Re: [PATCH 1/3 v3] dma: document dma_{un}map_{single|sg}_attrs() interface

Michael Ellerman
2008-03-14T04:32:18Z

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On Tue, 2008-03-11 at 18:19 -0700, akepner@sgi.com wrote:
> I have a new proposal for the documentation portion of this
> patchset. The code changes essentially amount to
> s/DMA_ATTR_SYNC_ON_WRITE/DMA_ATTR_BARRIER/ so thought I'd send
> just the doc change for comments now.
>
> The description of the DMA_ATTR_BARRIER is now very short and
> generic - no particular architecture is even mentioned. I can add
> a sentence or two near the architecture-specific changes in
> arch/ia64/sn/pci/pci_dma.c about why the implementation works
> on ia64/sn, etc.

I like this better, sorry to keep nitpicking, but just a few comments
below ..

> diff --git a/Documentation/DMA-attributes.txt b/Documentation/DMA-attributes.txt
> index e69de29..a4106ec 100644
> --- a/Documentation/DMA-attributes.txt
> +++ b/Documentation/DMA-attributes.txt
> @@ -0,0 +1,24 @@
> + DMA attributes
> + ==============
> +
> +This document describes the semantics of the DMA attributes that are
> +defined in linux/dma-attrs.h.
> +
> +DMA_ATTR_BARRIER
> +----------------
> +
> +DMA_ATTR_BARRIER is a barrier attribute for DMA. DMA to a memory
> +region with the DMA_ATTR_BARRIER attribute forces all pending DMA
> +writes to complete, and thus provides a mechanism to strictly order
> +DMA from a device across all intervening busses and bridges. This
> +barrier is not specific to a particular type of interconnect, it
> +applies to the system as a whole, and so its implementation must
> +account for the idiosyncracies of the system all the way from the
> +DMA device to memory.

You say a "DMA to a memory region with the DMA_ATTR_BARRIER attribute
forces all pending DMA writes to complete". Does it force _all_ DMA
writes to complete, or just writes to the region, or just writes coming
from devices? What if something is writing to a device?

Does DMA_ATTR_BARRIER have any effect on reads?

cheers

--
Michael Ellerman
OzLabs, IBM Australia Development Lab

wwweb: http://michael.ellerman.id.au
phone: +61 2 6212 1183 (tie line 70 21183)

We do not inherit the earth from our ancestors,
we borrow it from our children. - S.M.A.R.T Person

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