linux-kernel - Re: [PATCH v4 0/4] Add lightweight memory barriers for coherent memory access

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Date:	Tue, 18 Nov 2014 14:47:15 -0800
From:	Alexander Duyck <alexander.h.duyck@...hat.com>
To:	Linus Torvalds <torvalds@...ux-foundation.org>
CC:	"linux-arch@...r.kernel.org" <linux-arch@...r.kernel.org>,
	Network Development <netdev@...r.kernel.org>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	Mathieu Desnoyers <mathieu.desnoyers@...ymtl.ca>,
	Peter Zijlstra <peterz@...radead.org>,
	Benjamin Herrenschmidt <benh@...nel.crashing.org>,
	Heiko Carstens <heiko.carstens@...ibm.com>,
	Ingo Molnar <mingo@...nel.org>,
	Michael Neuling <mikey@...ling.org>,
	Russell King - ARM Linux <linux@....linux.org.uk>,
	donald.c.skidmore@...el.com, matthew.vick@...el.com,
	Geert Uytterhoeven <geert@...ux-m68k.org>,
	Jeff Kirsher <jeffrey.t.kirsher@...el.com>,
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	Martin Schwidefsky <schwidefsky@...ibm.com>,
	Frédéric Weisbecker <fweisbec@...il.com>,
	David Miller <davem@...emloft.net>
Subject: Re: [PATCH v4 0/4] Add lightweight memory barriers for coherent memory
 access

On 11/18/2014 12:53 PM, Linus Torvalds wrote:
> On Tue, Nov 18, 2014 at 9:28 AM, Alexander Duyck
> <alexander.h.duyck@...hat.com> wrote:
>> These patches introduce two new primitives for synchronizing cache coherent
>> memory writes and reads.  These two new primitives are:
>>
>>          coherent_rmb()
>>          coherent_wmb()
>
> So I'm still not convinced about the name. I don't hate it, but if you
> ever want to do "read_acquire", then that whole "coherent_" thing does
> make for a big mouthful. I don't see why "dma" isn't simpler and more
> to the point, and has the advantage of lining up (in documentation
> etc) with "smp".

The problem is DMA is a broad brush.  There are multiple cases I can 
think of where DMA does not represent coherent memory.

> Why would you ever use "coherent_xyz()" on something that isn't about
> dma? If it's cache-coherent memory without DMA, you'd use "smp_xyz()",
> so I really do prefer that whole "dma-vs-smp" issue, because it talks
> about what is actually the important issue. All sane memory is
> coherent, after all (and if it isn't, you have other issues than
> memory ordering).
>
>                    Linus

This barrier only applies to a subset of DMA memory types.  So yes, 
"coherent_xyz()" only applies to DMA, but not all DMA memory is coherent 
as it could be a non-coherent or streaming DMA mapping.

One spot where the name makes sense is in the headers themselves.  To 
avoid duplication of definitions in several spots if CONFIG_SMP was 
defined I simply defined smp_xyz() as coherent_xyz().  Defining it as 
dma_xyz() might have made that more difficult to read in terms of what 
was going on.

- Alex
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