From: Pankaj Suryawanshi <pankajssuryawanshi@gmail.com>
To: Christoph Hellwig <hch@lst.de>
Cc: pankaj.suryawanshi@einfochips.com, minchan@kernel.org,
linux-kernel@vger.kernel.org, Michal Hocko <mhocko@kernel.org>,
linux-mm@kvack.org, iommu@lists.linux-foundation.org,
minchan.kim@gmail.com, Robin Murphy <robin.murphy@arm.com>,
Vlastimil Babka <vbabka@suse.cz>
Subject: Re: cma_remap when using dma_alloc_attr :- DMA_ATTR_NO_KERNEL_MAPPING
Date: Thu, 25 Jul 2019 23:39:08 +0530 [thread overview]
Message-ID: <CACDBo56RWh=kjhEm_eOpzkTuZ+A-VEuCYPnVJW1BYAXrP6LERg@mail.gmail.com> (raw)
In-Reply-To: <20190716121026.GB2388@lst.de>
On Tue, Jul 16, 2019 at 5:40 PM Christoph Hellwig <hch@lst.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
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prev parent reply other threads:[~2019-07-25 18:09 UTC|newest]
Thread overview: 4+ messages / expand[flat|nested] mbox.gz Atom feed top
2019-07-12 18:30 cma_remap when using dma_alloc_attr :- DMA_ATTR_NO_KERNEL_MAPPING Pankaj Suryawanshi
2019-07-16 12:02 ` Robin Murphy
2019-07-16 12:10 ` Christoph Hellwig
2019-07-25 18:09 ` Pankaj Suryawanshi [this message]
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