linux-kernel - Re: [RFC] avoid indirect calls for DMA direct mappings

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Date:   Fri, 7 Dec 2018 02:21:42 +0100
From:   Christoph Hellwig <hch@....de>
To:     Robin Murphy <robin.murphy@....com>
Cc:     Christoph Hellwig <hch@....de>,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        iommu@...ts.linux-foundation.org, brouer@...hat.com,
        tariqt@...lanox.com, ilias.apalodimas@...aro.org, toke@...e.dk,
        Linux List Kernel Mailing <linux-kernel@...r.kernel.org>
Subject: Re: [RFC] avoid indirect calls for DMA direct mappings

On Thu, Dec 06, 2018 at 08:24:38PM +0000, Robin Murphy wrote:
> On 06/12/2018 20:00, Christoph Hellwig wrote:
>> On Thu, Dec 06, 2018 at 06:54:17PM +0000, Robin Murphy wrote:
>>> I'm pretty sure we used to assign dummy_dma_ops explicitly to devices at
>>> the point we detected the ACPI properties are wrong - that shouldn't be too
>>> much of a headache to go back to.
>>
>> Ok.  I've cooked up a patch to use NULL as the go direct marker.
>> This cleans up a few things nicely, but also means we now need to
>> do the bypass scheme for all ops, not just the fast path.  But we
>> probably should just move the slow path ops out of line anyway,
>> so I'm not worried about it.  This has survived some very basic
>> testing on x86, and really needs to be cleaned up and split into
>> multiple patches..
>
> I've also just finished hacking something up to keep the arm64 status quo - 
> I'll need to actually test it tomorrow, but the overall diff looks like the 
> below.

Nice.  I created a branch that picked up your bits and also the ideas
from Linus, and the result looks reall nice.  I'll still need a signoff
for your bits, though.

Jesper, can you give this a spin if it changes the number even further?

  git://git.infradead.org/users/hch/misc.git dma-direct-calls.2

  http://git.infradead.org/users/hch/misc.git/shortlog/refs/heads/dma-direct-calls.2