linux-kernel - Re: [PATCH 2/3] dma: override "dma_flags_set

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Date:	Tue, 21 Aug 2007 15:55:29 -0500
From:	James Bottomley <James.Bottomley@...senPartnership.com>
To:	Randy Dunlap <randy.dunlap@...cle.com>
Cc:	akepner@....com, Jes Sorensen <jes@....com>,
	linux-kernel <linux-kernel@...r.kernel.org>, rdreier@...co.com,
	linux-ia64 <linux-ia64@...r.kernel.org>
Subject: Re: [PATCH 2/3] dma: override "dma_flags_set_dmaflush" for sn-ia64

On Tue, 2007-08-21 at 13:05 -0700, Randy Dunlap wrote:
> > diff --git a/Documentation/DMA-API.txt b/Documentation/DMA-API.txt
> > index cc7a8c3..e117b72 100644
> > --- a/Documentation/DMA-API.txt
> > +++ b/Documentation/DMA-API.txt
> > @@ -392,6 +392,28 @@ Notes:  You must do this:
> >  
> >  See also dma_map_single().
> >  
> > +int
> > +dma_flags_set_dmaflush(int dir)
> > +
> > +Amend dir (one of the enum dma_data_direction values), with a platform-
> 
> no comma.
> 
> > +specific "dmaflush" attribute. Unless the platform supports "posted DMA" 
> 
> add comma after "posted DMA" and drop lots of trailing spaces.
> 
> > +this is a no-op

Almost every platform supports posted DMA ... its a property of most PCI
bridge chips.

> > +On platforms that support posted DMA, dma_flags_set_dmaflush() causes 
> > +device writes to the associated memory region to flush in-flight DMA. 
> > +This can be important, for example, when (DMA) writes to the memory 
> > +region indicate that DMA of data is complete. If DMA of data and DMA of 
> > +the completion indication race, as they can do when the platform supports 
> > +posted DMA, then the completion indication may arrive in host memory 
> > +ahead of some data.

This isn't possible on most platforms.  PCI write posting can only be
flushed by a read transaction on the device (or sometimes any device on
the bridge).  Either this interface is misnamed and misdescribed, or it
can't work for most systems.

> > +To prevent this, you might map the memory region used for completion 
> > +indications as follows:
> > +
> > +	int count, flags = dma_flags_set_dmaflush(DMA_BIDIRECTIONAL);
> > +	.....
> > +	count = dma_map_sg(dev, sglist, nents, flags);

James


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