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

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Date:   Tue, 15 Jan 2019 19:31:33 +0100
From:   "hch@....de" <hch@....de>
To:     Thomas Hellstrom <thellstrom@...are.com>
Cc:     "hch@....de" <hch@....de>,
        "christian.koenig@....com" <christian.koenig@....com>,
        "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, Jan 15, 2019 at 06:03:39PM +0000, Thomas Hellstrom wrote:
> 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)

And there is nothing preventing dma-buf sharing of these buffers.
Unlike the get_sgtable mess it can actually work reliably on
architectures that have virtually tagged caches and/or don't
guarantee cache coherency with mixed attribute mappings.

> 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.

I know of no architectures that can do cache maintainance on a less
than cache line basis.  Either the instructions require you to
specifcy cache lines, or they do sometimes more, sometimes less
intelligent rounding up.

Note that as long dma non-coherent buffers are devices owned it
is up to the device and the user space driver to take care of
coherency, the kernel very much is out of the picture.