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

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Message-ID: <41d0616e95fb48942404fb54d82249f5700affb1.camel@vmware.com>
Date:   Tue, 15 Jan 2019 18:03:39 +0000
From:   Thomas Hellstrom <thellstrom@...are.com>
To:     "hch@....de" <hch@....de>,
        "christian.koenig@....com" <christian.koenig@....com>
CC:     "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "yong.zhi@...el.com" <yong.zhi@...el.com>,
        "daniel.vetter@...ll.ch" <daniel.vetter@...ll.ch>,
        "linux-rdma@...r.kernel.org" <linux-rdma@...r.kernel.org>,
        "linux-media@...r.kernel.org" <linux-media@...r.kernel.org>,
        "bingbu.cao@...el.com" <bingbu.cao@...el.com>,
        "jian.xu.zheng@...el.com" <jian.xu.zheng@...el.com>,
        "tian.shu.qiu@...el.com" <tian.shu.qiu@...el.com>,
        "shiraz.saleem@...el.com" <shiraz.saleem@...el.com>,
        "sakari.ailus@...ux.intel.com" <sakari.ailus@...ux.intel.com>,
        "dri-devel@...ts.freedesktop.org" <dri-devel@...ts.freedesktop.org>,
        "jgg@...pe.ca" <jgg@...pe.ca>
Subject: Re: [PATCH] lib/scatterlist: Provide a DMA page iterator

On Tue, 2019-01-15 at 16:20 +0100, hch@....de wrote:
> 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://na01.safelinks.protection.outlook.com/?url=https%3A%2F%2Flists.linuxfoundation.org%2Fpipermail%2Fiommu%2F2018-December%2F031985.html&amp;data=02%7C01%7Cthellstrom%40vmware.com%7Cb1799c4073024a824f9408d67afcfcea%7Cb39138ca3cee4b4aa4d6cd83d9dd62f0%7C0%7C0%7C636831624340834140&amp;sdata=JiRBfzZMvN3joJ4vKiErbzXAHaNzuBcLapRJDL%2Bt6Hc%3D&amp;reserved=0
> 
> 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.

In the graphics case, it's probably because it doesn't fit the graphics
use-cases:

1) Memory typically needs to be mappable by another device. (the "dma-
buf" interface)
2) DMA buffers are exported to user-space and is sub-allocated by it.
Mostly there are no GPU user-space kernel interfaces to sync / flush
subregions and these syncs may happen on a smaller-than-cache-line
granularity.

So to help the graphics driver, that coherent flag would be much more
useful.

/Thomas