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Subject: Re: [PATCH 4/8] dma-buf: add peer2peer flag
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From:   =?UTF-8?Q?Christian_K=c3=b6nig?= <ckoenig.leichtzumerken@gmail.com>
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Date:   Fri, 20 Apr 2018 10:58:50 +0200
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Am 20.04.2018 um 09:13 schrieb Daniel Vetter:
> On Thu, Apr 19, 2018 at 01:16:57AM -0700, Christoph Hellwig wrote:
>> On Mon, Apr 16, 2018 at 03:38:56PM +0200, Daniel Vetter wrote:
>>> We've broken that assumption in i915 years ago. Not struct page backed
>>> gpu memory is very real.
>>>
>>> Of course we'll never feed such a strange sg table to a driver which
>>> doesn't understand it, but allowing sg_page == NULL works perfectly
>>> fine. At least for gpu drivers.
>> For GPU drivers on x86 with no dma coherency problems, sure.  But not
>> all the world is x86.  We already have problems due to dmabugs use
>> of the awkward get_sgtable interface (see the common on
>> arm_dma_get_sgtable that I fully agree with), and doing this for memory
>> that doesn't have a struct page at all will make things even worse.
> x86 dma isn't coherent either, if you're a GPU :-) Flushing gpu caches
> tends to be too expensive, so there's pci-e support and chipset support to
> forgo it. Plus drivers flushing caches themselves.
>
> The dma_get_sgtable thing is indeed fun, right solution would probably be
> to push the dma-buf export down into the dma layer. The comment for
> arm_dma_get_sgtable is also not a realy concern, because dma-buf also
> abstracts away the flushing (or well is supposed to), so there really
> shouldn't be anyone calling the streaming apis on the returned sg table.
> That's why dma-buf gives you an sg table that's mapped already.
>
>>> If that's not acceptable then I guess we could go over the entire tree
>>> and frob all the gpu related code to switch over to a new struct
>>> sg_table_might_not_be_struct_page_backed, including all the other
>>> functions we added over the past few years to iterate over sg tables.
>>> But seems slightly silly, given that sg tables seem to do exactly what
>>> we need.
>> It isn't silly.  We will have to do some surgery like that anyway
>> because the current APIs don't work.  So relax, sit back and come up
>> with an API that solves the existing issues and serves us well in
>> the future.
> So we should just implement a copy of sg table for dma-buf, since I still
> think it does exactly what we need for gpus?
>
> Yes there's a bit a layering violation insofar that drivers really
> shouldn't each have their own copy of "how do I convert a piece of dma
> memory into  dma-buf", but that doesn't render the interface a bad idea.

Completely agree on that.

What we need is an sg_alloc_table_from_resources(dev, resources, 
num_resources) which does the handling common to all drivers.

Christian.

> -Daniel