linux-kernel - Re: [PATCH 1/3 v3] dma: document dma_{un}map_{single|sg}

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Date:	Fri, 29 Feb 2008 13:45:46 +1100
From:	"Michael Ellerman" <michael@...erman.id.au>
To:	akepner@....com
Cc:	"Tony Luck" <tony.luck@...el.com>,
	"Jesse Barnes" <jbarnes@...tuousgeek.org>,
	"Jes Sorensen" <jes@....com>,
	"Randy Dunlap" <randy.dunlap@...cle.com>,
	"Roland Dreier" <rdreier@...co.com>,
	"James Bottomley" <James.Bottomley@...senpartnership.com>,
	"David Miller" <davem@...emloft.net>,
	"Benjamin Herrenschmidt" <benh@...nel.crashing.org>,
	"Grant Grundler" <grundler@...isc-linux.org>,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH 1/3 v3] dma: document dma_{un}map_{single|sg}_attrs() interface

On Thu, Feb 28, 2008 at 2:24 PM,  <akepner@....com> wrote:
>
>  Document the new dma_{un}map_{single|sg}_attrs() functions.
>
>  diff --git a/Documentation/DMA-attributes.txt b/Documentation/DMA-attributes.txt
>  index e69de29..36baea5 100644
>  --- a/Documentation/DMA-attributes.txt
>  +++ b/Documentation/DMA-attributes.txt
>  @@ -0,0 +1,29 @@
>  +                       DMA attributes
>  +                       ==============
>  +
>  +This document describes the semantics of the DMA attributes that are
>  +defined in linux/dma-attrs.h.
>  +
>  +
>  +DMA_ATTR_SYNC_ON_WRITE
>  +----------------------
>  +
>  +DMA_ATTR_SYNC_ON_WRITE is used on the IA64_SGI_SN2 architecture.
>  +It provides a mechanism for devices to explicitly order their DMA
>  +writes.
>  +
>  +On IA64_SGI_SN2 machines, DMA may be reordered within the NUMA
>  +interconnect. Allowing reordering improves performance, but in some
>  +situations it may be necessary to ensure that one DMA write is
>  +complete before another is visible. For example, if the device does
>  +a DMA write to indicate that data is available in memory, DMA of the
>  +"completion indication" can race with DMA of data.
>  +
>  +When a memory region is mapped with the DMA_ATTR_SYNC_ON_WRITE attribute,
>  +a write to that region causes all in-flight DMA to be flushed to memory.
>  +Any pending DMA will complete and be visible in memory before the write
>  +to the region with the DMA_ATTR_SYNC_ON_WRITE attribute becomes visible.

I'm not clear how this is all meant to work. Your intial patch says
this is an interface to pass "architecture-specific
attributes" from drivers through to the DMA mapping code, which is
fair enough - we want to do something similar.

But it's not clear that DMA_ATTR_SYNC_ON_WRITE is architecture
specific. If I was a driver writer might assume it works on all
platforms. And in fact in patch 3/3 you define it in
linux/dma-attrs.h, so it's not architecture specific.

What is architecture specific is the semantic. DMA_ATTR_SYNC_ON_WRITE
is defined entirely in terms of what it does on ia64 hardware, and a
particular flavour thereof.

It just seems to me we're going to end up with a gazillion flags,
because DMA_ATTR_SYNC_ON_WRITE isn't quite the same semantic as what
Power can do. So we'll have to add
DMA_ATTR_SYNC_ON_WRITE_WRT_OTHER_SYNC_ON_WRITE_MAPPINGS_ONLY and so
on. Or, we end with subtle driver bugs because the semantics aren't
clear across platforms.

I guess I think the attributes should either be truly arch-specific,
ie. DMA_ATTR_IA64_SYNC_ON_WRITE. Or we need to come up with some well
defined, and architecture neutral semantics for what the flags mean.

cheers
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