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From: "Michael Ellerman" <michael@ellerman.id.au>
To: akepner@sgi.com
Cc: "Tony Luck" <tony.luck@intel.com>,
	"Jesse Barnes" <jbarnes@virtuousgeek.org>,
	"Jes Sorensen" <jes@sgi.com>,
	"Randy Dunlap" <randy.dunlap@oracle.com>,
	"Roland Dreier" <rdreier@cisco.com>,
	"James Bottomley" <James.Bottomley@hansenpartnership.com>,
	"David Miller" <davem@davemloft.net>,
	"Benjamin Herrenschmidt" <benh@kernel.crashing.org>,
	"Grant Grundler" <grundler@parisc-linux.org>,
	linux-kernel@vger.kernel.org
Subject: Re: [PATCH 1/3 v3] dma: document dma_{un}map_{single|sg}_attrs() interface
Date: Fri, 29 Feb 2008 13:45:46 +1100	[thread overview]
Message-ID: <dc1166600802281845t1613beedj7a285399a21cd@mail.gmail.com> (raw)
In-Reply-To: <20080228032448.GS11012@sgi.com>

On Thu, Feb 28, 2008 at 2:24 PM,  <akepner@sgi.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

  reply	other threads:[~2008-02-29  2:45 UTC|newest]

Thread overview: 18+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2008-02-28  3:24 akepner
2008-02-29  2:45 ` Michael Ellerman [this message]
2008-02-29 18:25   ` Grant Grundler
2008-02-29 18:37     ` James Bottomley
2008-03-01  2:56       ` Benjamin Herrenschmidt
2008-03-01  3:11         ` Jesse Barnes
2008-03-01  7:18         ` Grant Grundler
2008-03-05 18:13       ` akepner
2008-03-05 19:02         ` Jesse Barnes
2008-03-06  6:01         ` Michael Ellerman
2008-03-12  1:19           ` akepner
2008-03-14  4:13             ` Grant Grundler
2008-03-14  4:30             ` Michael Ellerman
2008-03-14  5:21               ` Grant Grundler
2008-03-14 16:40                 ` Jesse Barnes
2008-03-18  1:08                 ` Michael Ellerman
2008-03-20  0:32               ` akepner
2008-02-29 21:23   ` akepner

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