linux-kernel - Re: [PATCH 09/10] swiotlb: add support for non-coherent DMA

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Date:   Fri, 26 Oct 2018 10:04:21 +0200
From:   Christoph Hellwig <hch@....de>
To:     Robin Murphy <robin.murphy@....com>
Cc:     Christoph Hellwig <hch@....de>, Will Deacon <will.deacon@....com>,
        Catalin Marinas <catalin.marinas@....com>,
        Konrad Rzeszutek Wilk <konrad.wilk@...cle.com>,
        linux-arm-kernel@...ts.infradead.org,
        iommu@...ts.linux-foundation.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH 09/10] swiotlb: add support for non-coherent DMA

On Mon, Oct 22, 2018 at 06:11:04PM +0100, Robin Murphy wrote:
>>   +	if (!dev_is_dma_coherent(dev) &&
>> +	    (attrs & DMA_ATTR_SKIP_CPU_SYNC) == 0)
>
> Nit: other parts of the file are already using the "!(...)" style rather 
> than "(...) == 0".

Indeed.  Probably some kind of copy and past error..

> I had a quick play at moving the cache maintenance here out into the 
> callers, which comes out arguably looking perhaps a little cleaner (only +1 
> source line overall, and actually reduces text size by 32 bytes for my 
> build), but sadly I can't really see any way of doing the equivalent for 
> map/unmap short of duplicating the whole 3-line arch_sync_*() block, which 
> just makes for a different readability problem. As you mentioned on patch 
> #7, I guess this really is just one of those things which has no nice 
> solution, so cosmetics aside,

It looks pretty ok, but given that I want to send this version to
Linus for the merge window I don't really dare to touch it.

The master plan for 4.21 is to actually merge swiotlb into dma-direct
and to allow direct calls to dma-direct given how expensive indirect
calls are with spectre, and dma mappings show up badly in various
benchmarks.  I'll see if I can do that in a way following what you've
done.