Christoph Hellwig <hch@....de> wrote:
>
> On Tue, Jul 16, 2019 at 01:02:19PM +0100, Robin Murphy wrote:
> >> Lets say 4k video allocation required 300MB cma memory but not required
> >> virtual mapping for all the 300MB, its require only 20MB virtually mapped
> >> at some specific use case/point of video, and unmap virtual mapping after
> >> uses, at that time this functions will be useful, it works like ioremap()
> >> for cma_alloc() using dma apis.
> >
> > Hmm, is there any significant reason that this case couldn't be handled
> > with just get_vm_area() plus dma_mmap_attrs(). I know it's only *intended*
> > for userspace mappings, but since the basic machinery is there...
>
> Because the dma helper really are a black box abstraction.
>
> That being said DMA_ATTR_NO_KERNEL_MAPPING and DMA_ATTR_NON_CONSISTENT
> have been a constant pain in the b**t.  I've been toying with replacing
> them with a dma_alloc_pages or similar abstraction that just returns
> a struct page that is guaranteed to be dma addressable by the passed
> in device.  Then the driver can call dma_map_page / dma_unmap_page /
> dma_sync_* on it at well.  This would replace DMA_ATTR_NON_CONSISTENT
> with a sensible API, and also DMA_ATTR_NO_KERNEL_MAPPING when called
> with PageHighmem, while providing an easy to understand API and
> something that can easily be fed into the various page based APIs
> in the kernel.
>
> That being said until we get arm moved over the common dma direct
> and dma-iommu code, and x86 fully moved over to dma-iommu it just
> seems way too much work to even get it into the various architectures
> that matter, never mind all the fringe IOMMUs.  So for now I've just
> been trying to contain the DMA_ATTR_NON_CONSISTENT and
> DMA_ATTR_NO_KERNEL_MAPPING in fewer places while also killing bogus
> or pointless users of these APIs.


I agree with you Christoph, users want page based api, which is useful
in many scenarios, but
As of now i think we have to move with this type of api which is
useful when dma_alloc (for cma )call with DMA_ATTR_NO_KERNEL_MAPPING,
and mapped again to kernel space, this api is useful mostly for 32-bit
system which has 4GB of limited virtual memory (its very less for
android devices) as we have already dma_mmap_attr() for user space
mapping.
This api is also useful for one who directly want to use cma_alloc()
in their own drivers. For example ion-cma.c.
Please let me know if any recommendation/suggestion/improvement required ?

Regards,
Pankaj