linux-kernel - Re: Flushing writes to PCI devices

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Message-id: <4511CD43.6060507@shaw.ca>
Date:	Wed, 20 Sep 2006 17:22:43 -0600
From:	Robert Hancock <hancockr@...w.ca>
To:	Alan Stern <stern@...land.harvard.edu>
Cc:	Kernel development list <linux-kernel@...r.kernel.org>
Subject: Re: Flushing writes to PCI devices

Alan Stern wrote:
> I've heard that to insure proper synchronization it's necessary to flush
> MMIO writes (writel, writew, writeb) to PCI devices by reading from the
> same area.  Is this equally true for I/O-space writes (inl, inw, inb)?  
> What about configuration space writes (pci_write_config_dword etc.)?

Technically, according to the PCI specification, I/O space writes may be 
posted in the host bus bridge (though unlike MMIO space, not at any 
other PCI bridges) and would require a read in order to flush them. 
However, there seem to be few if any chipsets ever made which actually 
do this for I/O writes, and most code in the kernel seems to just assume 
it won't happen, so it's probably safe to assume the same.

-- 
Robert Hancock      Saskatoon, SK, Canada
To email, remove "nospam" from hancockr@...pamshaw.ca
Home Page: http://www.roberthancock.com/

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