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

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Date:   Tue, 15 Jan 2019 19:13:11 +0000
From:   "Koenig, Christian" <Christian.Koenig@....com>
To:     "hch@....de" <hch@....de>, Thomas Hellstrom <thellstrom@...are.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

Am 15.01.19 um 19:31 schrieb hch@....de:
> 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.

Thomas is correct that the interface you propose here doesn't work at 
all for GPUs.

The kernel driver is not informed of flush/sync, but rather just setups 
coherent mappings between system memory and devices.

In other words you have an array of struct pages and need to map that to 
a specific device and so create dma_addresses for the mappings.

Regards,
Christian.