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

From: Thomas Hellstrom <thellstrom@vmware.com>
To: "hch@lst.de" <hch@lst.de>,
	"christian.koenig@amd.com" <christian.koenig@amd.com>
Cc: "linux-rdma@vger.kernel.org" <linux-rdma@vger.kernel.org>,
	"daniel.vetter@ffwll.ch" <daniel.vetter@ffwll.ch>,
	"jian.xu.zheng@intel.com" <jian.xu.zheng@intel.com>,
	"linux-kernel@vger.kernel.org" <linux-kernel@vger.kernel.org>,
	"dri-devel@lists.freedesktop.org"
	<dri-devel@lists.freedesktop.org>, "jgg@ziepe.ca" <jgg@ziepe.ca>,
	"sakari.ailus@linux.intel.com" <sakari.ailus@linux.intel.com>,
	"bingbu.cao@intel.com" <bingbu.cao@intel.com>,
	"yong.zhi@intel.com" <yong.zhi@intel.com>,
	"shiraz.saleem@intel.com" <shiraz.saleem@intel.com>,
	"tian.shu.qiu@intel.com" <tian.shu.qiu@intel.com>,
	"linux-media@vger.kernel.org" <linux-media@vger.kernel.org>
Subject: Re: [PATCH] lib/scatterlist: Provide a DMA page iterator
Date: Tue, 15 Jan 2019 18:03:39 +0000	[thread overview]
Message-ID: <41d0616e95fb48942404fb54d82249f5700affb1.camel@vmware.com> (raw)
In-Reply-To: <20190115152029.GB2325@lst.de>

On Tue, 2019-01-15 at 16:20 +0100, hch@lst.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

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