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

[Christoph Hellwig]

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.