linux-kernel - Re: [PATCH] lib/scatterlist: Provide a DMA page iterator

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Message-ID: <20190115152029.GB2325@lst.de>
Date:   Tue, 15 Jan 2019 16:20:29 +0100
From:   "hch@....de" <hch@....de>
To:     christian.koenig@....com
Cc:     Thomas Hellstrom <thellstrom@...are.com>,
        "hch@....de" <hch@....de>, "jgg@...pe.ca" <jgg@...pe.ca>,
        "syeh@...are.com" <syeh@...are.com>,
        "linux-rdma@...r.kernel.org" <linux-rdma@...r.kernel.org>,
        "daniel.vetter@...ll.ch" <daniel.vetter@...ll.ch>,
        "jian.xu.zheng@...el.com" <jian.xu.zheng@...el.com>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "dri-devel@...ts.freedesktop.org" <dri-devel@...ts.freedesktop.org>,
        "sakari.ailus@...ux.intel.com" <sakari.ailus@...ux.intel.com>,
        "bingbu.cao@...el.com" <bingbu.cao@...el.com>,
        "yong.zhi@...el.com" <yong.zhi@...el.com>,
        "shiraz.saleem@...el.com" <shiraz.saleem@...el.com>,
        "tian.shu.qiu@...el.com" <tian.shu.qiu@...el.com>,
        "linux-media@...r.kernel.org" <linux-media@...r.kernel.org>
Subject: Re: [PATCH] lib/scatterlist: Provide a DMA page iterator

On Tue, Jan 15, 2019 at 03:24:55PM +0100, Christian König wrote:
> Yeah, indeed. Bounce buffers are an absolute no-go for GPUs.
>
> If the DMA API finds that a piece of memory is not directly accessible by 
> the GPU we need to return an error and not try to use bounce buffers behind 
> the surface.
>
> That is something which always annoyed me with the DMA API, which is 
> otherwise rather cleanly defined.

That is exactly what I want to fix with my series to make
DMA_ATTR_NON_CONSISTENT more useful and always available:

https://lists.linuxfoundation.org/pipermail/iommu/2018-December/031985.html

With that you allocate the memory using dma_alloc_attrs with
DMA_ATTR_NON_CONSISTENT, and use dma_sync_single_* to transfer
ownership to the cpu and back to the device, with a gurantee that
there won't be any bouncing.  So far the interest by the parties that
requested the feature has been rather lacklustre, though.