linux-kernel - Re: [RFC] disable PCIE 'Enable No Snoop' bit by default

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Date:	Sun, 9 Sep 2007 09:43:43 -0700
From:	Greg KH <greg@...ah.com>
To:	David Miller <davem@...emloft.net>
Cc:	matthew@....cx, shaohua.li@...el.com, linux-kernel@...r.kernel.org,
	linux-pci@...ey.karlin.mff.cuni.cz, akpm@...ux-foundation.org,
	gregkh@...e.de
Subject: Re: [RFC] disable PCIE 'Enable No Snoop' bit by default

On Thu, Sep 06, 2007 at 05:40:38AM -0700, David Miller wrote:
> From: Matthew Wilcox <matthew@....cx>
> Date: Thu, 6 Sep 2007 05:57:31 -0600
> 
> > I'm not sure your analysis is correct.  Here's what my draft copy of
> > the pcie 2.0 spec says:
> > 
> >   Enble No Snoop ? If this bit is Set, the Function is permitted to
> >   Set the No Snoop bit in the Requester Attributes of transactions it
> >   initiates that do not require hardware enforced cache coherency (see
> >   Section 2.2.6.5). Note that setting this bit to 1b should not cause
> >   a Function to Set the No Snoop attribute on all transactions that it
> >   initiates. Even when this bit is Set, a Function is only permitted
> >   to Set the No Snoop attribute on a transaction when it can guarantee
> >   that the address of the transaction is not stored in any cache in
> >   the system.  This bit permitted to be hardwired to 0b if a Function
> >   would never Set the No Snoop attribute in transactions it initiates.
> >   Default value of this bit is 1b.
> > 
> > That implies that devices are only allowed to set it when it's safe to
> > do so ... and we don't need to turn it off.
> 
> This is my understanding of this area of PCI-E as well, and I
> also agree that therefore we should not turn this bit off.

I agree.  But Shaohua, do you see any problems that this patch fixes?

thanks,

greg k-h
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